Literatura científica selecionada sobre o tema "Duricrust"

The lateritic mantle overlying ultramafic rocks at Itacupim Island (state of Pará, northern Brazil) comprises 5 horizons. From bottom to top they are: (1) altered rock (hornblende, phlogopite, apatite, magnetite, fissural wavellite, and smaller amounts of smectite or kaolinite), (2) a smectitic saprolite (smectite, wavellite, goethite, and traces of hornblende), a kaolinitic saprolite (kaolinite, wavellite, goethite, and traces of phlogopite), a phosphatic duricrust (crandallite, goethite and hematite), and a ferruginous duricrust (hematite, goethite and crandallite). The mineralogical and chemical composition of the saprolitic and duricrust horizons indicate a discontinuity in the weathering evolution: the P-rich crandallitic duricrusts cannot be derived from the P-poor wavellitic saprolites. Most probably, the duricrusts developed during an earlier stage of the weathering evolution, under a drier climate, when the hypogene phosphates (wavellite and apatite) transformed into crandallite. Under subsequent more humid conditions, the weathering alteration of the parent rock led to the dissolution of the hypogene phosphates, resulting in the P-depleted saprolites.

Abstract. Duricrusts are hard mineral layers forming in climatically contrasted environments. They form in tropical to arid environments and can be currently observed all around the world in areas such as Europe, Africa, South America, India, and Australia. In most cases, they cap hills and appear to protect softer layers beneath. Two main hypotheses have been proposed for the formation of duricrusts; i.e. the hydrological or transported model, where the enrichment in the hardening element (iron for ferricretes, silica for silcretes, or calcium carbonates for calcretes) is the product of leaching and precipitation through fluctuations in the water table during contrasted seasonal cycles, and the laterization or in situ model, where the formation of duricrusts is the final compacting stage of laterization. In this article, we present the first numerical geomorphological model for the formation of duricrusts based on the hydrological hypothesis. The model is an extension to an existing regolith formation model, where the position of the water table is used to predict the formation of a hardened layer at a rate set by a characteristic timescale, τ, and over a depth set by the range of fluctuations in the water table, λ. Hardening causes a decrease in surface erodibility, which we introduce in the model as a dimensionless factor, κ, that multiplies the surface transport coefficient of the model. Using the model, we show under which circumstances duricrusts form by introducing two dimensionless numbers that combine the model parameters (λ and τ), as well as parameters representing external forcing like precipitation rate and uplift rate. We demonstrate that when using model parameter values obtained by independent constraints from field observations, hydrology, and geochronology, the model predictions reproduce the observed conditions for duricrust formation. We also show that a strong feedback exists due to duricrust formation on the shape of the regolith and the position of the water table. Finally, we demonstrate that although duricrusts protect elements of the landscape, their efficiency in doing so is significantly lower than their inherent strength.

Abstract The Tertiary sequence begins with fossiliferous marl and is overlain by carbonaceous clay, china clay and laterite. The thin (3 cm thick), hard, compact duricrust layer, which is found interbedded within the carbonaceous clays, shows abnormal geochemical anomalies. Major elements such as Fe, Mn and Ca and the trace element Co show about 2-3 fold and 1-2 fold increase respectively in the duricrust than the upper and lower sections. However, the content of Na, K, Ni, Cu and organic carbon show an opposite trend. Cd does not show any marked variation along the profile. The CaCO3 content is remarkably high in the duricrust and marl than the other layers. Fe and Mn contents are much higher in the laterite than china clay, other parameters do not show much variation. The high content of Fe, Mn, Ca and CaCO3 and low Ni/Co ratio and organic carbon content in the duricrust layer indicate an oxidizing environmental condition which existed at the time of the formation of this layer, while the converse trend of these parameters in the carbonaceous clays indicate the prevalence of a reducing condition.

