Gotowa bibliografia na temat „Vanthoffite Mineral”

The crystal structure of vanthoffite {hexasodium magnesium tetrakis[sulfate(VI)]}, Na6Mg(SO4)4, was solved in the year 1964 on a synthetic sample [Fischer & Hellner (1964). Acta Cryst. 17, 1613]. Here we report a redetermination of its crystal structure on a mineral sample with improved precision. It was refined in the space group P21/c from a crystal originating from Surtsey, Iceland. The unique Mg (site symmetry \overline{1}) and the two S atoms are in usual, only slightly distorted octahedral and tetrahedral coordinations, respectively. The three independent Na atoms are in a distorted octahedral coordination (1×) and distorted 7-coordinations intermediate between a `split octahedron' and a pentagonal bipyramid (2×). [MgO6] coordination polyhedra interchange with one half of the sulfate tetrahedra in <011> chains forming a (100) meshed layer, with dimers formed by edge-sharing [NaO7] polyhedra filling the interchain spaces. The other [NaO7] polyhedra are organized in a parallel layer formed by [010] and [001] chains united through edge sharing and bonds to the remaining half of sulfate groups and to [NaO6] octahedra. The two types of layers interconnect through tight bonding, which explains the lack of morphological characteristics typical of layered structures.

Abstract A new mineral, metathénardite, ideally Na2SO4, the high-temperature hexagonal dimorph of thénardite, a natural analogue of the synthetic phase Na2SO4(I), was found in the sublimates of active fumaroles at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure eruption, Tolbachik volcano, Kamchatka, Russia. The holotype originates from the Glavnaya Tenoritovaya fumarole in which metathénardite is associated with hematite, tenorite, fluorophlogopite, sanidine, anhydrite, krasheninnikovite, vanthoffite, glauberite, johillerite, and lammerite. The cotypes 1 and 2 are from the Arsenarnaya (with hematite, tenorite, fluorophlogopite, sanidine, euchlorine, wulffite, anhydrite, fluoborite, johillerite, nickenichite, calciojohillerite, badalovite, tilasite, cassiterite, and pseudobrookite) and the Yadovitaya (with tenorite, euchlorine, fedotovite, dolerophanite, langbeinite, krasheninnikovite, anhydrite, and hematite) fumaroles, respectively. All specimens with metathénardite were collected from areas with temperatures of 350–400 °C. Metathénardite forms hexagonal tabular, lamellar, or dipyramidal crystals (forms: {001}, {100}, {102}, and {201}) up to 3 mm combined in crusts up to several hundred cm2 in area. The mineral is transparent to semitransparent, colorless, white, light-blue, greenish, yellowish, grayish or brownish, with vitreous luster. Dmeas. = 2.72(1), Dcalc. = 2.717 g/cm3. Metathénardite is optically uniaxial (–), ω = 1.489(2), ε = 1.486(2). The empirical formulae are (Na1.92K0.05Ca0.02Zn0.01)[S0.99O4] (holotype), (Na1.54K0.22Ca0.09Cu0.01Mg0.01)[S1.00O4] (cotype 1), and Na1.65K0.11Ca0.05Cu0.04Mg0.01)[S1.01O4] (cotype 2). Admixed K and bivalent cations probably stabilize the hexagonal aphthitalite-like structure of metathénardite at room temperature. The crystal structure was solved using single crystals of all three samples, R1 = 0.0852, 0.0452, and 0.0449 for holotype and cotypes 1 and 2, respectively. The space group is P63/mmc, and the unit-cell parameters of the holotype are a = 5.3467(9), c = 7.0876(16) Å, V = 157.47(6) Å3, and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 4.667(27)(100), 3.904(89)(101), 3.565(33)(002), 2.824(94)(102), 2.686(100)(110), and 1.939(35)(202). Metathénardite and thénardite clearly differ from one another in X-ray diffraction data and infrared and Raman spectra.

The genesis of vanthoffite, loeweite and kieserite in the potassium salts deposits of Precarpathian region is characterized on the basis of mineralogical, geochemical and experimental studies. Vanthoffite and loeweite are rare minerals. Vanthoffite associates with astrakhanite and loeweite. It is proved due to experimental studies that dehydration of astrakhanite occurs at the stages of late diagenesis–katagenesis under conditions of increasing temperature. From the products of its decomposition, loeweite is formed first, and then vanthoffite. This is facilitated by an increase of pressure, under which minerals with a higher density are more stable (the density of vanthoffite (2.69 g/cm3 ) is much higher than that of loeweite (2.37) and astrakhanite (2.32)). Kieserite is found in sylvinite, kainite and kainite-langbeinite rocks in the amount of small impurities up to 10–33 %. It is unstable on the day surface: it absorbs water from the atmosphere and transforms into hexahydrite. Experimental studies have shown that kieserite does not precipitate in salt sedimentary basins, but is formed from epsomite (hexahydrite) due to its dehydration Петро Білоніжка 60 ISSN 2078-6220. Мінералогічний збірник. 2021. № 71 under conditions of increasing temperature and pressure at the stages of late diagenesis and katagenesis. Increasing temperature values (up to 200 °С), which existed in the potassium salts deposits of Precarpathian region during the thrust of the Skybova zone to the Precarpathian Foredeep Inner zone and the formation of folding in salt deposits, are sufficient for phase transformation of astrakhanite to loeweite and vanthoffite, and epsomite (hexahydrite) – to kieserite.

ABSTRACT Six different exsolution types are found in crystals of aphthitalite-group alkali sulfates from exhalations of the active Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. The coexisting minerals in these exsolutions are metathénardite, ideally Na2SO4 (P63/mmc), and vanthoffite, Na6Mg(SO4)4 (P21/c) (Type I); metathénardite and belomarinaite, KNaSO4 (P3m1) (Type II); thénardite, Na2SO4 (Fddd), and aphthitalite, K3Na(SO4)2 (Pm1) (Type III); aphthitalite and arcanite, K2SO4 (Pnma) (Type IV); metathénardite and natroaphthitalite, KNa3(SO4)2 (Pm1) (Type V); and two chemical varieties of metathénardite (Type VI). The exsolution processes occur in crystals belonging to the high-temperature, hexagonal Na2SO4(I) (= metathénardite, P63/mmc) structure type with different K:Na ratios formed at temperatures higher than 500 °C. The similarity and hexagonal close-packed nature of the crystal structures of the coexisting phases, all representatives of aphthitalite-like structure types, cause the coherent conjugation of domains during diffusion and cation ordering in the parent phase. The breakdown of solid solution can be facilitated by the mosaic character of crystals of a parent phase (incoherent grain boundaries) and the presence of coherent twin boundaries. The heating of samples with exsolution Types II and V up to 700 °C over 24 h shows that diffusion of K and Na through the domain borders does not result in the complete disorder of these cations and the extinction of domains with different crystal structures.