Anhydrous Phases Research Articles

Multiple intrusions and remelting-remobilization events in a magmatic arc: The St. Peter Suite, South Australia

Granitoids with juvenile signatures are common in arc environments and contribute to growth of the continental crust. Thermo-mechanical models of arcs suggest that intermittent intrusion of magma batches leads to magma hybridization, remelting, and remobilization of earlier intrusive rocks driven by fluctuations in temperature and water fluxing. While there are numerous examples in the literature of multiple intrusions and magma hybridization, field examples of remelting and remobilization of earlier intrusive rocks within an arc are rare. Here, we investigate the evolution of magmatic rocks of the Paleoproterozoic St. Peter Suite, emplaced along the SW margin of the Gawler craton, South Australia, a typical calc-alkaline arc suite. Magmatic rocks recording multiple intrusions and multiple magma interactions have undergone in situ remelting and remobilization forming migmatites. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon dating yielded crystallization ages of 1647 ± 12 Ma for a tonalitic gneiss representing the oldest intrusive suite, and 1604 ± 12 Ma for a leucogranite representing the youngest intrusive suite. Both these suites developed synmagmatic foliation and magmatic banding defined by broadly parallel dikes, elongated enclaves, and schlieren. The rock suites record two deformation events associated with anatexis. The first event, D 1 , was responsible for a dominant approximately E-W–striking foliation (S 1 ) parallel to magmatic foliation, and associated with a dominantly sinistral shearing that affected the older suite. This deformation was associated with the first anatectic event, as demonstrated by the association between leucosomes and structures. The second deformation event, D 2 , affected both suites and was characterized by isolated F 2 folds and shear planes filled with leucosomes subparallel to axial planar foliation. Leucosomes interconnected and gave rise to magma extraction channels tens of meters long. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb titanite dating of a late leucosome sample, collected from within the older magmatic suite, yielded an age of 1605 ± 7 Ma, coinciding with the crystallization age of the younger suite rather than postdating it, as expected. We interpret these results to indicate that crystallization of the younger suite and the second anatectic event occurred in the time encompassed by the error of these young ages. Leucosomes from both anatectic phases lack anhydrous peritectic phases and are interpreted to represent low-temperature anatexis resulting from water fluxing. Combined, these results suggest that a protracted and complex intrusive history can be made significantly more complex by anatexis, giving rise to evolved magmas after older ones, erasing earlier intrusive relationships, and establishing new ones. Rocks of the St. Peter Suite record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger and more fractionated magmas.

Generation of oligonucleotides under hydrothermal conditions by non-enzymatic polymerization.

We previously reported that 5'-mononucleotides organized within a multilamellar lipid matrix can produce oligomers in the anhydrous phase of hydration-dehydration (HD) cycles. However, hydrolysis of oligomers can occur during hydration, and it is important to better understand the steady state in which ester bond synthesis is balanced by hydrolysis. In order to study condensation products of mononucleotides and hydrolysis of their polymers, we established a simulation of HD cycles that would occur on the early Earth when volcanic land masses emerged from the ocean over 4 billion years ago. At this stage on early Earth, precipitation produced hydrothermal fields characterized by small aqueous pools undergoing evaporation and refilling at elevated temperatures. Here, we confirm that under these conditions, the chemical potential made available by cycles of hydration and dehydration is sufficient to drive synthesis of ester bonds. If 5'-mononucleotides are in solution at millimolar concentrations, then oligomers resembling RNA are synthesized and exist in a steady state with their monomers. Furthermore, if the mononucleotides can form complementary base pairs, then some of the products have properties suggesting that secondary structures are present, including duplex species stabilized by hydrogen bonds.