This study aims to investigate the effects of parent rock and minerals on lateritic weathering. The study presents X-ray diffraction (XRD), whole-rock geochemistry, and Nd-Sr isotopic data for examining two profiles, 10 and 12 m thick, respectively, that illustrate the regional tropical weathering status in the Midwest of Brazil. The profiles, developed from metasedimentary and sedimentary rocks, are constituted by saprolite, mottled horizon, lateritic duricrust, and oxisol. Across the profiles, the minerals controlling the weathering geochemistry are muscovite, microcline, quartz, kaolinite, hematite, goethite, and gibbsite. Red and yellow zones in the saprolite and mottled horizon as well as the lateritic duricrust with breccia/fragmental, pisolitic, and oolitic textures make profile 1 more complex. In contrast, profile 2 has an oxisol that mantles the homogeneous vermiform lateritic duricrust. Fe2O3, accumulated during surface weathering, is a potent element in the geochemical profile control since it forms the harder goethite to hematite lateritic duricrust, bearing most of the trace elements (As, Cu, Cs, Pb, Sc, Sr, Th, U, V, and Zn) with similar ionic radii and electrovalence. The LREE have affinity for the elements of the Fe2O3 group of the lateritic duricrust. On the other hand, the K2O group together with Zr and TiO2 e in the phyllite, saprolite, and mottled horizon of profile 1, are associated with the HREE. Additionally, in profile 2, the HREE are mostly associated with the Al2O3 group and the residual minerals in the oxisol. The indication that REE is associated with phosphates, zircon, rutile/anatase, cereanite, and muscovite/illite, which have variable weathering behavior, caused the REE fractionation to occur across and between the profiles. Despite the REE fractionation, the ƐNd(0) values along the profiles consistently maintain the signature of the parent rock. Muscovite and microcline weathering, in profiles 1 and 2, respectively, control the decrease in 87Sr/86Sr signatures of both profiles and the distinct radiogenic ratios. The development of lateritic duricrust in both profiles indicates a similar weathering intensity, although the gibbsite–kaolinite predominance in the oxisol of profile 2 highlights a geochemical reorganization under humid conditions, as well as near-intense soluble silica leaching.

The morphologically distinct materials in Darling Range lateritic duricrust (i.e. loose and cemented pisoliths, concretions, matrix, pisolith coatings and void coatings) exhibit goethite/(goethite + hematite) ratios ranging from 0.15 for individual pisoliths to 1.0 for void coatings. Mole % Al substitution ranged from 20 to 34% in goethite and from 2 to 15% in hematite. Goethite and hematite in pisoliths and concretions were mostly highly Al substituted. Al substitution in goethite was positively related (P < 0.01) to Al substitution in hematite. Al substitution in maghemite was less than 5%. Goethite crystals ranged in size from 130 to 260 A. Hematite crystals ranged from 140 to 520 A, and were systematically smaller in pisoliths. Crystal size of goethite and hematite decreased with increasing Al substitution. Hematite crystals were usually about 50% larger than goethite crystals in the same sample, and crystal sizes of goethite and hematite were positively correlated (P < 0.01). Goethite and hematite occurred as aggregates of subrounded platy crystals. Differences in the properties of goethite and hematite between morphologically distinct materials in single hand specimens are indicative of the complex history of these duricrusts.

The bauxite duricrust in the Bangam and surroundings is classified into three major storey levels as follows: the upper level on top of the plateaus where the continuous bauxites outcrop in slabs over distances of about 1 km, the intermediate level on top and at the flanks of the hills where the bauxites flush in metric blocks, the lower level at the talwegs where bauxites duricrust are gravels. The average thickness of the duricrust is 9.6m and extends over an area of about 9.5km2. Five types of facies are encountered here: nodular, pisolitic, pseudobreccia, vacuolar and massive. These different facies were carried out by X-ray fluorescence spectrometry, X-ray diffractometric, thermal analysis (ATD / ATG) and by polarizing microscope observation. The results of these analyses show that, gibbsite is the dominant mineral; kaolinite, anatase, hematite, goethite, magnetite and ilmenite are also present. The chemical composition of the major elements that characterize bauxites are 38-60% Al2O3; 2 -22% SiO2 and 9-42% Fe2O3. These values are almost similar to those obtained in Doumbouo-Fokoué, Ghana, Ngaoundal, and Minim-Martap precisely at Briskok prospect. The different minerals paragenesis and geochemistry data show that, the Bangam locality is a bauxite ore deposit in spite of a high content of iron and silicate of some facies, nevertheless, these can be used in some industries.

