Dissertations / Theses on the topic 'Macles de la calcite'

La compréhension et la simulation des mécanismes de déformation dans la croûte supérieure sont des enjeux scientifiques et techniques importants. La calcite étant un minéral commun de la croûte supérieure se déformant essentiellement par maclage sous 200 °C, c'est pour cette raison que l'on s'intéresse plus particulièrement au maclage de la calcite dans le cadre de cette thèse. L'utilisation du maclage de la calcite n'en est pas à son coup d'essai et de nombreuses méthodes d'inversions existent que ce soit pour remonter aux tenseurs des contraintes comme aux tenseurs des déformations. Cette thèse propose une nouvelle méthode d'inversion basée sur l'inversion d'Etchopar permettant de reconstruire 5 sur 6 paramètres du tenseur des contraintes avec une quantification fine des incertitudes. Cette méthode permet de détecter automatiquement l'existence d'un ou de plusieurs évènements tectoniques enregistrés par le maclage de la calcite. Un deuxième volet de la thèse consiste en l'amélioration de la méthode d'acquisition des données de macles en utilisant un EBSD (electron backscatter diffraction). En effet, l'utilisation traditionnelle de la platine universelle a des limitations techniques et amène aussi à un doute visuel sur l'appréciation du statut non-maclé de certains plans, en plus du côté long et fastidieux. Le dernier volet de la thèse consiste à déterminer le seuil de maclage de la calcite pour différentes tailles de grains à l'aide de tests mécaniques sous une presse uniaxiale. Qui plus est, le suivi en continu des déformations sur les monocristaux de calcite a permis de mettre en évidence le comportement macroscopique d'un cristal de calcite et la séquence de maclage
The understanding and modelling of deformation mechanics in the upper crust are important scientific and technical issues. The calcite is a common mineral in the upper crust and mainly deforms by twinning under 200°C. That is why we are interested by calcite twinning as part of this thesis. It is not the first time that calcite twinning is used to reconstruct paleostress or paleostrain tensors. This thesis propose a new inversion method based on the Etchecopar’s one allowing to reconstruct 5 among 6 parameters of the stress tensor with an accurate quantification of the uncertainties. This method allows to automatically detect the realness of one or several tectonic events recorded by calcite twinning. A second part of the thesis consists in the improvement of the data acquisition by using EBSD (electron backscatter diffraction). In fact, the traditional use of the universal stage has technical limitations and brings some optical doubt about the untwinned status of few planes in addition to the long and tedious side. The last part of the thesis consists in the establishing the threshold of calcite twinning for different grain sizes by mechanical tests under a uniaxial press. Moreover, the continuous monitoring of the experiments using single crystals of calcite allowed to highlight the macroscopic behavior of a single crystal and the sequence of twinning

La compréhension et la simulation des mécanismes de déformation dans la croûte supérieure sont des enjeux scientifiques et techniques importants. La calcite étant un minéral commun de la croûte supérieure se déformant essentiellement par maclage sous 200 °C, c'est pour cette raison que l'on s'intéresse plus particulièrement au maclage de la calcite dans le cadre de cette thèse. L'utilisation du maclage de la calcite n'en est pas à son coup d'essai et de nombreuses méthodes d'inversions existent que ce soit pour remonter aux tenseurs des contraintes comme aux tenseurs des déformations. Cette thèse propose une nouvelle méthode d'inversion basée sur l'inversion d'Etchopar permettant de reconstruire 5 sur 6 paramètres du tenseur des contraintes avec une quantification fine des incertitudes. Cette méthode permet de détecter automatiquement l'existence d'un ou de plusieurs évènements tectoniques enregistrés par le maclage de la calcite. Un deuxième volet de la thèse consiste en l'amélioration de la méthode d'acquisition des données de macles en utilisant un EBSD (electron backscatter diffraction). En effet, l'utilisation traditionnelle de la platine universelle a des limitations techniques et amène aussi à un doute visuel sur l'appréciation du statut non-maclé de certains plans, en plus du côté long et fastidieux. Le dernier volet de la thèse consiste à déterminer le seuil de maclage de la calcite pour différentes tailles de grains à l'aide de tests mécaniques sous une presse uniaxiale. Qui plus est, le suivi en continu des déformations sur les monocristaux de calcite a permis de mettre en évidence le comportement macroscopique d'un cristal de calcite et la séquence de maclage
The understanding and modelling of deformation mechanics in the upper crust are important scientific and technical issues. The calcite is a common mineral in the upper crust and mainly deforms by twinning under 200°C. That is why we are interested by calcite twinning as part of this thesis. It is not the first time that calcite twinning is used to reconstruct paleostress or paleostrain tensors. This thesis propose a new inversion method based on the Etchecopar’s one allowing to reconstruct 5 among 6 parameters of the stress tensor with an accurate quantification of the uncertainties. This method allows to automatically detect the realness of one or several tectonic events recorded by calcite twinning. A second part of the thesis consists in the improvement of the data acquisition by using EBSD (electron backscatter diffraction). In fact, the traditional use of the universal stage has technical limitations and brings some optical doubt about the untwinned status of few planes in addition to the long and tedious side. The last part of the thesis consists in the establishing the threshold of calcite twinning for different grain sizes by mechanical tests under a uniaxial press. Moreover, the continuous monitoring of the experiments using single crystals of calcite allowed to highlight the macroscopic behavior of a single crystal and the sequence of twinning

La présente thèse a pour objectif d'éprouver des méthodologies permettant de reconstituer l'évolution des propriétés réservoirs et de la pression subit par un réservoir. Ce manuscrit présente (1) une caractérisation pétrologique des roches réservoirs avec une estimation du calendrier de l'évolution de la porosité, (2) les résultats une étude paléopiézométrique, et (3) une évolution des contraintes effectives et potentielles (sur)pressions de fluides ayant prévalu dans le réservoir carbonaté au cours de son évolution. Ces éléments permettent de reconstituer l'histoire d'enfouissement et les paléocontraintes subies par des réservoirs carbonatés dans un contexte de marge passive. Pour cela ce travail s'est basé sur une approche couplée de deux outils paléopiézométrique complémentaires combinant l'inversion des macles de la calcite et l'inversion de la rugosité des stylolites. Cette approche a été combinée avec des analyses pétrographiques, géochimiques, géomécaniques et géochronologiques pour bien caractériser le réservoir. Le matériel étudié lors de cette thèse consiste en des carottes de forage offshore récupérées de puits profonds fournis par TotalEnergies situés dans les bassins du Bas Congo et de Kwanza sur la marge ouest africaine de l'océan Atlantique Sud. Ces bassins ont subi un épisode de rifting au Crétacé inférieur (145.5 - 112 Ma). L'étude de la destruction de la porosité des réservoirs pré-sel de la formation TOCA syn-rift d'âge Barrémien (130-125 Ma) à partir de carottes prélevées dans le bassin du Bas Congo a révélé que la porosité initiale avait été réduite à sa valeur actuelle de 4-8% au cours des 35 premiers Ma de son histoire d'enfouissement, atteignant ~10% après seulement 10 Ma, i.e. dans les 400-500 premiers mètres d'enfouissement et que la porosité actuelle n'avait pas évolué de manière significative depuis 95 Ma (fin de la stylolitisation). Cette étude a ainsi montré que les propriétés des réservoirs dans les formations carbonatées bioclastiques telles que la formation TOCA peuvent être largement contrôlés par des processus diagénétiques précoces et très superficiels plutôt que par des réactions mésogénétiques qui se produisent ultérieurement en profondeur dans l'histoire de l'enfouissement. Une étude paléopiézométrique a été réalisée dans le but de reconstituer l'histoire d'enfouissement et des paléocontraintes des formations carbonatées de la TOCA (syn-rift) du Barrémien et Sendji (post-rift) de l'Albien sur la marge ouest africaine. La paléopiézométrie basée sur l'inversion de la rugosité des stylolites et l'inversion des macles de calcite a été combinée à l'analyse de la fracturation, à la datation géochronologique U-Pb du ciment calcitique et à la modélisation de l'enfouissement des deux formations pour reconstruire les orientations et les grandeurs des contraintes horizontales et verticales ayant affecté les deux formations au cours du temps. L'inversion des macles de la calcite réalisée sur des ciments précoces a révélé que les carbonates syn-rift et post-rift avaient enregistré une histoire des paléocontraintes polyphasée, incluant (1) des régimes de contraintes extensifs liés à l'ouverture l'océan Atlantique Sud (145.5-112 Ma), avec un σ3 orienté NE-SW à E-W, et à l'échelle du bassin des failles normales orientées N-S et NE-SW. Cette phase d'extension n'est observée que dans les formations syn-rift. (2) des régimes de contraintes extensifs associés à une tectonique salifère locale affectant les formations post sel, trouvés uniquement dans la formation Sendji (101 à 80 Ma) avec un σ3 orienté ~N-S et ~E-W. (3) Des régimes de contraintes compressifs et décrochants avec un σ1 horizontal orienté ~N-S à NE-SW probablement liés à la collision Afrique-Eurasie. (4) Des régimes de contraintes compressifs et décrochants avec un σ1 horizontal orienté ~E-W que nous avons proposé d’attribuer à la poussée à la ride médio-Atlantique et qui prévalent depuis ~15 - 10 Ma
This thesis aims to test methodologies allowing the reconstruction of the evolution of reservoir properties and their pressure. This manuscript presents (1) a petrological characterization of the reservoir rocks with an estimation of the timing of porosity evolution, (2) the results of a paleopiezometric study, and (3) an evolution of effective stresses and potential fluid (over)pressures that prevailed in the carbonate reservoir during its evolution. These tools allow the reconstruction of the burial history and the paleostresses experienced by carbonate reservoirs in a passive margin. For this purpose, this work was based on a coupled approach of two complementary paleopiezometric tools combining the inversion of calcite twin data and stylolite roughness data. This approach was combined with petrographic, geochemical, geomechanical and geochronological analyses to fully characterize the reservoir. The studied material during this thesis consists of offshore cores recovered from deep wells provided by TotalEnergies located in the Lower Congo and Kwanza basins on the West African margin of the South Atlantic Ocean. These basins underwent a rifting event in the early Cretaceous times (145.5 - 112 Ma). The study of the porosity destruction of the pre-salt reservoirs of the syn-rift TOCA formation of Barremian age (130-125 Ma) from offshore core located in the Lower Congo basin revealed that the initial porosity was reduced to its current value of 4-8% during the first 35 Ma of its burial history, reaching ~10% after only 10 Ma, i.e. in the first 400-500 meters of burial and that the current porosity has not evolved significantly since 95 Ma (end of stylolitization). This study has shown that the outcome of reservoir properties in bioclastic carbonate formations such as the TOCA formation may be largely controlled by early and very shallow diagenetic processes rather than by mesogenetic reactions that occur later in the burial history. A paleopiezometric study was carried out in order to reconstruct the burial and paleostress history of the TOCA (syn-rift) Barremian and Sendji (post-rift) Albian carbonate formations on the West African margin. Paleopiezometry based on stylolite roughness inversion and calcite twins inversion was combined with fracture analysis, U-Pb geochronological dating of the calcite cement and burial modelling of the both formations to unravel the orientations and magnitudes of horizontal and vertical stresses that affected the TOCA and Sendji formations over time. The inversion of calcite macles on early diagenetic cements revealed that syn-rift and post-rift carbonates recorded a complex, polyphase paleostress history, (1) extensional stress regimes related to the opening of the South Atlantic ocean (145.5-112 Ma), with a σ3 oriented NE-SW to E-W, and at the basin scale N-S and NE-SW oriented normal faults. This extensional phase is only recorded by the TOCA syn-rift formation. (2) Extensional stress regimes associated with local salt tectonics that affect the post-salt formations, only found in the Sendji formation (101 to 80 Ma) with a σ3 oriented ~N-S and ~E-W. (3) Compressional and strike-slip stress regimes with horizontal σ1 oriented ~N-S to NE-SW probably related to the Africa-Eurasia collision at ~67-60 Ma. (4) Compressional stress regimes with horizontal σ1 oriented ~E-W that we proposed to attribute to the mid-Atlantic ridge push and that have prevailed since 15-10 Ma onwards