Reversible Dehydration in Polyoxometalate-Based Hybrid Compounds: A Study of Single-Crystal to Single-Crystal Transformations in Keggin-Type Germanotungstates Decorated with Copper(II) Complexes of Tetradentate N-Donor Ligands

Hydrothermal reaction of the Keggin-type [α-GeW11O39]8– anion with copper(II) complexes of the tetradentate N-donor ligands N,N′-dimethyl-N,N′-bis-(pyridine-2-ylmethyl)-1,2-diaminoethane (bpmen) or N,N′-dimethyl-N,N′-bis-(pyridine-2-ylmethyl)-1,3-diaminopropane (bpmpn) results in three new hybrid compounds [Cu(bpmen)(H2O)][GeW12O40{Cu(bpmen)}]·3.5H2O (1), [GeW12O40{Cu(bpmpn)(H2O)}2]·3H2O (2), and [Cu(bpmpn)(H2O)][GeW12O40{Cu(bpmpn)}]·8H2O (3). All compounds have been characterized by elemental analyses and infrared spectroscopy, and their structures have been established by single-crystal X-ray diffraction. Both 1 and 3 contain monodecorated polyanions and additional complex cations, but their layered structures show noticeable differences. For 1, the polyanions are arranged in rows whose stacking generates intralamellar cavities where complex cations and lattice water molecules are hosted, whereas double-layers of hybrid anions pillared by the cations occupying the interlamellar space are observed for 3. In the case of 2, the packing of trans-didecorated clusters generates honeycomb-like metalorganic layers and perpendicular hexagonal channels where Keggin moieties are nested. A combination of thermogravimetric and variable temperature powder X-ray diffraction analyses show that dehydration of 1 and 2 results in thermally stable, crystalline phases. Dehydrations are fully reversible upon exposure to air and proceed via single-crystal to single-crystal transformations implying rupture/formation processes in the copper(II) bonding and consequent geometrical and conformational modifications in the complexes. Transformations have been followed by single-crystal X-ray diffraction, allowing for the structures of the anhydrous phases [GeW12O40{Cu(bpmen)}2] (1a) and [GeW12O40{Cu(bpmpn)}2] (2a) to be determined. Dehydration of 1 into 1a with consequent formation of neutral, didecorated clusters does not result in the collapse of the intralamellar cavities but rather in an open-framework structure with slit-shaped micropores. Structural variations between 1 and 2 and their corresponding anhydrous phases are reflected in the electron paramagnetic resonance spectra.

Topologically identical, but geometrically isomeric layers in hydrous α-, β-Rb[UO2(AsO3OH)(AsO2(OH)2)]·H2O and anhydrous Rb[UO2(AsO3OH)(AsO2(OH)2)

The hydrothermal reaction of uranyl nitrate with rubidium nitrate and arsenic (III) oxide results in the formation of polymorphic α- and β-Rb[UO2(AsO3OH)(AsO2(OH)2)]·H2O (α-, β-RbUAs) and the anhydrous phase Rb[UO2(AsO3OH)(AsO2(OH)2)] (RbUAs). These phases were structurally, chemically and spectroscopically characterized. The structures of all three compounds are based upon topologically identical, but geometrically isomeric layers. The layers are linked with each other by means of the Rb cations and hydrogen bonding. Dehydration experiments demonstrate that water deintercalation from hydrous α- and β-RbUAs yields anhydrous RbUAs via topotactic reactions.

Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica

Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. It occurs in veins or in disseminated form in spinel lherzolites. Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe3+/ΣFe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870–0.994 O3(O2−) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of aH2O (0.014–0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58 % in determination of aH2O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphibole equilibrium) is between −2.52 and −1.32 log units below the fayalite–magnetite–quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine–orthopyroxene–spinel equilibria (fO2 between −1.98 and −0.30 log units). A comparative evaluation of the two methods suggests that when amphibole is present in mantle peridotites, the application of oxy-amphibole equilibrium is preferred, because ol–opx–sp oxy-calibrations are not “sensitive” enough in recording the effects (if any) of amphibole in the peridotite matrix. Amphibole acts as the main H acceptor among the peridotite minerals and may prevent fluid circulation and buffer oxygen fugacity. The important conclusion of this study is that amphibole within the lithospheric mantle does not always means high water activity and oxidizing conditions.