AbstractThe Capim Kaolin District (eastern Brazilian Amazon), is one of the largest kaolin deposits in the world; with the kaolin used mainly for paper coating. The kaolin developed at the expense of Cretaceous sandy-clayey sediments of the Ipixuna formation, through intense lateritization from the Mesozoic to Cenozoic times.This work describes the morphological, mineralogical, crystallochemical and geochemical evolution of the Capim kaolin facies. Based on the profile analysis in the open pit fronts, it encompasses X-ray diffraction, thin-section optical analysis, EDS-assisted scanning electron microscopy and transmission electron microscopy, chemical analysis, infrared and Mössbauer spectroscopies.Six facies were defined as different stages of the supergene process. Ferruginization led to a thick duricrust on the soft kaolin, which in turn evolved from sandy-clayey sediments of the Ipixuna Formation. A subsequent deferruginization event degraded the duricrust, resulting in the flint kaolin facies.

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Observações de campo, petrografia, geoquímica e mineralogia em dois perfis de alteração de solo dos arenitos provenientes do Membro Serra da Galga, Formação Marília, foram utilizados para avaliar a possível relação genética entre os materiais lateríticos e os solos enriquecidos com argilas aluminosas (argilas refratárias). O estudo focou um sistema Latossolo-Gleissolo presente na superfície da chapada sedimentar. O afloramento do Latossolo, com crosta ferruginosa, situa-se na borda da chapada e o Gleissolo, onde estão concentradas as argilas aluminosas, está localizado na depressão topográfica hidromórfica. As fácies petrográficas mostram que a couraça ferruginosa é autóctone, formada no nível saprolítico do arenito. O desmantelamento atual deste horizonte está associado com a desferruginização parcial em conseqüência ao aumento da umidade. A estrutura pisolítica é reconhecida tanto no perfil do Latossolo quando do Gleissolo, após a desferruginização. A associação caulinita e gibbsita predomina em todas as profundidades dos dois perfis, porém a caulinita é mais expressiva nas maiores profundidades e a gibbsita nas camadas superficiais. A desferruginização é o processo que acompanha a elevação do nível freático, seja formando horizontes manchados na base do Latossolo ou formando horizontes brancos no Gleissolo nos quais se concentram o alumínio. O enriquecimento de alumínio está associado com a dessilicificação, possivelmente relacionado com a diminuição do pH e a desestabilização da caulinita. Quartzo, raros minerais de turmalina, cianita, zircão e rutilo compõem a fase dos minerais resistentes ao intemperismo químico. A goethita é o óxido de ferro predominante, resultado de meio hidratado. O modelado plano da chapada, que corresponde à Superfície Sul-Americana descrita por King (1956), sob condições climáticas adequadas e prolongadas, favoreceu a formação de cobertura encouraçada ferruginosa. Após o término da deposição sedimentar, no Paleoceno inferior (aproximadamente 65 milhões de anos AP), a superfície foi truncada por longos períodos de erosão que aplainaram o relevo. Atualmente, o clima é tropical com forte sazonalidade e aumento da umidade. Esta nova condição hídrica, de saturação, somada às condições de estabilidade tectônica e vegetação esparsa exerce influência na lixiviação e a perda ferro no horizonte de argila aluminosa. Os resultados sugerem que é possível a formação de material aluminoso a partir de uma crosta ferruginosa, propondo assim uma hipótese autóctone para o desenvolvimento das argilas aluminosas.