In this dissertation I have created and applied a parametric model for bulk carbonate materials. The new empirical model for carbonates is stable for a wide range of carbonate structures and reproduces experimental results with reasonable accuracy. To study the surface of calcite the ab initio code SIESTA has been used. New implementation has been introduced into the SIESTA code to allow the calculation of effective charges using the modern theory of polarisation. Using these charges the calculation of the long range electrostatic effects, which are removed by the zero electric field boundary conditions, have been introduced into the phonon methodology, reproducing the LO-TO splitting within the calculated phonon modes near the F-point. Furthermore the effective charges have been used in the calculation of the infrared intensity for each phonon mode. The SIESTA implementation of DFT relies upon the evaluation of electron density on a real-space grid. Such discretization of the real-space integrals introduces an oscillatory error in the energy and forces, with the periodicity of the real-space grid. A method for reducing this error has been introduced. The SIESTA code with the new methodology has been used to study bulk calcite, {211} calcite surface and the interaction of water with the {211} sur­face. The structure and phonon frequencies for the bulk match well with experimental values. The {211} surface has been calculated showing the response of the crystal in both distortion of the ion position and the electronic configuration. Surface relaxations and phonon frequencies show no symmetry breaking reconstruction of the calcite {211} surface. Calculation of the interaction of water molecules with the {211} surface predicts the optimum position for water on the surface.

The aim of this thesis is to develop an atomistic model of the mineral-water interface taking calcium carbonate as an example. This model would enable us to gain insight into the microscopic processes involved in the growth and dissolution of minerals, In Chapter 1, major experimental and computational studies of calcite surfaces arc reviewed and the main topics of this thesis arc presented. Chapters 2 and 3 discuss the methodology used throughout this work. In Chapter 2, the potential model is explained in great detail. Chapter 3 gives an introduction to the computational techniques, namely energy minimisation and molecular dynamics, used throughout this study, together with a brief description of the density functional theory, also used in this thesis. Chapters 4 to 8 present the main results obtained during this project. In Chapter 4, energy minirnisations of the main low-index surfaces of calcite reveal that the {1014} surface is the most stable surface in both dry and hydrated conditions. In addition, it was found that the reaction of water with surface carbonate groups, which results in the dissociation of water molecules, would occur at step edges and defective surfaces rather than at the {1014} surface. In Chapter 5, a comparison of the surface energies obtained from electronic structure calculations with those described in the previous chapter shows that the relative energies are in very good agreement. Also, surface phase diagrams of three surfaces in contact with a gaseous phase suggest that calcium poor surfaces are the dominant terrninations under ambient conditions.In Chapter 6, potential parameters developed to model the interactions of carbonate surfaces with water are shown to also reproduce the structure and dynamics of the hydration shell of metal cations. This chapter also presents new potential parameters for modelling halide ions in solution. In Chapter 7, the free energy of adsorption of water on the {lOI4} surface is calculated and found to be small when compared to the enthalpy of adsorption, therefore implying a large entropy of adsorption. Also, it is shown that the free energy profiles of metal ions adsorbing on the surface are correlated with the solvent density. In Chapter 8, the distribution of ions near the surface, calculated from molecular dynamics simulations, is found to differ from the classical view of the electrical double layer. Finally, Chapter 9 gives a summary of the main results presented in this thesis

Le maclage thermique est un défaut cristallographique largement discuté dans les métaux de type CFC à faible énergie de faute d'empilement. Malgré une importante littérature scientifique dédiée à ce sujet, les mécanismes expliquant précisément la formation de ces macles thermiques ne sont pas totalement élucidés à ce jour. Dans ce travail, nous avons cherché à améliorer notre compréhension de ce phénomène fondamental en métallurgie physique. Différents matériaux de type CFC (acier inoxydable 304L, nickel pur et Inconel 718) ont été considérés. Nous avons confirmé, grâce à des expériences de traitement thermique in situ couplées à des cartographies d'orientation, que la majorité des macles thermiques sont générées durant la recristallisation. De la même manière, par une expérience réalisée sur l'Inconel 718, nous avons mis en évidence que la croissance de grains pure n'était pas source de joints de macle. Par conséquent, il semble évident que les phénomènes de recristallisation et de croissance de grains ont des régimes totalement distincts associés à des mécanismes spécifiques du point de vue de la formation des macles thermiques, et doivent donc absolument être étudiés séparément. Nous avons ainsi proposé un nouveau modèle, dans lequel l'effet du signe de la courbure moyenne du front de recristallisation est pris en compte. Les influences de différents facteurs thermomécaniques, y compris le niveau de déformation, la taille de grains initiale, la température de recuit et la vitesse de montée en température, ont été étudiées à travers deux séries d'expériences. Suite à l'effet du signe de la courbure moyenne du joint de grain, nous avons proposé une méthode pour quantifier la tortuosité du front de recristallisation. Dans cette étude, nous montrons que cette quantité est corrélée à la densité de macles post-recristallisation. En sus des analyses expérimentales, des outils numériques de type champ moyen et champ complet ont également été développés dans cette étude afin de modéliser l'évolution des macles thermiques tout en tenant en compte des mécanismes physiques mis en évidence expérimentalement. Les bases d'un nouveau modèle de type champ moyen ont été proposées afin de modéliser l'évolution de la densité de macles moyenne durant le phénomène de croissance de grains. Ce modèle, dans lequel seulement un paramètre doit être identifié par des donnés expérimentales, semble mieux décrire les résultats expérimentaux obtenus pour l'inconel 718 comparé au modèle de Pande, référence en la matière. Deux méthodes implicites i.e. la méthode level-set et la méthode champ de phase ont été comparées au niveau de leurs formulations et de leurs performances numériques pour des simulations de croissance de grains anisotrope. C'est la première fois que ces deux méthodes sont comparées dans le contexte de l'utilisation de maillages éléments finis non stucturés et hétérogènes en terme de taille de maille. Une nouvelle méthodologie a été ainsi proposée dans le cadre de l'approche level-set pour simuler l'évolution de macles thermiques durant le phénomène de croissance de grains. Dans cette nouvelle méthodologie, les joints de macles peuvent être insérés dans des microstructures synthétiques. De plus, les joints de macles peuvent être distingués selon leur nature cohérente ou incohérente. Nous avons montré à travers les différentes simulations réalisées que les propriétés spéciales des joints de macles peuvent être prises en compte avec ce nouveau formalisme
Annealing twin is a crystallographic defect that is largely reported in F.C.C. metals especially those with low stacking fault energy. Despite the amount of work dedicated to the subject, the understanding of annealing twin formation mechansims is not complete in the literature. In the present work, by applying both experimental and numerical tools, we tried to have a more profound understanding of this phenomenon, which is essential to Physical Metallurgy. For this purpose, different F.C.C. Materials including 304L stainless steel, commercially pure nickel and nickel based superalloy Inconel 718 are investigated. We confirmed that annealing twins are mainly formed in the recrystallization regime, especially driven by the migration of recrystallization front into deformed regions by using in situ EBSD technique. In addition, we found in the in situ observations that there are almost no twins generated in the grain growth regime. This observation is confirmed by another grain growth experiment performed on Inconel 718. Therefore, curvature driven grain boundary migration by itself is not sufficient to generate annealing twins. A new atomistic model to explain annealing twin formation mechanism, in which the effect of migrating boundary curvature is considered, is proposed. The effects of different thermo-mechanical factors, including prior deformation level, initial grain size, annealing temperature and the heating velocity, on annealing twin formation are determined via two experiments performed on commercially pure nickel. Based on the idea of grain boundary curvature, we proposed a method to quantify recrsytallization front tortuosity. In the present study, we show evidence that this quantity is positively correlated with the twin density at the end of the recrystallization regime. In addition to experimental studies, numerical tools including both mean field and full field approaches are applied to model annealing twin evolution during grain growth by taking into account the revealed mechanisms. A basis of a new mean field model is proposed to model annealing twin density evolution during grain growth. This model, which has only one parameter to be identified, provides a better consistency with the experimental data of Inconel 718 compared to the Pande's model. Besides, full field approaches are also applied to simulate the overall microstructure evolution during grain growth. Two implicit methods i.e. the level set and the multi-phase-field methods are compared in terms of their formulations and their numerical performance in anisotropic grain growth simulations. It is the first time that these two methods are compared in the finite element context with non-structural mesh. In the present numerical context, the level set method is more suitable to describe strong anisotropy in grain boundary energy. A new methodology is thus developed in the level set framework to simulate annealing twin evolution during grain growth. This methodology, in which we can insert annealing twin boundaries into synthetic microstructures and distinguish coherent and incoherent twin boundaries, is proven to be able to counting for the strong anisotropy introduced by coherent annealing twin boundaries