Preparation and application of zirconium sulfate supported on SAPO-34 molecular sieve as solid acid catalyst for esterification

Zirconium sulfate (ZS) was supported on SAPO-34 molecular sieve by using an incipient wetness impregnation method with zirconium sulfate as the precursor. The as-prepared catalysts were used as solid acid catalyst for esterification reaction of acetic acid with ethanol. The influence of calcination temperature on the acidic property, catalytic activity, and reusability of ZS/SAPO-34 catalysts were mainly investigated. FT-IR, SEM, EDS and TG analysis have been carried out to demonstrate the characteristics of ZS/SAPO-34 catalysts. It was found that the 30wt%ZS/SAPO-34 catalysts display the property of superacid irrespective of calcination temperature. The ZS/SAPO-34 catalyst treated at 200°C can enhance the interaction between the supported ZS and SAPO-34 and keep the catalyst remaining substantially active after several reaction cycles. However, further increasing calcination temperature will cause the transfer of ZS from hydrate to anhydrous phase, and thus the decrease of activity.

Stability of hydrous silicate at high pressures and water transport to the deep lower mantle

Hydrous magnesium-rich silicates play an important role in transporting water into the deep mantle when oceanic plates subduct as slabs, but were thought to dissociate at pressures of 44 GPa. In situ X-ray measurements in conjunction with a multi-anvil apparatus show that hydrous phases of magnesium-rich silicate are stable under lower mantle conditions up to 50 GPa, and may transport water to deeper layers of the mantle. The presence of water strongly influences the structure, composition and dynamics of the Earth’s deep mantle1,2,3,4. Hydrous magnesium-rich silicates play an important role in transporting water into the deep mantle when oceanic plates subduct as slabs. The highest-pressure form of such hydrous silicates, phase D, was reported to dissociate into an assemblage of nominally anhydrous phases plus water at pressures of about 44 GPa, equivalent to 1,250 km depth5. In this way, free water would be released in the middle region of the lower mantle, which suggests that no water can reach the deeper regions of the lower mantle. Here we use in situ X-ray measurements in conjunction with a multi-anvil apparatus using sintered diamond anvils to show that hydrous phases are stable under lower mantle conditions up to 50 GPa. We found that phase D transforms to an assemblage with another hydrous silicate (MgSiH2O4), named phase H, at pressures above about 48 GPa. Our results suggest that phase H is the dominant hydrous silicate in descending slabs, and may be found at depths far deeper than 1,250 km. We conclude that a significant amount of water is retained in the descending slabs and may be delivered to the deepest part of the lower mantle.

Four salt phases of theophylline

The structures of two anhydrous salt phases of theophylline, namely 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium tetrafluoroborate, C7H9N4O2(+)·BF4(-), and 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium chloride, C7H9N4O2(+)·Cl(-), are reported together with the structures of two monohydrate salt forms, namely 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium chloride monohydrate, C7H9N4O2(+)·Cl(-)·H2O, and 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium bromide monohydrate, C7H9N4O2(+)·Br(-)·H2O. The monohydrate structures are mutually isostructural, with the cations and anions lying on crystallographic mirror planes (Z' = ½). The main intermolecular interaction motif is a hydrogen-bonding network in the same mirror plane. The tetrafluoroborate structure is based on planar hydrogen-bonded theopylline cation dimers; the anions interact with the dimers in a pendant fashion. The anhydrous chloride structure has Z' = 2 and in contrast to the other species it does not form planar hydrogen-bonded constructs, instead one-dimensional chains of cations and anions propagate parallel to the crystallographic c direction. An earlier report claiming to describe an anhydrous structure of theophylline hydrochloride is re-examined in light of these results. It is concluded that the earlier structure has been reported in the wrong space group and that it has been chemically misidentified.

Preparation and characterization of polymorphs of the glucocorticoid deflazacort

The polymorphism of new and old active pharmaceutical ingredients (APIs) is of great importance due to performance, stability and processability aspects. The objective of this study was to investigate the polymorphism of deflazacort (DEF), a glucocorticoid discovered >40 years ago, since this phenomenon has not been previously investigated for this API. Using different methods for solid form screening, it was determined for the first time that DEF is able to exist as three forms: a crystalline (DEF-1); a hydrated X-ray amorphous (DEF-t-bw) and an anhydrous amorphous phase (DEF-g) obtained from manually grinding DEF-1. The in vitro and in vivo dissolution rates (DRs) of DEF-1 and DEF-t-bw, which were measured using the rotating disk method in water at 37 °C and the pellet implantation technique in rats, respectively, indicated that DEF-t-bw exhibited slightly faster in vitro and in vivo DRs than those of the crystalline form, but the values were not significantly different. In addition, it was determined that DEF-t-bw devitrifies to DEF-1 by the effect of pressure, humidity and heat. It was concluded that DEF is glucorticoid with low tendency to exhibit different crystalline forms and that DEF-t-bw has no advantages over DEF-1 in terms of solubility, DRs and solid-state stability.