Field observation, petrography, geochemistry and mineralogy in two weathered soil profiles of the sandstones from Member Serra da Galga, Marilia Formation, were used to evaluate the possible genetic relation between the lateritic material and the soils enriched in aluminous clays (refractory clays). The study focus was an Oxisol-Gleysol system present in the surface of the sedimentary plateau. The Oxisol outcrop, with ferruginous duricrust, is situated on the edge of the plateau and the Gleysol, where the aluminous clay is concentrated, is located in the hydromorphic topographic depression. The petrographical facies show that the ferruginous duricrust is authochthonous, formed in the saprolitic level of the sandstone. The current dismantling of this horizon is associated with the partial deferruginization due to humidity increase. The pisolitic structure is recognized both in the Oxysol profile and Gleysol, after deferruginization. The kaolinite and gibbsite association prevails in all depth of both profiles, albeit kaolinite is more significant in deeper depths and gibbsite in the superficial layers. Deferruginization is a process that accompanies groundwater level rise, might develop mottled horizons at the Oxysol base or forming white horizons at the Gleysol in which the aluminum is concentrated. The aluminum enrichment is associated with the dessilicification, possibly related with decrease in pH and the kaolinite destabilization. Quartz, rare tourmaline, kyanite, zircon and rutile minerals make up the phase of minerals resistant to chemical weathering. Goethite is the dominant iron oxide, outcome of a hydrated environment. The flat shape of the plateau, which corresponds to the South American Surface described by King (1956), under appropriate and prolonged weather conditions, favored the formation of the hard ferruginous coverage. After the end of sedimentary deposition, in early Paleocene (approximately 65 million years BP), the surface was truncated by long erosion periods that flattened the relief. Currently the weather is tropical with strong seasonality and humidity increase. This new hydrological condition, saturated, combined with stable tectonic conditions and sparse vegetation exerts influence in leaching and loss of iron in the aluminous clay horizon. The results suggests that is possible the formation of a aluminous material from a ferruginous duricrust, proposing an autochthonous hypothesis for the development of the aluminous clays.

La Formation Barreiras comprend des séquences continentales et marines synchrone à l'ouverture de l'Océan Atlantique. La plupart des études sur la Fm. Barreiras sont concentrées sur les régions Nord et Nord-Est du Brésil, tandis que la recherche dans la région Sud-Est est encore limitée sur l'enregistrement du système de dépôt sédimentaire. Cette thèse vise à caractériser les cuirasses et leurs transformations au sein de la Fm. Barreiras afin de comprendre comment les horizons supergènes s'intègrent dans le paquet sédimentaire. La zone d'étude est située dans l'État de l'Espírito Santo, sud-est du Brésil. Les travaux de terrain comprennent la description stratigraphique et l'échantillonnage de trois profils : P1 (Praia das Falésias), situé sur le littoral, et P2 (Praia de Marobá), situé à 20 m d'altitude à l'intérieur des terres, tous deux dans la partie émergée du bassin de Campos ; et P3 (Presidente Kennedy), situé à 80 m d'altitude. Un total de 36 échantillons a été analysé à l'aide d'une série de techniques, comprenant l'analyse texturale, la pétrographie et la micromorphologie, complétée par une étude au microscope électronique à balayage (MEB) et par spectroscopie de rayons X dispersifs. De plus, la diffraction des rayons X (DRX), la fluorescence des rayons X et les techniques de plasma à couplage inductif. Cette étude adopte une approche inédite pour la Fm. Barreiras, notamment par la caractérisation fine de la kaolinite et l'estimation de son âge via la spectroscopie de résonance paramagnétique électronique (RPE). Le quartz, l'hématite, la kaolinite et la goethite sont les principales phases identifiées par DRX. Les analyses optiques ont également