La deformation des metaux a symetrie hexagonale tels que le zinc (c/a=1,856) et le titane (c/a=1,588) entraine la formation de macles. Leurs structures atomiques ont ete identifiees a la fois theoriquement a l'aide de calculs de minimisation d'energie (relaxation) et experimentalement par microscopie electronique haute resolution. On retrouve la structure plane et symetrique decrite pour un metal ayant un rapport parametrique c/a proche de la valeur ideale (1,633). Experimentalement, une interface metastable, microfacettee, a ete observee au voisinage des defauts de macle. De plus, nous avons tout particulierement examine les marches interfaciales associees aux dislocations de macle. Nous avons propose une nouvelle nomenclature pour exprimer les vecteurs de burgers. Les vecteurs b#p#/#q expriment les vecteurs de burgers ayant une composante hors du plan d'interface et b#p#/#p les vecteurs de burgers paralleles a l'interface, ou p et q sont definis par les hauteurs respectives des marches dans le cristal 1 et le cristal 2 telles que h(1)=pd et h(2)=qd, d etant l'espacement des plans paralleles a l'interface. Nous avons mis en evidence l'origine et la stabilite de chacune des dislocations repertoriees, d'une part, grace aux parametres topologiques (vecteur de burgers et hauteur de marche) associes, et d'autre part, en considerant le flux de matiere engendre par le mouvement des dislocations. Les mecanismes d'interaction entre dislocations et de decomposition des dislocations de matrice dans le joint de macle ont ete decrits

In this thesis, the effect of dissolution on the chemistry and structure of perforate planktonic foraminiferal tests was investigated. Atomistic simulations were undertaken to study the energetics of magnesium ions in calcite and their structural ordering in the {104} surface. Shell mass, δ¹⁸O_c and Mg/Ca measurements were obtained on core-top tests of foraminifera from depth transects in the Indian and Pacific oceans. The degree to which dissolution biased the temperature determined from δ¹⁸O_c and Mg/Ca measurements was found to be similar for G. sacculifer and G. ruber but greater for the Mg/Ca-temperature estimates for P. obliquiloculata and N. dutertrei. Corrections to the Mg/Ca ratios based on core-top depth, bottom water carbonate ion concentration, and shell mass were developed. For all species, bottom water carbonate ion concentration provided the best correction for the dissolution artifact. Crystallinity of the planktonic foraminiferal tests was investigated using x-ray powder diffraction from the {104} plane of calcite. Species offsets in crystallinity were attributed to differences in test structure, in particular the proportions of primary to secondary calcite composing the test wall. A correlation between tests which had undergone increased dissolution and a narrowing of the 104 Bragg peak was observed. This change was attributed to the preferential removal of smaller crystallites (the primary calcite). The crystallinity of undissolved tests correlated with growth rate. Faster test calcification could broaden the 104 Bragg peak because overall smaller crystallites were deposited or because more primary calcite was initially deposited. An empirical potential set was generated for calcium carbonate. Trends in metal cation defect energies between calcite and aragonite stemmed from the differences in bulk geometries. The {104} calcite surface, in vacuo and hydrated, was generated from the same potential set. Surface relaxation of the in vacuo surface was investigated by calculating within layer polarization and between layer strain.

Les compositions chimiques et isotopiques traces des métaux de traces dans la calcite sont à la base de la plupart des reconstructions de températures passées, de CO2 atmosphérique et de pH océanique. Les compositions isotopiques des métaux divalents incorporées dans la structure de calcite ont également la capacité d'enregistrer des informations précieuses reflétant les conditions géochimiques de la formation. (Galy et al., 2002; Chang et al., 2004; Beard et al., 2012; Mavromatis et al., 2013; Fantle and Tipper, 2014; Mavromatis et al., 2015; Mavromatis et al., 2019). De plus, comme la répartition à l'équilibre et la cinétique des métaux divalents entre la calcite et le fluide présentent des comportements contrastés, on peut s'attendre à ce que leur composition isotopique dans la calcite présente des tendances distinctes (Schott et al., 2014). Comme la spéciation aqueuse de Ni dépend fortement du pH et de l'alcalinité, la composition isotopique de Ni adsorbée à la surface de la calcite et / ou incorporée dans le réseau de calcite pourrait fournir des informations précieuses sur l'environnement chimique dans lequel ce minéral s'est formé à l'origine. Pour calibrer cet outil potentiellement utile, des expériences ont été effectuées à 25° C pour déterminer le fractionnement isotopique du Ni pendant 1) l'adsorption de Ni à la surface de la calcite en fonction du pH de la solution (de 7,5 à 8,7) et 2) la coprécipitation de Ni avec de la calcite à pH 6,2, pCO2 = 1 atm en fonction de taux de croissance de la calcite allant de 10-8,2 à 10-7,3 mol m-2 s-1. Les résultats montrent que le facteur de fractionnement à l'équilibre, solution Δ60Nisolid-solution, pour l'adsorption de Ni sur la calcite est égal à -0,49 ± 0,16 et indépendant du pH. Les isotopes de Ni léger sont également incorporés de manière préférentielle dans la calcite pendant sa coprécipitation. L'ampleur du fractionnement isotopique du Ni diminue de -0.3 and -1 lorsque le taux de précipitation de la calcite augmente de 10-8,2 à 10-7,3 mol m-2 s-1. Ce comportement, dû à la forte hydratation de l'ion aqueux Ni2+, peut servir à estimer les taux de précipitation de la calcite et la pCO2 dans les océans du passé
The chemical and isotopic compositions of trace elements in calcite are the basis for most past temperature, atmospheric CO2, and ocean pH paleo reconstructions. The isotope compositions of divalent metals incorporated into the calcite structure also have the ability to record valuable information that reflects the geochemical conditions of formation (Galy et al., 2002; Chang et al., 2004; Beard et al., 2012; Mavromatis et al., 2013; Fantle and Tipper, 2014; Mavromatis et al., 2015; Mavromatis et al., 2019). Moreover, as the equilibrium and kinetic partition of divalent metals between calcite and fluid exhibit contrasting behaviors, it can be expected that their isotope composition in calcite exhibits distinct trends (Schott et al., 2014). Because Ni aqueous speciation strongly depends on fluid pH and alkalinity, the isotopic composition of Ni adsorbed on calcite surface and/or incorporated into the calcite lattice could provide information on the chemical environment in which this mineral originally formed. To calibrate this potentially useful tool, experiments were performed at 25°C to determine Ni isotope fractionation during 1) Ni adsorption on calcite surface as a function of solution pH (from 7.5 to 8.7) and 2) Ni coprecipitation with calcite at pH = 6.2, pCO2 = 1 atm as a function of calcite growth rates (ranging from 10-8.2 to 10-7.3 mol m-2 s-1). Results show that the equilibrium fractionation factor, Δ60Nisolid-solution, for Ni adsorption on calcite is equal to -0.49 ± 0.16‰ and is pH - independent. Light Ni isotopes are also preferentially incorporated in calcite during its coprecipitation. The extent of Ni isotope fractionation decreases from -0.3 to -1 ‰ as the calcite precipitation rate increases from 10-8.2 to 10-7.3 .mol m-2 s-1. This behavior, due to the strong hydration of the Ni2+ aqua ion, may serve to approximate calcite precipitation rates and pCO2 in past oceans

Le principal objectif de ce travail est l’investigation et la quantification de l’influence des interactions parent-macle et macle-macle sur la réponse mécanique des métaux hexagonaux, et en particulier sur celle du Mg et du Zr. Une nouvelle approche à champs moyens est tout d’abord proposée pour étudier l’interaction parent-macle. Cette dernière est déclinée en deux modèles micromécaniques à double-inclusion : un premier modèle généralisant le résultat de Tanaka-Mori aux milieux élastiques hétérogènes avec déformations libres de maclage appliqué au Mg pour du maclage primaire et secondaire, et un second modèle consistant en une extension du premier modèle à un milieu élasto-plastique appliqué à des polycristaux d’alliage de Mg subissant un maclage primaire important. Leurs résultats mettent notamment en valeur l’influence des interactions de type parent-macle sur l’évolution des contraintes internes ainsi que sur l’activation des modes de glissement et de maclage. L’influence des interactions macle-macle sur les phénomènes de nucléation et de croissance de macles est ensuite étudiée de façon statistique à partir de données EBSD extraites à partir d’un logiciel, basé sur la théorie des graphes et développé lors de la thèse. La première étude réalisée sur le Zr montre que les interactions macle-macle sont défavorables à la nucléation de nouvelles macles et que les mécanismes de croissance peuvent différer de façon conséquente d’un mode de maclage à l’autre. Une seconde étude, effectuée à partir d’échantillons d’alliage AZ31 Mg, discute des macles d’extension primaires {10-12} à faible facteur de Schmid et des double-macles {10-12}-{10-12}
The objective of this thesis is to investigate and quantify the influence of parent-twin and twin-twin interactions on the mechanical response of h.c.p. metals. To study parent-twin interactions, a mean-field continuum mechanics approach has been developed based on a double inclusion topology. A first model generalizing the Tanaka-Mori scheme to heterogeneous elastic media is applied to first and second generation twinning in Mg. The model is capable of reproducing the trends in the development of backstresses within the primary twin and enables the identification of the most likely second-generation twin variants to grow in a primary twin domain. A second self-consistent model, consisting of an extension of the first one to the case of elasto-plasticity, is applied to AZ31 Mg polycrystals. The results show that deformation system activities and plastic strain distributions within twins drastically depend on the interaction with parent domains. The influence of twin-twin interactions on nucleation and growth of twins is being statistically studied from Zr and Mg electron back-scattered diffraction (EBSD) scans. A twin recognition software relying on graph theory analysis has been developed for data extraction. The results obtained from Zr EBSD maps reveal that twin-twin interactions hinder subsequent twin nucleation and that mechanisms involved in twin growth may differ for each twinning mode. A second study performed on AZ31 Mg presents statistics about low Schmid factor {10-12} tensile twins and about {10-12}-{10-12} double twins coupled with a simplified version of the Tanaka-Mori scheme generalized to heterogeneous elasticity with plastic incompatibilities