Three new structural forms of thiocolchicoside, a muscle relaxant, as shown by X-ray powder diffraction

Thiocolchicoside (THC) is an important active pharmaceutical ingredient (API) used as a muscle relaxant because of its anti-inflammatory and analgesic effects. The only entry for a THC-related compound present in the Cambridge Structural Database (CSD) corresponds to a THC ethanol solvate hydrate (refcode: THCLCS). The diffraction pattern recorded for the THC raw material (C27H33NO10S·xH2O) is different from the pattern calculated using the THCLCS crystallographic data contained in the CSD. The indexing of the THC raw material pattern, produced an orthorhombic unit cell with a = 28.018(7) Å, b = 12.519(2) Å, c = 8.519(1) Å, and V = 2988.01 Å3. All the diffraction maxima of the powder pattern of a phase recrystallized in water (C27H33NO10S·2H2O) can be indexed in an orthorhombic cell with a = 25.264(4) Å, b = 13.537(3) Å, c = 8.553(1) Å, and V = 2925.12 Å3. Thermogravimetric analysis shows that this compound is a dihydrate phase. Upon heating, a new anhydrous phase (C27H33NO10S) with a monoclinic cell and unit cell parameters: a = 17.090(5) Å, b = 19.485(5) Å, c = 8.526(3) Å, β = 100.30(2)°, and V = 2793.34 Å3 is obtained.

Dehydration of Gypsum Rock by Solar Energy: Preliminary Study

The dehydration process of gypsum rock was studied under concentrated solar energy by using a Fresnel lens with power density of 260 Wcm-2. Temperatures higher than 700。C were attained for 1 min of solar exposure. The effect of grain size of sample and radiation exposure time on the formation of bassanite and anhydrite was studied by XRD. The complete transformation of dihydrate into hemihydrate and/or anhydrate phases is complete for the finer size sample. Plaster composed of 92.7% of anhydrite and 7.3% of bassanite was obtained for 5 min of solar exposure. Morphological and textural modifications were followed by SEM and interferometric/confocal profilometer.

Dehydration of mildronate dihydrate: a study of structural transformations and kinetics

The dehydration of mildronate dihydrate (3-(1,1,1-trimethylhydrazin-1-ium-2-yl)propionate dihydrate) was investigated by powder X-ray diffraction, thermal analysis, hot-stage microscopy, water sorption–desorption studies and dehydration kinetic studies. It was determined that mildronate dihydrate dehydrated in a single step, directly transforming into the anhydrous form. In order to understand the reasons for a one step dehydration mechanism, crystal structures of dihydrate, monohydrate and anhydrous forms were compared, proving the similarity of the dihydrate and anhydrous forms. In order to understand the reasons for molecule reorganization during dehydration, the energy of the anhydrous form was compared with that of a theoretical dihydrate structure without water molecules. It was proven that the experimentally observed anhydrous phase AP was thermodynamically more stable. By analyzing the effect of the particle size and sample weight on the dehydration kinetic parameters it was determined that besides the main rate limiting step, phase boundary advancement, contribution from the water diffusion outside the crystal and the water diffusion outside the powdered sample also appeared to affect the dehydration kinetics and contribution from these processes could be changed by changing the aforementioned factors.

Development of Thermal Energy Storage Materials from Waste-process Salts

TES is a technology that has been taken more importance in the last decades because of the increasing demand of clean energies, to avoid environment polluting by energy production from fossil sources. For that reason has been developed many materials for energy storage so far, but many of them are still much expensive and produce lesser energy than conventional fuels.In this research the potential use of waste-process salts, coming from the non-metallic mining industry in northern Chile, as heat storage materials were studied. The cost of these salts is low, and it corresponds to the transport cost to the application place mainly. The stability and thermophysical properties of two hydrated double salts, astrakanite and kainite, were determined by thermogravimetry and differential scanning calorimetry techniques. That information was useful to establish the range of application for these waste-salts as materials for sensible energy storage. The results proved that both salts are suitable for using as heat storage materials at low temperatures, 0°C to 120°C. Also it was found that the anhydrous phase of astrakanite can be used as solid TES material at higher temperatures, 350°C to 530°C.