identifié du zircon, du feldspath, du rutile, de la muscovite, de la biotite, de l'ilménite, de l'allanite et de l'épidote. Les profils sélectionnés ont permis de mettre en évidence sept horizons ferrugineux: (i) Massive Duricrust (MD) caractérisée par une morphologie cavernicole et colonne; (ii) une Fragmentary Duricrust (FD) lobée et cohésive avec une matrice similaire à celle du MD ; (iii) deux couches de Platy Duricrust, (PD) continues séparées par un grès massif et une argilite ; (iv) Clayey (C) constitué d'argilite à silte et de grains opaques; (v) un horizon Sand-clay Mottled caractérisé par une c/f porphyrique ouverte avec des hypocoatings; (vi) une Nodular Duricrust (ND) définie par des nodules qui passent progressivement à faciès supérieur ; et (vii) Oxisol/Latosol (Sand clayey, SC) avec une transition graduelle du sol rouge au sol jaune et des nodules de taille centimétrique. Les processus de dégradation de l´horizon MD et d'altération des minéraux primaires contribuent à la formation des horizons superposés (FD, SCM, C et ND), menant au développement des horizons pédogéniques (SC, post-Barreiras) au sommet de tous les profils étudiés. La caractérisation RPE a révélé que le Fe3+ structural remplace l'aluminium, indiquant que les kaolinites sont désordonnées, et la concentration des défauts induits par irradiation varie entre 0,46 ± 0,07 et 7,48 ± 1,12 u.a. L'augmentation du signal isotrope associée à la kaolinite a subi une altération pédogénique et le composant g ⊥ (perpendiculaire) indique un faible niveau de radiation et un âge de cristallisation jeune pour les kaolinites. La paléodose; dose de rayonnement accumulée au cours du temps, varie entre 5,87 Gy/ka et 93,28 Gy/ka pour des débits de dose corrigés variant entre 6,04 et 72,12 Gy/ka. L'âge des horizons ferrugineux, telles que les cuirasses et la zone tache, varie entre 4,13 ± 1,2 et 0,29 ± 0,1 Ma, tandis que celui de la zone pâle et des horizons pédologiques est compris entre 2,41 ± 0,7 et 0,73 ± 0,2 Ma. Ils indiquent des périodes des durées de formation importantes de kaolinite durant le Pliocène/Pleistocène et révèlent deux fronts de dégradation liés à des périodes paléoclimatiques et à des enregistrements géodynamiques depuis l'établissement de la configuration actuelle de la plateforme continentale
The Barreiras Formation (Fm.) represents continental and shallow-marine sequences that are widespread along the entire Brazilian Atlantic margin. The extensive ferruginization is conspicuous as evidenced by numerous ferruginous horizons with duricrust layers. This study aims to investigate the genesis and degradation of duricrusts within the Barreiras Fm. on the southeastern Atlantic coast, with a focus on the characterization of the ferruginous horizons and their integration into the sedimentary sequence. Survey activities involved the identification and stratigraphic description. Three profiles were selected: P1, located on the seashore; P2, onshore at 20 m, both in the emerged portion of the Campos basin; and P3, at 80 m on the crystalline basement. A total of 36 bulk and preserved samples were analyzed using classical methods such as, textural analysis, petrography and micromorphology complemented by scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction, X-ray fluorescence and inductively coupled plasma were carried out. The results indicate seven ferruginous horizons, from the bottom to top: (i) massive duricrust (MD) composed mainly of quartz grains disseminated in a ferruginous micromass, characterizing a close to single-spaced porphyric c/f related, (ii) a fragmentary duricrust (FD) with a similar groundmass to the MD and iron depletion pedofeatures towards fragments edges, (iii) two layers of platy duricrust (PD) separated by a sandstone, (iv) a clayey level (C) that exhibits opaque grains disseminated in a undifferentiated b-fabric, (v) a sand-clayey mottled (SCM) exhibiting an open porphyric c/f, hypocoatings and relict iron mottles, (vi) a nodular duricrust (ND) characterized by nodules with dispersed quartz grains disseminated in a friable micromass that gradually leads to the formation of the upper horizon, the (vii) Oxisol/Latosol (SC). Quartz, hematite, kaolinite and goethite are the main mineral phases, also zircon, epidote, biotite, muscovite and feldspar dispersed in an iron-rich, aluminous micromass. The spatial distribution and correlation indicate a ferruginization process progressing from the continent towards the coast. The weathering processes, which were accompanied by a marked deferruginization, led to kaolinite recrystallization constituting a kaolinitic micromass and hosting rare remains of a vermicular structure or typical booklets replacing the feldspars. The EPR revealed structural Fe replacing Al, indicating that the kaolinites are disordered. The g ⊥ component indicates a low radiation background and a young crystallization age of the kaolinites. The age of the ferruginous facies, such as the duricrusts and mottled zone, varies between 4.13 ± 1.2 and 0.29 ± 0.1 Ma and that of the pale zone and pedologic horizons between 2.41 ± 0.7 and 0.73 ± 0.2 Ma. They indicate significant periods of duricrust formation during the Pliocene/Pleistocene and reveal two degradation fronts leading to the transformation of kaolinite crystals and related to paleoclimatic periods since the establishment of the current configuration of the continental platform. The first, from the base of the profiles to the middle section, is controlled by the water table and a hydraulic discontinuity. The second, from the top to the middle profiles, is due to the evolution of pedogenetic alteration, i.e. mineralogical transformations of the ND horizon, especially by infiltration of rainwater. The results show a polygenetic weathering in the Barreiras Fm. with different cycles of ferruginization and degradation. There is no evidence of a simple history of vertical weathering. Instead, different processes affected specific areas of the regolith while other parts were preserved
A Formação Barreiras (Fm.) abrange sequências continentais e marinhas rasas amplamente distribuídas ao longo da margem atlântica do Brasil. A intensa ferruginização é evidente, com numerosos horizontes contendo couraças. Este estudo investigou a origem e a degradação das couraças ferruginosas da Fm. Barreiras, na costa sudeste do Brasil, com foco na caracterização mineralógica, micromorfológica e cronológica e na sua integração na sequência sedimentar. As atividades de campo incluíram a identificação e descrição estratigráfica de três perfis principais: BFP1, na linha da costa e; BFP2, a 20 m no continente, ambos na porção emersa da bacia de Campos; e BFP3, a 80 m, Fm. Barreiras sobre o embasamento cristalino do orógeno Ribeira. Foram coletadas 36 amostras brutas e preservadas, com base em variações de cor, textura e mineralogia. As análises laboratoriais incluíram textura, petrografia e micromorfologia, complementadas por microscopia eletrônica de varredura (MEV) e espectroscopia de raios-X por dispersão de energia (EDS). Foram também realizadas difração de raios-X (DRX), fluorescência de raios-X (FRX) e espectrometria de plasma acoplado indutivamente (ICP-SFMS e -OES). A metodologia inclui um nível de abordagem ainda não aplicado na Fm. Barreiras, especialmente a caracterização cristaloquímica das caulinitas e a estimativa das idades de cristalização via espectroscopia de ressonância paramagnética eletrônica (EPR). Foram identificados sete horizontes ferruginosos, da base para o topo: (i) couraça maciça com estrutura cavernosa (MD); (ii) couraça fragmentada (FD), com arcabouço semelhante ao da MD e feições de depleção de ferro; (iii) duas camadas de couraça placoidal (PD) separadas por um arenito e um lutito; (iv) um nível de argiloso (C), constituído por caulinita e minerais opacos; (v) uma camada argilo-arenosa mosqueada (SCM), exibindo um estrutura porfírica aberta, hiporrevestimentos e relictos de ferro; (vi) couraça nodular (ND), caracterizada por nódulos com grãos de quartzo dispersos em uma matriz friável, transitando para o horizonte superior; (vii) Latossolo (SC). As principais fases minerais são quartzo, hematita, caulinita e goetita, além de zircão, epídoto, biotita, muscovita e feldspato, dispersos numa micromassa rica em ferro e alumínio. A