Engineering
Ph.D.
Heavy metals like cadmium, lead and zinc pose a significant threat to human health and environment. Several factors such as pH, EH, organic matter and clay content of the soil affect the bioavailability of such heavy metals in the environment. The presence of several naturally occurring minerals such as calcite (calcium carbonate, CaCO3) and calcium oxalate (CaC2O4.) in the environment significantly influence the fate and transport of some of the heavy metal cations. Sequestration of heavy metals such as cadmium (Cd) and zinc (Zn) from solution by calcite has been clearly demonstrated in the literature. However, studies on heavy metal sequestration by calcite and calcium oxalate from a multi-metal environmental that represents real world situations are rare. In this contribution, column flow studies of Cd and Zn sequestration by calcite exposed to influent solutions saturated or non-saturated with calcite and containing either 1 mg/L of Cd, 1 mg/L of Zn or combined 1 mg/L of Zn and Cd, followed by desorption of the sequestered metals were conducted. Complementary scanning electron microscopy (SEM) coupled with electron dispersive x-ray spectroscopy (EDS) data were generated. Kinetic studies show that reaction rates of Cd and Zn with calcite are governed by a simple rate law with reaction orders of less than 1 (0.02 - 0.07) indicating at least mathematically, the occurrence of reactions that went to completion if the reaction orders did not change. When the influent solution contains a single cation, the rate of Zn removal from solution by calcite and calcium oxalate is greater than Cd removal rate. However in a multi-cation environment, cadmium removal rate was greater than zinc removal rate. MINTEQA2 a geochemical equilibrium speciation model was used to compute the equilibrium between the various species in the cation-calcite environment. Complimentary desorption studies and surface SEM/EDS analysis indicate that the removal of Cd and Zn from solution by calcite and calcium oxalate is probably due to precipitation/complexation reaction. The SEM and EDS results appear to confirm the presence of a precipitate on the mineral surface in the case of the influent solution containing Zn. The current research also examines the effect of citrate, a commonly present urinary tract species on calcium oxalate dissolution. The dissolution studies indicate that citrate solution is capable of dissolving sodium oxalate at high pH. The dissolution of calcium oxalate results in the release of heavy metals that were previously sequestered within the mineral. Results show that a greater percentage of zinc was removed than cadmium, from calcium oxalate due to its dissolution by citrate.
Temple University--Theses

Thesis (M.S.)--Miami University, Dept. of Geology, 2002.
Title from first page of PDF document. Document formatted into pages; contains vi, 78 p. : ill. Includes bibliographical references (p. 37-40).

Carbonate minerals play a very important role in nature, they represent some of the most diverse and common mineral species on the Planet. They are directly involved in the carbon dioxide (CO2) cycle acting as relatively stable long term chemical storage reservoirs, moderating both global warming trends and oceanaquatic chemistry through carbonate buffering systems. A range of synthetic metal carbonates have been synthesised for analysis under multiple experimental conditions, in order to study the variation in physical and chemical properties such as phase specificity, metal substitution, hydration/hydroxy carbonate formation under varying partial pressures of CO2 and thermal stability. Synthetic samples were characterised by a variety of instrumental analysis techniques in order to investigate chemical purity and phase specificity. Some of the techniques included, vibrational spectroscopy (IR/Raman), thermal analysis (TGA-MS) (thermal Raman), X-Ray diffraction (XRD) and electron microscopy (SEM-EDX). From the instrumental characterisation techniques, it was found that single phase smithsonite, hydrozincite, calcite and nesquehonite could successfully be synthesised under the conditions used. Minor impurities of other minerals and / or phases were found to form under specific chemical or physical conditions such as in the case of hydrozincite / simonkolleite if zinc chloride was used during hydrothermal synthesis.

Carbonate minerals play a very important role in nature, they represent some of the most diverse and common mineral species on the Planet. They are directly involved in the carbon dioxide (CO2) cycle acting as relatively stable long term chemical storage reservoirs, moderating both global warming trends and oceanaquatic chemistry through carbonate buffering systems. A range of synthetic metal carbonates have been synthesised for analysis under multiple experimental conditions, in order to study the variation in physical and chemical properties such as phase specificity, metal substitution, hydration/hydroxy carbonate formation under varying partial pressures of CO2 and thermal stability. Synthetic samples were characterised by a variety of instrumental analysis techniques in order to investigate chemical purity and phase specificity. Some of the techniques included, vibrational spectroscopy (IR/Raman), thermal analysis (TGA-MS) (thermal Raman), X-Ray diffraction (XRD) and electron microscopy (SEM-EDX). From the instrumental characterisation techniques, it was found that single phase smithsonite, hydrozincite, calcite and nesquehonite could successfully be synthesised under the conditions used. Minor impurities of other minerals and / or phases were found to form under specific chemical or physical conditions such as in the case of hydrozincite / simonkolleite if zinc chloride was used during hydrothermal synthesis.

[Truncated abstract] Characterising the behaviour of calcareous sediments that possess some degree of bonding between their constituents has attracted worldwide research interest in recent years. Although many recent studies have made significant contributions in delineating the behaviour of these sediments, there is still paucity of information particularly on the cyclic behaviour of cemented calcareous soils. This thesis describes in detail the characteristic features of cemented calcareous soils and proposes methods for characterising their cyclic behaviour. Two different calcareous soils Goodwyn (GW) and Ledge Point (LP) soils representing extreme depositional environments were examined in this study. Artificially cemented sample were created using the CIPS (Calcite Insitu Precipitation Systems) technique, considering its superiority over other most commonly available cementation techniques in replicating the natural pattern of cementation, and the behaviour of natural calcarenite under monotonic loading conditions. The experimental program involved triaxial testing of both uncemented and calcite-cemented calcareous soils under different loading conditions, i.e. isotropic compression tests to high-pressure (16 MPa), monotonic shearing tests, undrained cyclic shearing tests and undrained monotonic post-cyclic shearing tests. Significant emphasis has been placed on the cyclic behaviour of these soils. Internal submersible LDVTs were used for the accurate and continuous measurement of strain down to about 10-5

In natural aqueous systems, adsorption and co-precipitation (sorption) are regarded as possible metal sequestration mechanisms that would promote in situ passive remediation. Calcite is a ubiquitous mineral on the Earth's surface, whose propensity to scavenge trace metals has been extensively investigated. Since Nickel (Ni) appears on the EPA "priority list" of pollutants, identifying factors that may control its fate in calcite-rich natural waters (e.g. groundwaters, carbonate-rich coastal and marine sediments) is critical. Unfortunately, scarce literature is dedicated to the influence of mineral properties and solution chemistry on the affinity and sorption mechanisms of Ni on the calcite surface. With that in mind, batch sorption experiments were conducted, under controlled laboratory conditions (ambient temperature of 25 ± 2ºC, and atmospheric pCO2= 10-3.41atm) in calcite-saturated 0.1M and 0.7M NaCl solutions over a range of pH (7.5 to 8.9) and total Ni concentrations- [Ni]total- (1.2 x 10-4 to 3.5 x 10-6 M), to characterize the affinity of Ni for the calcite surface. Our experimental results revealed that the first 24 hours of reaction are critical to the metal-mineral interaction and the fractional Ni(II) sorption varied with solution pH and Ni(II) aqueous speciation. Notably, Ni sorption onto the calcite surface increased with pH and was well correlated to the relative abundance of the NiCO30(aq) species (ion-pair) over the pH range studied. After 24 hours, the overall Ni fractional sorption was attenuated with an increase of the background electrolyte (NaCl) concentration. The application of adsorption isotherm models (Langmuir and Freundlich) allowed us to highlight the dependence of adsorption to the [Ni]total and solution pH. Finally, irrespective of the solution ionic strength (0.1 and 0.7 M NaCl), Ni did not desorb readily when the total Ni(II) concentration in solution exceeded 4.0 x 10-5M.
Dans les systèmes aquatiques naturels, l'adsorption et la co-précipitation (sorption) sont considérés comme des processus passifs de séquestration in-situ des métaux lourds. La calcite est un minéral très abondant dans la croûte terrestre et un puits pour plusieurs métaux. Par conséquent, sa capacité de séquestration de métaux a été largement étudiée. Puisque le nickel (Ni) est considéré comme un polluant dangereux pour la santé humaine et environnementale, l'identification des facteurs qui peuvent contrôler son devenir dans les milieux naturels riches en calcite (eaux souterraines riches en carbonate ou sédiments côtiers et marins) est essentielle. Malheureusement, il y a peu de littérature pertinente sur l'influence des propriétés du minéral et la composition de la solution sur l'affinité et les mécanismes de sorption du Ni(II) à la surface de la calcite. Vu le manque d'information sur le sujet, nous avons mené une étude sous des conditions contrôlées en laboratoire (température ambiante de 25 (±2°C) et une pression partielle de CO2 = 10-3.41atm) dans des solutions aqueuses de NaCl saturées en calcite. Afin de caractériser l'affinité du Ni(II) pour la surface de la calcite, les expériences ont été effectuées dans des solutions de force ionique (0.1M et 0.7M), pH et concentration totale de Ni ([Ni]total) variées. Les résultats révèlent que les 24 premières heures de réaction sont critiques pour la sorption du Ni(II). La sorption du Ni(II) sur la calcite augmente avec le pH de la solution mais diminue avec l'augmentation de la concentration de NaCl. Indépendamment de la force ionique, la sorption fractionnelle du Ni est fortement corrélée avec l'abondance relative du complexe NiCO3o(aq) dans l'intervalle de pH étudié. Quelle que soit la force ionique (0.1M et 0.7M NaCl) de la solution, le Ni(II) ne se désorbe pas suite à son adsorption dans les solutions dont la concentration totale en Ni est supérieure à 4.0 x 10-5M.

Chemistry
Ph.D.
On surfaces and within interfaces occur some of the most important reactions in chemistry, from world changing industrial reactions to critical environmental processes. It is even hypothesized that the chiral nature of life arose from reactions occurring on chiral mineral surfaces. In any case adsorption typically plays a key role. Adsorption can occur on rapid time scales, particularly in catalytic systems, and it can be the precursor to highly stable surface interaction mechanisms such as surface precipitation. Surface adsorption can have a dramatic affect on the resulting surface increasing or decreasing the propensity for further reactivity or adsorption. In order to understand the processes occurring on a surface both the surface and the adsorbate must be understood. This includes a surface with any prior adsorbates. This is why many catalytic studies are done in UHV environments where clean surfaces are prepared for each experiment. The same is true with environmental surfaces, but obtaining pristine surfaces can be problematic, and systems are often extremely complicated involving organic, inorganic, and biological components. Often research is focused on just one component. A significant portion of this dissertation is focused on the adsorption of organic and inorganic species on pristine mineral surfaces. While there is significant research done on environmental surfaces, often times the surface used in studies is not well characterized. In essence lesser attention is paid to the substrate then the adsorbate. This is particularly true of infrared studies similar to the type presented in chapter 5 where carbonate is shown to exist in significant quantity on all ferrihydrite surfaces. Furthermore, chapter 4 highlights the potential for ion mobility on calcite surfaces under ambient conditions and the effect the adsorbates in chapter 3 have on the mobility process. The principal of this dissertation is to characterize fundamental surface processes which occur on calcite and ferrihydrite surfaces under ambient conditions. The hope is that this can lay the ground work for future studies where native adsorption and restructuring is taken into account on mineral surfaces during experimental studies.
Temple University--Theses

Microbial Induced Calcite Precipitation (MICP) is an attractive alternative for a variety geotechnical ground improvement practices commonly used today and has a variety of potential applications. This research focuses primarily on its use as a soil stabilization technique using the bacteria Sporosarcina Pasteurii and a single injection point percolation method adapted from previous research in granular soils. This method, and most published data, show an inherent variability in both physical and engineering properties due to the distribution of precipitated calcite within the specimen. The focus of this research is on the quantification of the variability in shear strength parameters induced by MICP treatment in sand. Also, on the initial development of a new treatment method which aims to reduce this inherent variability and offer a more feasible option for field applications. The MICP treated soil columns were sampled at constant intervals from the injection point and then subject to direct shear testing (DST) and calcite distribution analysis. This analysis reiterates previously documented reduction in cementation as distance from injection point increases. The reduction in cementation results in reduced shear strength parameter improvements. This research also concluded a minimum of two percent mass of calcite per total mass of treated soil for significant strength improvements.

Nature aggregates granular materials such as sand, silt and clay into form beach rocks, and other forms of microbialites. A common bio-geo-chemical cementation occurs due to the mineralization of calcium through bacterial enzymatic action, often called Microbial Induced Calcium Carbonate Precipitation (MICP). It is a sustainable construction method. Experimental and Numerical models are presented to study the aggregated granular materials formed due to MICP. The models are applied to investigate different levels of aggregation of sand. The numerical results are compared against experiments and the most important parameters are discussed.

The frictional behavior of a fault and the physical and mechanical properties of the fault rocks have important implications for earthquake nucleation, propagation and arrest. To further understand the mechanical behavior of carbonate-bearing faults, low- to high-velocity experiments on calcite gouge have been conducted using three rotary-shear apparatus: ROSA, located at the Department of Geosciences of the University of Padua, Italy; SHIVA, at the Istituto Nazionale die Geofisica e Vulcanologia (INGV) in Rome, Italy, and the Phv-apparatus of the Physical Property Research Group of the Kochi Institute for Core Sample Research, Kochi, Japan. Three main topics were addressed with the experimental approach: 1) The formation of clast-cortex aggregates in natural and experimental calcite-bearing fault zones (Chapter I); 2) Localization of strain in gouge layers (Chapter II); and 3) The effect of fluids on the frictional behavior of calcite gouge (Chapter III). Clast-cortex aggregates (CCAs) are composite grains found in the slipping zones of faults hosted in calcite- and clay-rich rocks that were previously suggested to be textural evidence of seismic slip on a fault. Experimental investigation of the dependence of CCA formation in calcite gouge layers on the applied slip rate, normal stress, total displacement and ambient humidity showed that CCAs formed at all investigated slip rates (100 µm/s to 1 m/s) but only at relatively low normal stresses (≤5 MPa). The aggregates were better developed with increasing displacement (up to 5 m) and did not form in experiments with water-dampened gouges. In the experiments, aggregates formed in low-strain regions within the gouge layers, adjacent to the highest-strain slip zones. We propose that CCAs in calcite-bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions, but our experiments suggest that they cannot be used as indicators of seismic slip. Formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces. To further understand strain localization during seismic slip, which is a fundamental mechanical process that has implications for frictional heating and the earthquake energy budget, we performed intermediate- to high-velocity rotary-shear experiments on calcite gouge containing strain markers made of dolomite gouge. Sheared markers should provide microstructural information of the strain distribution in the gouge layer and its dependence on the applied total strain, normal stress, slip rate, and ambient conditions. Microstructural analysis revealed that in both dry and water-dampened gouges strain localization at 1 m/s occurs progressively and rapidly. The strain accommodated in the bulk of the gouge layer does not change significantly with increasing total displacement, suggesting that, once formed, the high-strain slipping zone and principal slip surface accommodate most of the ongoing displacement. This is supported by the presence of sintered and recrystallized grains and zones of calcite decarbonation adjacent to the principal slip surface, indicating localized frictional heating. Faster localization in water-dampened conditions, although suggested by the faster dynamic weakening, was not reflected in the investigated microstructures. Instead, weakening in water-dampened gouges may be enhanced by faster subcritical crack growth in the presence of fluids. When extrapolated to natural conditions, our results suggest that calcite-bearing gouge slip zones are more prone to slip in the presence of water than in relatively dry conditions. The effect of fluids on the frictional behavior of calcite gouge was further investigated by conducting intermediate- to high-velocity experiments with controlled fluid pressure. Consistent with our results from experiments with strain-markers, slip appears to be localized on one or more slip surfaces adjacent to which zones of recrystallization are found. Grey or black material covering the slip surface of several samples was identified as disordered carbon by Raman spectroscopy indicating decarbonation of the calcite. In low-velocity (1 mm/s) experiments, a lower shear stress of water-saturated gouges as compared to room-dry conditions is attributed to intergranular lubrication and to the lower calcite fracture surface energy accelerating subcritical crack growth, consistent with a high degree of compaction. At the initiation of sliding at high velocity, weakening in saturated gouges occurs abruptly, while the room-dry gouges show a pronounced strengthening phase before the onset of weakening. For a given effective normal stress, the peak stress is lower, and the strengthening phase is longer, for higher pore-fluid factors. The weakening in room-dry and water-saturated gouges sheared at high velocity likely occurs by flash heating, which is accelerated in the presence of fluids by subcritical crack growth. Consistent with flash heating, the presence of carbon on the slipping surface of our calcite samples indicated that decarbonation has occurred even though the bulk temperature of the gouge layer was lower than the decarbonation temperature. At high velocity, intense frictional heating leads to thermal pressurization and subsequent decrease of the shear stress in the experiments performed in undrained conditions. Zones of recrystallized grains adjacent to the principal slip surfaces are possibly featuring microstructures characteristic for grain boundary sliding aided by diffusion creep which suggests that strain was not only accommodated by frictional processes, but possibly by superplasticity. The experimental results suggest that the presence of water in carbonate-bearing faults facilitates earthquake nucleation and even more so if the fluids present are pressurized. This might explain the long-lasting earthquake sequences e.g. of Umbria-Marche and L’Aquila hosted on carbonate-bearing faults. Additionally, some of the slip distribution complexity during earthquakes occurring in carbonate sequences might be due to a difference in the degree of fluid saturation in different fault patches.
Le proprietà frizionali delle faglie e le proprietà fisiche e meccaniche delle rocce di faglia influenzano in modo importante la nucleazione, la propagazione e l’arresto dei terremoti. Per capire più approfonditamente il comportamento meccanico delle faglie in rocce carbonatiche, sono stati fatti esperimenti a diverse velocità usando gouge (rocce granulari) di calcite, con tre diverse macchine rotary shear: ROSA, installata presso il Dipartimento di Geoscienze dell’Università di Padova, Italia; SHIVA, presso l’Istituto Nazionale di Geofisica e Vulcanologia (INGV), Roma, Italia, e il Phv-apparatus del Physical Property Research Group del Kochi Institute of Core Sample Research, Kochi, Giappone. Tre sono gli obiettivi principali indagati con il metodo sperimentale: 1) La formazione di clast-cortex aggregates (aggregati aventi al nucleo un clasto e una corteccia composta da detrito granulare ultrafine) nelle zone di faglia ricche in calcite sia naturali che sperimentali (Capitolo I); 2) Localizzazione della deformazione nei livelli di gouge (Capitolo II); e 3) L’effetto dei fluidi (acqua) nel comportamento frizionale del gouge di calcite (Capitolo III). I Clast-cortex aggregates (CCAs) sono clasti compositi che si trovano nelle zone di slip delle faglie ricche in calcite e minerali argillosi, precedentemente candidati sulla base di evidenze tessiturali ad essere indicatori di scivolamento cosismico. Esperimenti mirati sono stati fatti per trovare la correlazione tra la formazione di CCA in gouge di calcite e velocità, sforzo normale, rigetto totale e condizioni ambientali (umidità atmosferica e saturazione in acqua). I risultati sperimentali mostrano che i CCA si formano a tutte le velocità di scivolamento (da 100 µm/s a 1 m/s) ma solo a sforzi normali relativamente bassi (<5 MPa). Gli aggregati sono più abbondanti e meglio sviluppati per grandi rigetti (massimo rigetto imposti pari a 5 m) e non si formano negli esperimenti con gouge saturo d’acqua. Negli esperimenti, gli aggregati si sono formati in regioni poco deformate del livello di gouge, ma adiacenti alle zone con elevata localizzazione della deformazione. Da queste osservazioni sperimentali concludiamo che i CCA si formino nelle parti più superficiali delle faglie durante lo la deformazione per taglio in condizioni relativamente asciutte, ma non necessariamente durante lo scivolamento cosismico. Di conseguenza i CCA non possono essere usati come indicatori di slip cosismico. Il meccanismo di formazione dei CCA è per rotazione dei clasti dovuta al flusso granulare accompagnato ad accrescimento per cattura di particelle più piccole della matrice, probabilmente a causa di forze di natura elettrostatica. Per meglio comprendere i meccanismi di localizzazione della deformazione durante lo scivolamento cosismico, che controlla, p.e., lo sviluppo di calore per attrito su faglia e il bilancio energetico di un terremoto, abbiamo condotto esperimenti imponendo velocità di scivolamento da intermedie ad elevate con macchine tipo rotary shear su gouge di calcite. All'interno dello spessore del gouge abbiamo posizionato dei marker (indicatori) di deformazione per taglio composti da gouge di dolomite. I marker, deformandosi unitamente alla matrice di calcite, consentono di misurare la distribuzione della deformazione per taglio nel livello di gouge negli esperimenti. Le analisi microstrutturali hanno dimostrato che sia in condizioni asciutte che in presenza d’acqua la deformazione a velocità di scivolamento di 1 m/s, è molto rapida e si localizza in una zona principale di scivolamento (ZPS) dallo spessore di poche decine di micrometri e sulla adiacente superficie principale di scivolamento (SPS). La deformazione per taglio accomodata nella parte rimanente del livello di gouge non cambia significativamente all’aumentare del rigetto, suggerendo che, una volta localizzata, la ZPS e la SPS accomodano la maggior parte del rigetto. Questa conclusione è supportata dalla presenza di granuli sinterizzati e ricristallizzati e zone di decarbonatazione della calcite adiacenti alla SPS, che indicano lo sviluppo, estremamente localizzato, di calore per attrito. I dati meccanici indicano che i gouge saturi in acqua si indeboliscono (l'attrito diminuisce più rapidamente con il rigetto) di quelli asciutti, ma le microstrutture sono sostanzialmente simili per quanto riguarda la velocità di localizzazione della deformazione. L'indebolimento frizionale nei gouge saturi d'acqua può essere innescato dal meccanismo di crescita sub-critica delle microfratture, più efficiente in presenza d'acqua. L'estrapolazione di questi risultati alle condizioni naturali, suggerisce che i gouge ricchi in calcite sono più favorevoli allo scivolamento se saturi in acqua, piuttosto che in condizioni relativamente più asciutte. L’effetto dei fluidi sul comportamento frizionale di gouge di calcite è stato ulteriormente studiato attraverso esperimenti in controllo di pressione di fluidi a velocità da intermedie ad elevate. Coerentemente con i nostri esperimenti con gli indicatori di deformazione, il rigetto appare localizzato su una o più superfici di scivolamento che sono spesso contornate da zone di ricristallizzazione. La microspettroscopia Raman ha evidenziato la presenza di carbonio amorfo sulla superficie di scivolamento, indicatore di processi di decarbonatazione nella calcite. In esperimenti condotti a basse velocità di scivolamento (1 mm/s), la minore resistenza al taglio dei gouge saturi d’acqua rispetto ai gouge deformati in presenza di sola umidità atmosferica, è attribuita a lubrificazione intergranulare operata dalla acqua e alla bassa energia di superficie della calcite. Quest'ultima consente l'accelerazione dei processi di crescita sub-critica delle microfratture cui corrisponde un alto grado di compattazione. Nelle prime fasi di scivolamento ad alte velocità, l’indebolimento nei gouge saturi avviene improvvisamente, mentre i gouge in presenza di umidità atmosferica mostrano una fase di aumento di resistenza al taglio prima della fase di indebolimento. Per un dato sforzo normale efficace, per rapporti più elevati di pressione di poro su sforzo normale, lo sforzo di taglio di picco è minore e la fase di aumento di resistenza che precede l'indebolimento più lunga. La riduzione della resistenza per attrito ad alte velocità di scivolamento (cosismiche, ca. 1 m/s), sia in condizioni di umidità atmosferica che sature d’acqua, occorre verosimilmente per meccanismo di "riscaldamento istantaneo" (flash heating) alla scala delle asperità (decine di micrometri). L'indebolimento per "flash heating" è accelerato in presenza di fluidi per il meccanismo di crescita subcritica delle microfratture. Coerentemente con il verificarsi di flash heating, la presenza di carbonio sulla superficie di scivolamento dei nostri campioni di calcite indica che la decarbonatazione è avvenuta nonostante le temperature medie nell'intera zona di scivolamento, misurate con termocoppia, fossero più basse di quella di decarbonatazione. Ad alte velocità di scivolamento, in esperimenti in condizioni sature non drenate, la presenza di un intenso riscaldamento frizionale comporta la pressurizzazione termica del livello di gouge, con conseguente diminuzione dello sforzo di taglio. Nella zona di scivolamento, la formazione di nanoparticelle, grani ricristallizati di calcite e microcavità adiacenti alla superficie di scivolamento principale può essere associata a processi di grain boundary sliding sostenuti da processi diffusivi dipendenti dalla granulometria. Di conseguenza, la deformazione cosismica non è accomodata da soli processi prettamente frizionali, ma anche di tipo superplastico. I risultati degli esperimenti indicano che la presenza d’acqua in faglie all’interno di litologie carbonatiche facilita l’enucleazione di terremoti, ancor più se i fluidi presenti sono in pressione. Questa potrebbe essere una possibile spiegazione delle lunghe sequenze sismiche all’interno di successioni carbonatiche, ad esempio Umbria-Marche e L’Aquila. In aggiunta, la complessa distribuzione dei rigetti durante un singolo terremoto potrebbe essere causata da differenze nel grado di saturazione in fluidi in diverse zone della faglia.