normalização dos elementos terras raras (ETR) evidencia maiores concentrações de ETRL e filiação entre os horizontes. A distribuição espacial, cotas topográficas e correlações entre os horizontes indicam que o processo de ferruginização progride do continente em direção à costa. O intemperismo, acompanhado por uma ação de deferruginização, levou à formação de caulinitas com, pelo menos, duas populações: i- livretos típicos que substituem os feldspatos e as micas e; ii- uma micromassa caulinítica com vestígios de estrutura vermicular. A caracterização EPR revelou a presença de ferro trivalente estrutural e alumínio no domínio superparamagnético, o que indica que as caulinitas são desordenadas. O componente g ⊥ indica um baixo nivel de radiação e uma idade de cristalização recente das caulinitas. A idade dos horizontes ferruginosos, como as couraças e a zona mosqueada, varia entre 4,13 ± 1,2 e 0,29 ± 0,1 Ma, enquanto a da zona pálida e dos horizontes pedológicos varia entre 2,41 ± 0,7 e 0,73 ± 0,2 Ma. Materiais parentais datam entre 6.56 ± 2,0 e 4,16 ± 1.2 Ma. Os dados indicam períodos significativos de formação de couraças ferruginosas durante o Plioceno/Pleistoceno e de formação de solos durante o Quaternário. Duas frentes de intemperismo levam à transformação dos cristais de caulinita e estão relacionadas a períodos paleoclimáticos desde o estabelecimento da configuração atual da plataforma continental. A primeira, da base dos perfis até a seção média, é controlada pelo nível do mar, pelo lençol freático e por uma descontinuidade hidráulica entre arenitos e lutitos. A segunda, do topo até o meio dos perfis, resulta da evolução da alteração pedogenética do horizonte [...]

This work takes a look at some of the literature on calcretes and especially the problem of geochemical exploration in calcrete terrains. The conclusion that will be reached is that exploration in calcrete terrains is not futile and that provided the explorationist is aware of the types of calcrete and their genetic implications calcrete can be used as a sampling medium and anomalies can be detected through calcrete (p.1.)
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Silica cemented pedogenic horizons known as duripans occur on relict landforms in climates having limited leaching potential. Under the influence of tephra, incipient silica cementation may also occur in late Quaternary soils. The source of silica for cementation in Holocene soils is rapidly weatherable volcanic glass. In response to wetting and drying cycles volcanic glass weathers to form sand-size composite particles composed of silt, clay and redeposited silica. A portion of the hydrolyzed silica is eluviated as monosilicic acid which accumulates as the wetting front evaporates. Illuvial silica augments existing composite particles eventually forming a continuously cemented duripan. In actively forming late Quaternary soils, the cementation process is a complex interaction between illuvial silica, clay and calcium carbonate and the soil matrix. Surface reactions between monosilicic acid and illuvial clay or soil matrix particles provide nucleation sites for polymerization of silica concentrated by evaporation. The resulting opaline silica bonds adjacent soil grains without necessarily plugging intervening pore spaces. In contrast, calcium carbonate preferentially precipitates in large pores and interped voids. Cementation occurs by the plugging of progressively smaller pores with relatively pure calcite rather than by heterogeneous bonding of mineral grains. In illuvial zones containing both silica and calcium carbonate, cementation may occur rapidly because the former holds small soil particles in place while the latter plugs large pores. The mineralogy of silica cement is determined by identification of varying amounts of crystal order using X-ray diffraction. Opal-A is recently polymerized, noncrystalline, highly hydrated silica gel. The more prevalent, partly crystalline opal-CT forms where surface reaction with clays create crystal orientation, where silica gel dehydrates or when silica precipitates from soil solutions having high concentrations.