Depuis quelques dizaines d'années, l'élaboration de matériaux nanostructurés tend de plus en plus à se développer. En effet, ces matériaux présentent souvent des propriétés intéressantes et en particulier des propriétés mécaniques surprenantes vis-à-vis de leurs homologues sous forme massive. Les métaux nano-maclés ou nano-lamellaires par exemple, sont connus pour avoir une bonne résistance mécanique, une bonne stabilité thermique et une excellente résistance aux radiations. Au fur et à mesure que l'espacement entre les interfaces diminue, leur densité augmente de manière significative et les propriétés macroscopiques du matériau sont de plus en plus dépendantes des interactions défaut-interface. Dans ce contexte, nous avons étudié, via des simulations atomistiques, la formation de macles de déformation et les mécanismes d'interaction d'une macle nouvellement formée avec une interface préexistante (un joint de macle ou une interface entre 2 métaux), pour une configuration modèle de film mince auto-supporté. Des premiers résultats montrent l'influence de marches de surface sur le maclage, pour un cas modèle sans interface. Puis nous avons identifié un mécanisme inédit aboutissant à la formation d'une dislocation de Lomer suite à l’interaction d'une macle en formation avec un joint de macle préexistant. En faisant varier la densité de défauts de surface, nous montrons l'influence particulière d'un joint de macle sur la taille et le nombre de macles formées. Enfin, pour les systèmes bimétalliques Cu/Ag, nos résultats mettent en évidence le rôle des dislocations d'épitaxie (à l'interface) dans la nucléation et l'extension des macles ainsi qu'une influence directe du type d'interface considéré sur la propagation de ces macles
For several decades, the elaboration of nano-structured materials tends to develop more and more. Indeed, these materials often show interesting properties, and in particular surprising mechanical properties when compared to their bulk counterparts. For example, nano-twinned or nano-layered metals are known to have ultra-high mechanical strength, good thermal stability, and very good radiation resistance. As the interface spacing decreases to the nanometer-scale, the density of interfaces increases significantly and subsequently the macroscopic properties become largely governed by the interface-defect interactions. In that context, we have studied deformation twin formation and mechanisms of interaction between a new formed twin and a preexisting interface (a twin boundary or a bimetallic interface), using atomistic simulations and a thin film model configuration. First results show the influence of surface steps on mechanical twinning, for a model system without interface. Then we identify a new mechanism leading to the formation of a Lomer dislocation, following the interaction of a newly formed twin and a preexisting twin boundary. By varying the density of surface defects, we show the particular influence of a preexisting twin boundary on twin size and number. Finally, for the Cu/Ag bimetallic system, our results highlight the role of epitaxial dislocations (at the interface) in twin nucleation and extension as well as a direct influence of the interface type in twin propagation

La calcite (carbonate de calcium) est un matériau extrêmement répandu dans les roches telles que le marbre et comme constituant dans des domaines variés (bâtiment, pharmacie, papier, art). La compréhension des propriétés mécaniques des suspensions de calcite constitue une étape importante pour améliorer à la fois leur maniabilité ainsi que les propriétés finales du matériau fabriqué. Cette étude relie les propriétés rhéologiques (élasticité, écoulement) de ces suspensions à leurs interactions microscopiques. Les interactions attractives entre particules de calcite confèrent aux pâtes les propriétés de gels colloidaux caractérisés par un module élastique et une déformation critique, et ce pour une large gamme de concentrations. L'étude de ces grandeurs en fonction de la concentration a permis de mettre en évidence pour la première fois l'existence de deux régimes de déformation (liens forts et faibles) prédits théoriquement et de caractériser la dimension fractale. L'étude des interactions a été réalisée grâce à la mesure du potentiel Zeta des pâtes, du pH et au calcul de la longueur de Debye résultant de la force ionique. L'ajout d'additifs tels que la chaux ou la soude modifie les interactions. La chaux réduit fortement l'élasticité initiale des pâtes, facilitant leur maniabilité et renforçant la réactivité de la pâte en présence de CO2. L'ajout de soude augmente fortement l'attraction entre les particules ce qui se traduit à l'échelle macroscopique par l'existence de bandes de cisaillement. Cette manifestation de l'attraction entre colloïdes à l'échelle macroscopique avait été observée dans des émulsions concentrées mais encore jamais dans les gels colloïdaux
Calcite (calcium carbonate) is an extremely widespread material that can be found naturally in rocks (i.e. marble, limestone) and is employed in many industrial fields such as paper filling, pharmaceutical, art or construction. Understanding the mechanical properties of calcite suspensions is a first step to improve the workability of the paste as well as the final properties of solid mineral materials. Macroscopic characterization of calcite suspensions via rheological measurements are linked to microscopic interactions, via DLVO analysis. Our calcite pastes are weakly attractive systems showing a typical colloidal gel behavior and characterized by an elastic shear modulus and a critical strain. The elastic domain of pure calcite suspensions is characterized for a wide range of volume concentrations. The deformation at the end of linearity exhibits a minimum versus concentration, a major prediction of colloidal gel theory, never verified so far. The interaction forces between particles are tuned by addition of simple ionic species. Rheological measurements are analyzed through DLVO calculations, obtained by chemical speciations and ζ potential measurements on dense suspensions. Addition of calcium hydroxide improves initially the workability of the paste, enhancing the reactivity when in contact with CO2. The role of interaction forces is also evaluated with flow measurements. The addition of sodium hydroxide increases strongly the attraction between particles, inducing shear bands at the macroscopic scale. This correlation is well known for emulsions but never verified so far for colloidal gels

Growth and dissolution of the mineral calcite is important for prediction and control of surface and subsurface water quality, calculation of past sea-surface temperatures using paleoenvironmental proxies, and sequestration of contaminants through engineered calcite precipitation. At high concentrations of strontium, calcite growth is known to be inhibited, but the mechanism by which strontium inhibits growth is not well understood. Seeking to better understand this mechanism, atomic force microscopy is used with a flow-through fluid cell to measure real time growth rates of the obtuse and acute monomolecular step orientations on the calcite surface. Growth was measured at two saturation indices as a function of the ratio of the concentrations of aqueous calcium-to-carbonate and varying aqueous strontium concentration. It was found that the amount of strontium required to inhibit growth correlated with the aqueous calcium concentration, but did not correlate with carbonate concentration. This suggests that strontium inhibits attachment of calcium, but not carbonate, during growth. Analytical models of nucleation and propagation of steps are expanded from previous studies to capture multiple saturation indices.

Le présent travail est consacré à l'analyse de l’effet de la vitesse de déformation et de la taille des grains sur les mécanismes de plasticité activés lors de la déformation plastique dans deux types de matériaux polycristallins: (i) le zinc (Zn), de structure hexagonale compacte, et ayant des tailles de grains dans le domaine micrométrique (300 µm) et dans le domaine des grains ultrafins (200 nm). Ces matériaux sont déformés en compression quasi-statique et dynamique (vitesse de déformation atteignant 10⁵ s⁻¹) à l’aide de bars de Hopkinson (DIHPB) ; (ii) le nickel (Ni) électrodéposé à grains fins, de structure cubique à faces centrées ayant une taille moyenne de grains de l’ordre de 4 µm, également déformé en compression dynamique (DIHPB). Des différences significatives en termes de micro-mécanismes de déformation dans les deux régimes ont été mises en évidence: (i) pour des faibles vitesses de déformation et jusqu'à ~10² s⁻¹, la déformation a lieu principalement par glissement des dislocations à la fois dans Ni et dans Zn à grains nanométriques. Toutefois, un maclage important est observé dans le cas des échantillons Zn à grains micrométriques, indiquant ainsi un effet de la taille des grains sur le maclage; (ii) Dans le régime dynamique (> 10³ s⁻¹) la déformation plastique induit une augmentation de la température dans les échantillons (Ni ou Zn). Cette augmentation de la température est suffisamment importante pour induire à son tour la restauration et/ou la recristallisation dynamique. Comme conséquence, deux phénomènes sont observés en fonction du matériau: pour Ni, la microstructure et les propriétés mécaniques résultant sont similaires à celles de l'état initial, dominé par des macles de recuit et des grains équiaxes et orientés de façon aléatoire. Pour Zn un affinage important de la taille des grains est observé (de 300 µm à 6 µm) qui s’accompagne au passage de l’inhibition du phénomène de maclage. Pour clarifier ce point, des investigations complémentaires ont été effectuées sur des échantillons de Ti de pureté commerciale (CP-Ti) à grains micrométriques dans les deux régimes quasi-statique et dynamique. Les résultats montrent que le maclage est ici le principal mécanisme de déformation. La densité des macles est d’autant plus élevée que la vitesse de déformation est importante et les grains plus gros. Ces résultats confirment l’influence l'effet de la taille des grains sur le maclage mécanique dans les matériaux HC. Par contre, il a été observé que l'effet de la vitesse de déformation sur le maclage dans le régime dynamique est différent selon qu’il s’agisse de Zn ou de Ti. Dans le premier cas, il est probable que le faible niveau du rapport T/T f joue un rôle clé, en induisant le recristallisation dynamique dans les conditions expérimentales étudiées ici
The present work is devoted to the analysis of the strain rate and grain size effects on the deformation mechanism activated during plastic deformation of two polycrystalline materials: (i) zinc (Zn), a crystal with hexagonal compact packing structure, having grain size in the micro and ultrafine grain ranges (~ 300 µm and 200 nm, respectively), loaded under quasi-static and dynamic compression conditions, up to a strain rate of ~ 10⁵ s⁻¹ (by use of a Direct Impact Hopkinson Pressure Bars (DIHPB); (ii) electrodeposited nickel (Ni), a face-centered cubic structure with grain size of 5 µm deformed in compression under dynamic conditions using DIHPB. Significant differences in terms of micro-mechanisms of deformation in the two regimes were found: (i) At lower strain rates, up to ~ 10² s⁻¹, dislocation-based plasticity was observed in both Ni and Zn. Extensive twinning occurred only in the case of micrometer grain-sized Zn, indicating a grain size dependence of twinning; (ii) In the dynamic regime (> 10³ s⁻¹) plastic deformation induced a significant increase of the temperature within the samples. This increase of temperature was significant enough to induce recovery and/or dynamic recrystallization. As consequence two phenomena were observed depending on the structure under investigation: for Ni, the resulting microstructure and mechanical properties were similar to that of the initial state, dominated by annealing twins and equiaxed and randomly oriented grains. For micro-grained Zn a tremendous grain refining was found. As a consequence, twinning was inhibited. To clarify this point, additional investigations were carried out on coarse-grained CP-Ti deformed in both quasi-static and dynamic regimes. It was found that twinning was the main deformation mechanism. Indeed, the larger the strain rate and grains size, the larger the twin density. On the one hand, these results clearly demonstrate the grain size effect on the occurrence of mechanical twinning in HCP materials. On the other hand, the effect of the strain rate on twinning was found to depend on the material under investigation. Compared to Ti, the lower homologous temperature T/T m of Zn probably plays a key role, as it may induce dynamic recovery/recrystallization as far as the present experimental conditions are concerned

Sedimentary basins can contain close to 20% by volume pore fluids that are commonly classified as brines. These fluids can become undersaturated with respect to calcite as a result of processes such as migration, dispersive mixing, or anthropogenic injection of CO2. This study measured calcite solubility and dissolution rates in geologically relevant Na-Ca-Mg-Cl synthetic brines (35 to 200 g L-1 TDS). In brines < 50 g L-1 TDS, the EQPITZER calculated calcium carbonate ion activity product (IAP) at steady-state was in reasonable agreement (±10%) with the thermodynamic solubility constant for calcite (Kc). However, the IAP systematically exceeded Kc in more concentrated brines. The deviation was strongly correlated with calcium concentration and also was observed in magnesium-free solutions. This is interpreted as an uncertainty in the carbonate ion activity coefficient, and minor adjustment in stoichiometric association constants (K*M2+CO30) for the CaCO30 or MgCo30 ion pairs would correct for the error. The dissolution rate dependency on brine composition, pCO2 (0.1 to 1 bar), and temperature (25.0 to 82.5 °C) was modeled using the empirical rate equation ()nkRΩ−=1 where R is the rate, k and n are empirical fitting terms, and Ω the degree of disequilibrium with respect to calcite. When Ω was defined relative to an apparent kinetic solubility, n could be assumed first-order over the range of Ω investigated (Ω = 0.2 to 1.0). Rates increased with increasing pCO2 as did the sensitivity to brine concentration. At 0.1 bar, rates were nearly independent of concentration (k = 13.0 ±2.0 x 10-3 moles m-1 hr-1). However, at higher CO2 partial pressures rates became composition dependent and the rate constant, k, was shown to be a function of temperature, pCO2, ionic strength, and calcium and magnesium activity. The rate constant (k) can be estimated from a multiple regression (MR) model of the form k = B0 + B1(T) + B2(pCo2) + B4(aCa2+) + B5(aMg2+). A relatively high activation energy (Ea = 20 kJ mol-1) was measured, along with a stirring rate independence suggesting the dissolution is dominated by surface controlled processes at saturation states Ω > 0.2 in these calcium-rich brines. These findings offer important implications to reaction-transport models in carbonate-bearing saline reservoirs.

Published mineralogical and diagenetic studies of the Derbyshire Platform focus upon the source of Mississippi - Valley type (MVT) mineralising fluids and hydrocarbon deposits or upon the diagenetic history of the area. This study aims to integrate these fields by developing a complete model for the geochemical evolution of the Derbyshire Platform and the surrounding basins, in line with the regional Variscan tectonic history of the area and the controls this imposed upon fluid flow. The study area is compared to diagenetic events on the southern margin of the Askrigg Platform and the adjacent Craven Basin which are thought to have been contemporaneous with events on the Derbyshire Platform. A sequence of late diagenetic calcite cements can be recognised on the Derbyshire Platform within vein systems and cross cutting relationships between veins and variations in cement type illustrate there were several phases of cementation. On the Askrigg Platform, burial calcite cements can be identified within veins and intergranular pores. On both the Derbyshire and the Askrigg Platforms, burial calcite cements were contemporaneous with hydrocarbon emplacement and MVT mineralisation. Compacting Dinantian - Namuiran shales in basins adjacent to the Derbyshire and Askrigg platforms offer the most likely source of fluids. Trace metals, hydrocarbons and fluids were progressively released from overpressured basins onto the platforms along fault systems. Fluids circulated on the platform within minor faults and fractures. Restricted volumes of fluid were released from the basins during the waning effects of Carboniferous extension, but the onset of Variscan compression in the mid-late Westphalian led to reactivation of fault systems and massive dewatering of the basins onto the platform. A model can therefore be established relating fluid flow to Variscan tectonic events in northern Britain.