Algumas indicações morfológicas sugerem que as superfícies de paisagem tropicais onde encontram-se os duricrusts ferruginosos, podem apresentar idades que variam desde o Quaternário até o Cretáceo, e que a laterização sobre estas superfícies pode ter sido iniciada simultaneamente ou não, estabelecendo-se uma sequência cronológica de formação escalonada de acordo com a elevação, em função da evolução geomorfológica das superfícies. Sabendo-se que no Planalto de Diamantina na Serra do Espinhaço Meridional (SdEM) são reconhecidos três níveis geomorfológicos correlatos aos ciclos de aplainamento da plataforma Sul-americana, o objetivo deste trabalho foi estudar as variações mineralógicas, geoquímicas e morfológicas de duricrusts ferruginosos da Serra do Espinhaço Meridional buscando entender os processos genéticos envolvidos e as implicações disto sobre a distribuição e evolução da paisagem regional e suas relações com os ciclos de aplainamento do continente Sul-americano. Para atingir estes objetivos, foram realizadas análises de difratometria de raios X, suscetibilidade magnética, microscopia eletrônica de varredura com sistema de energia dispersiva acoplado e análise elementar a partir da dissolução total dos minerais constituintes. Os resultados mostraram que no Planalto de Diamantina na SdEM coexistem níveis de ferricretes e lateritas na superfície correspondente ao ciclo erosivo Pós-Gondwanico. As superfícies lateríticas são provavelmente as formações supérgenas mais antigas da paisagem regional, originadas durante os processos denudacionais que ocorreram ao longo do ciclo erosivo Pós-Gondwanico. A erosão parcial de seu perfil laterítico constituíram o material fonte de Fe e Al para a gênese dos ferricretes distribuídos em superfícies elevadas (> 1200 m), especialmente aqueles em superfícies em torno de 1400 m de altitude. As variações paleoclimáticas do ciclo Pós-Gondwanico também proporcionaram ciclos erosivos alternados que resultaram no subescalonamento desta superfície, criando condições geomorfológicas favoráveis à gênese de ferricretes mais recentes (1200 - 1400 m). Além disso, os duricrusts ferruginosos do Planalto Diamantina (SdEM) sob influência do maciço quartzítico do Supergrupo Espinhaço e situados superfícies erosivas elevadas (> 1200 m), além de policíclicos, também podem apresentar características poligenéticas.
Some morphological indications suggest that the tropical landscape surfaces where the iron-rich duricrust are present may have ages varying from the Quaternary to the Cretaceous, and that the laterization on these surfaces may have been initiated simultaneously or not, establishing a sequence chronological formation according to the elevation, according to the geomorphological evolution of the surfaces. Three geomorphological levels correlated to the South American platform planing cycles are known on the Diamantina Plateau, for this the objective of this work was to study the mineralogical, geochemical and morphological variations of the iron-rich duricrust of the SdEM, trying to understand the genetic processes involved and the implications of this on the distribution and evolution of the regional landscape and its relations with the planning cycles of the South American continent. In order to reach these objectives, X-ray diffraction, magnetic susceptibility, scanning electron microscopy with dispersive energy system coupled and elemental analysis carried out from the total dissolution of the constituent minerals. The results showed that in the Diamantina Plateau at the SdEM, coexist levels of ferricretes and laterites on the surface corresponding to the Post-Gondwanic erosive cycle. The laterite surfaces are probably the oldest surviving formations of the regional landscape, originated during the denudation processes that occurred along the Post-Gondwanic erosive cycle. Partial erosion of its lateritic profile was source material of Fe and Al for the genesis of ferricretes distributed on high surfaces (> 1200 m), especially those on surfaces at 1400 m of altitude. The paleoclimatic variations of the Post-Gondwanic cycle also provided alternating erosive cycles that resulted in sub-scaling of this surface, creating geomorphological conditions favorable for genesis of earlier ferricretes (1200 - 1400 m). In addition, the ferricretes of the Diamantina Plateau (SdEM) under influence of the Quartzite massif of the Espinhaço Supergroup and located in elevated erosive surfaces (> 1200 m), besides polycyclics, may also present polygenetic characteristics.