Le but de ce travail est de déterminer le cycle biogéochimique du sélénium, et plus particulièrement des sélénites, dans un environnement proche de la surface. Pour cela, nous avons réalisé des études de sorption sur différentes phases minérales en système mono et multi-phasiques. Nous avons également réalisé des expériences pour étudier le devenir du sélénium dans un système-sol_plantes_microorganismes. (i) Les expériences de sorption des sélénites sur différentes phases minérales ont validé la méthode dite des « membranes de dialyse » et ont montré qu'il n'y avait pas d'additivité des capacités de sorption dans des systèmes à plusieurs phases minérales. La présence de Si aqueux relâché par la bentonite réduit la sorption du Se sur la goethite en occupant les sites de sorption. (ii) L'étude de l'influence des substances humiques (HA, FA) sur les capacités de sorption des différentes phases minérales vis-à-vis des sélénites a montré que ces substances diminuent la sorption du Se sur la goethite. De plus, ces expériences ont montré que les HA/FA ne complexent pas le sélénium. (iii) Les expériences « rhizoplants » nous ont permis de calculer des facteurs de transfert du Se des sols vers le ray-grass. Une application de ces facteurs de transfert dans un scénario de migration et de transfert du Se à partir d'un site de stockage des déchets HAVL, a donné une dose effective annuelle vers l'Homme comprise entre2,3 x 10-6 et 5,9 x 10-6 Sv. An-1
The aim of this work is to determine the biogeochemical cycle of selenium, especially selenites, in a subsurface environment. To do so, we studied Se sorption on different mineral phases in single or multi-phase systems. We also conducted experiments to investigate the fate of selenium in a soil-plants microorganisms system. (i) Sorption experiments of selenites on different mineral phases validated the « dialysis membranes » method and showed that there was no additivity of the sorption capacity in systems composed of several minerals phases. The presence of aqueous Si released by bentonite reduced Se sorption on goethite by occupying the sorption sites. (ii) The study of the influence of humic substances (HA, FA) on the sorption capacities of different mineral phases has shown that these substances decrease Se sorption on goethite. Moreover, these experiments have shown that HA/FA do not complex selenium. (iii) « Rhizoplants experiments allowed us to calculate transfer factors of soil Se toward rye-grass. In a scenario of migration and transfer of Se from a radioactive waste disposal, the application of these transfer gave an annual effective dose to Humans comprised between 2,3 x 10-6 and 5,9 x 10-6 Sv. Y-1

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Page 238 blank.
Includes bibliographical references.
In nature, carbonates often accumulate large amounts of strain in localized shear zones. Such marble sequences play a key role in crustal deformation processes. Despite extensive field and laboratory investigation, many questions remain concerning the mechanical behavior of these rocks. For example, the mechanical behavior of different limestones and marbles differ greatly, possibly owing to the presence of chemical impurities or solid-solutes. Thus, Chapter 2 examines the effect of Mg solute, a common impurity, on the mechanical behavior of calcite rocks. The results indicate that increasing Mg content increases the strength of calcite rocks during dislocation creep. The anisotropic nature of crystal slip usually entails variations in reorientation of individual grains and heterogeneous deformation within the polycrystalline material. In Chapter 3, a new technique including a series of sample preparation and image analysis algorithms is developed to provide quantitative measurements of the scale of heterogeneities produced, and to gain fundamental insight into polycrystalline plasticity. We place particular attention on quantifying variations of strain within grain interiors and at grain boundaries, and on recognizing the relative activities of different slip systems. The quantification of grain-to-grain interactions during straining is relevant for the improvement and verification of models of polycrystalline plasticity. The strain measurements obtained from Chapter 3 are compared with predictions of grain strain and reorientation obtained from the self-consistent viscoplastic method (Chapter 4). The results suggest that the self-consistent model gives a good description of global texture, but does not always predict lattice rotation and deformation within individual grains. To predict the actual deformation of individual grains will require a quantitative consideration of the effects on local strain of grain-boundary misorientation, local strain/stress state, grain-boundary sliding, and deviations in grain geometry.
by Lili Xu.
Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1997.
Includes bibliographical references (leaves 233-244).
by Gunter Siddiqi.
Ph.D.

Microbial mat aggradation and morphology can be strongly influenced by sedimentation and light in ice-covered lakes of the McMurdo Dry Valleys, Antarctica. In Lake Joyce, mats transitioned from prostrate with widely spaced pinnacles to having densely spaced pinnacles with complex webs and ornamentation at greater distances from inflowing melt water streams. This transition is interpreted to result from decreasing mud sedimentation, which compacted delicate microbial structures such as pinnacle tips, webs, and surface ornamentation. Mud sedimentation also changed through time at sites adjacent to inflowing streams on one of the Lake Joyce deltas; sedimentation likely increased from 1947 through 2009 as lake levels rose. Although mud sedimentation demonstrably affected mat morphology in Lake Joyce, changes in sand and mud sedimentation associated with overhanging rocks in Lake Vanda were not sufficient to dramatically change mat morphology. Instead, microbial mat pinnacles and ridges had a variety of morphological responses to their light environment. Microbial mats growing with oblique directional light both grew down from overhanging rocks with pinnacle orientation independent from the directional light and grew up from the rock-sheltered mat with pinnacles and ridges oriented relative to incident light: asymmetrical pinnacles were inclined toward and flattened perpendicular to the incident light, and ridges were oriented parallel to the incident light. Changes in mat morphology and microbial processes are also preserved in Lake Joyce stromatolites that grew over decades. Stromatolites contain ?13Ccalcite records of variable photosynthetic fractionation of local DIC under lower lake levels, followed by decades of DIC pool 13C enrichment with lower rates of photosynthesis during lake level rise. These results demonstrate that microbial responses to their environments are complex and under the right conditions can be preserved in the rock record.

L'etude de l'adsorption des molecules d'acides gras satures (c#nh#2#n#+#1co#2h) et monoinsatures (c#nh#2#n##1co#2h) sur le carbonate de calcium (calcite) en milieu aqueux a ete entreprise dans le cadre de la caracterisation des interactions entre la matiere organique d'origine naturelle et les mineraux des sols et sediments. Les principaux resultats experimentaux sont issus des investigations par des techniques basees sur la vibration moleculaire, et utilisables dans les conditions in situ : spectrometrie d'absorption infrarouge par reflexion totale attenuee (ftir-atr) et diffusion raman. Ces resultats mettent en evidence une importante adsorption pour les acides gras a longue chaine alkyle (n = 17), les acides stearique et oleique. L'adsorption s'effectue par la fonction carboxylate qui complexe les ions calcium de la surface de la calcite. L'apport specifique de la microspectrometrie raman confocale est la mise en evidence de la repartition tres heterogene des depots ainsi que l'organisation des chaines alkyles, a la resolution du micrometre. La microscopie a force atomique (afm) a montre la nature heterogene de l'adsorption, sous la forme de vesicules d'oleate ou de stearate de calcium d'environ 20 nm. Ces vesicules se forment a partir de micelles d'oleate ou de stearate du milieu aqueux lors de la mise en presence avec la calcite. Les calculs de mecanique et de dynamique moleculaire ont conforte les resultats experimentaux. Le role des acides gras dans le transport des polluants organiques et metalliques a ete evalue, la presence de calcite pouvant par exemple provoquer un relargage de polluants.

The aim of this project is to investigate the suitability of specific polymers inclusions in improving some important mechanical properties of single crystals of calcite. The mechanical properties of synthetic and mineral single crystals of pure calcite are reported and compared with synthetic and biogenic crystals incorporating polystyrene particles or di-block co-polymer micelles and organic molecules respectively. The mechanical properties of these crystals are measured using nanoindentation and micro-compression techniques. It is shown that the crystals’mechanical properties obtained by the nanoindentation Oliver-Pharr method are influenced by the compliance of the resin substrate into which the crystal samples are embedded. The embedding of the crystals is shown to be necessary in order to achieve a flat surface onto which reliable indentation can be made. An estimate of the elastic moduli of the samples was obtained by using the Song-Pharr model, which takes into account substrate compliance. The hardness of all the tested samples was also recalculated by using conventional methods. By analyzing the obtained substrate independent data it was found that although the occlusion of co-polymer micelles reduced the modulus of pure calcite its hardness was not affected. This behaviour was not observed in the composites occluded with 200 nm polystyrene particles which showed a significant hardness reduction. The 20 nm co-polymer micelles also increased the specific hardness of the crystals whereas the 200 nm polystyrene particles showed no such behaviour. The micro-compression data showed that the 200 nm polystyrene particles could potentially be used to reduce the brittleness of calcite. Images of cracks, post-fracture of the crystals, indicated deformation of the polymer. This is evidence of the occurrence of a crack bridging mechanism. It was thought that this could lead to an enhancement of the strain at fracture and work of fracture of these composites when compared with pure calcite. Mechanical data from these tests showed however that the polystyrene particles’ occlusion was inadequate to increase the strain to fracture and the work of fracture of calcite. In addition to this the specific compressive strength, specific compressive modulus and specific compressive work of fracture of calcite also decreased with the addition of PS particles.

Carbon and oxygen isotopes in calcite crystals provide a record of the environmental conditions under which the crystals formed. To investigate the influence of temperature, pH, and growth rate on isotope discrimination by calcite, we measured carbon and oxygen isotope fractionation through a series of calcite precipitation experiments at T = 25C and pH = 7.5 - 9.3. We observe that neither the carbon nor oxygen isotope compositions correspond to the theoretical equilibrium isotope fractionation between calcite and solution. We also demonstrate that the fractionation of oxygen isotopes between calcite and water decreases with increasing pH, consistent with available data from experiments in which the enzyme carbonic anhydrase was used. Finally, we compare the carbon and oxygen isotopes of our calcite crystals to those of biogenic carbonates. This thesis includes previously unpublished co-authored material.