Synthesis and crystal growth of thienyl-substituted pyridinium derivatives for second-order nonlinear optics

Two series of thienyl-substituted pyridinium salts with different counteranions have been synthesized to investigate the effect of hydrous/anhydrous phase on crystal packing. Single crystals of two kinds of salts were successfully obtained from methanol by slow evaporation method. X-Ray studies exhibited that they crystallized in the monoclinic space group P1 and P, respectively. (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium3-nitrobenzenesulfonate (TPNS) forms a hydrated noncentrosymmetric phase with a macroscopic optical nonlinearity 0.40 times of that of DAST at the non-resonant wavelength of λ = 2109 nm. (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methyl-pyridinium2,5-dimethylbenzenesulfonate (TPDMS) forms an anhydrous phase and crystallizes in centrosymmetric structures without SHG signal.

Mineral abundances at the final four curiosity study sites and implications for their formation

A component of the landing site selection process for the Mars Science Laboratory (MSL) involved the presence of phyllosilicates as the main astrobiological targets. Gale crater was selected as the MSL landing site from among 4 down selected study sites (Gale, Eberswalde and Holden craters, Mawrth Vallis) that addressed the primary scientific goal of assessing the past habitability of Mars. A key constraint on the formation process of these phyllosilicate-bearing deposits is in the precise mineralogical composition. We present a reassessment of the mineralogy of the sites combined with a determination of the modal mineralogy of the major phyllosilicate-bearing deposits of the four final study sites from the modeling of near-infrared spectra using a radiative transfer model. The largest abundance of phyllosilicates (30–70%) is found in Mawrth Vallis, the lowest one in Eberswalde (<25%). Except for Mawrth Vallis, the anhydrous phases (plagioclase, pyroxenes and martian dust) are the dominant phases, suggesting formation conditions with a lower alteration grade and/or a post-formation mixing with anhydrous phases. The composition of Holden layered deposits (mixture of saponite and micas with a total abundance in the range of 25–45%) suggests transport and deposition of altered basalts of the Noachian crust without major chemical transformation. For Eberswalde, the modal mineralogy is also consistent with detrital clays, but the presence of opaline silica indicates that an authigenic formation occurred during the deposition. The overall composition including approximately 20–30% smectite detected by MSL in the rocks of Yellow-knife Bay area interpreted to be material deposited on the floor of Gale crater by channels (http://www.nasa.gov/mission_pages/msl/news/msl20130312.html) is consistent with the compositions modeled for the Eberswalde and Holden deltaic rocks. At Gale, the paucity, the small diversity and the low abundance of nontronite do not favor a complex and long drainage system. Localized aqueous processes in space and time environments could have produced both nontronites and sulfates. However, most materials in Gale are unfortunately dust covered, so that orbital data are limited by spatial resolution and surficial fines that could dilute and obscure the spectral influence of phyllosilicates in the rocks. Potential formation processes of diverse and abundant Mawrth Vallis deposits include low temperature hydrothermal alteration in marine environments and/or pedogenesis.

Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites

We present a comprehensive description of petrologic, chemical and spectroscopic features of thermally metamorphosed CI-like and CM (and CM-like) chondrites. Only two such CI chondrites have so far been discovered i.e. Y-86029 and Y-82162. Thermal metamorphism in these chondrites is apparent in their low contents of H2O, C and the most thermally labile trace elements, partial dehydration of matrix phyllosilicates and abundance of thermally decomposed Ca–Mg–Fe–Mn carbonates, which apparently resulted from heating of Mg–Fe carbonate precursors.The CM chondrites exhibit a wide range of aqueous and thermal alteration characteristics. This alteration was almost complete in Y-86720 and Y-86789, which also escaped alternating episodes of oxidation and sulfidization experienced by the others. Thermal metamorphism in the CM chondrites is apparent in loss of thermally labile trace elements and also in partial to almost complete dehydration of matrix phyllosilicates: heating was less uniform in them than in CI chondrites. This dehydration is also evident in strength and shapes of integrated intensities of the 3μm bands except in PCA 91008, which experienced extensive terrestrial weathering. Tochilinite is absent in all but Y-793321 probably due to heating. Textural evidence for thermal metamorphism is conspicuous in blurring or integration/fusion of chondrules with matrix in the more extensively heated (⩾600°C) CM chondrites like PCA 91008 and B-7904. TEM and XRD analyses reveal that phyllosilicate transformation to anhydrous phases proceeds via poorly crystalline, highly desiccated and disordered ‘intermediate’ phases in the least and moderately heated (400–600°C) carbonaceous chondrites like WIS 91600, PCA 91008 and Y-86029. These findings are significant in that they confirm that these phases occur in meteorites as well as terrestrial samples.Thermal alteration in these meteorites can be used to identify other carbonaceous chondrites that were thermally metamorphosed in their parent bodies. Combining RNAA trace element data for experimentally heated Murchison CM2 samples with petrographic and spectroscopic data, these thermally metamorphosed carbonaceous chondrites can be ordered by severity of open system heating as 400°C⩽Y-793321<WIS91600=EET90043=A881655<PCA91008<B-7904=Y-86029<Y-82162<Y-86720=Y-86789⩾700°C. Nearly all heated carbonaceous chondrites discovered so far have been found in Antarctica, which is known to have sampled the flux of near-Earth material for much longer than exemplified by current falls.

Identification and characterisation of stable phases of silicotungstic acid, H4SiW12O40·nH2O

Thermal analysis data recorded from silicotungstic acid show the existence of stable phases of composition H4SiW12O40·nH2O where n=24, 14, 6 and 0. FT-IR data confirms the presence of the Keggin anion, SiW12O404−, in each of these phases. In addition, dehydration of the higher hydrates (n>6) is shown to occur via the loss of zeolitic type water followed by (n<6) the removal of water associated with the acidic protons. Each of the hydrated phases, as well as the anhydrous phase, has been characterised by X-ray powder diffraction and the unit cell parameters are reported. 29Si MAS NMR spectroscopy is consistent with a single silicon site in each of the four single phases and deshielding of the silicon nucleus in the range n=6 to n=0 is associated with the transfer of acidic protons from the H5O2+ ions to random locations on oxygen atoms in the Keggin unit. The observed increase in magnitude of T1 (29Si) for the hexahydrate is interpreted in terms of a reduction in motional freedom of the water molecules in this hydrate compared to those in the higher hydrates. Nominal hydrated states between n=6 and 14, as well as those between n=14 and 24, are shown by X-ray powder diffraction to be composed of the stable hydrated phases in the given ranges.

Testing the cation-hydration effect on the crystallization of Ca-Mg-CO3 systems.

Dolomite and magnesite are simple anhydrous calcium and/or magnesium carbonate minerals occurring mostly at Earth surfaces. However, laboratory synthesis of neither species at ambient temperature and pressure conditions has been proven practically possible, and the lack of success was assumed to be related to the strong solvation shells of magnesium ions in aqueous media. Here, we report the synthesis of MgCO3 and MgxCa(1-x)CO3 (0 < x < 1) solid phases at ambient conditions in the absence of water. Experiments were carried out in dry organic solvent, and the results showed that, although anhydrous phases were readily precipitated in the water-free environment, the precipitates' crystallinity was highly dependent on the Mg molar percentage content in the solution. In specific, magnesian calcite dominated in low [Mg(2+)]/[Ca(2+)] solutions but gave way to exclusive formation of amorphous MgxCa(1-x)CO3 and MgCO3 in high-[Mg(2+)]/[Ca(2+)] and pure-Mg solutions. At conditions of [Mg(2+)]/[Ca(2+)] = 1, both nanocrystals of Ca-rich protodolomite and amorphous phase of Mg-rich MgxCa(1-x)CO3 were formed. These findings exposed a previously unrecognized intrinsic barrier for Mg(2+) and CO3(2-) to develop long-range orders at ambient conditions and suggested that the long-held belief of cation-hydration inhibition on dolomite and magnesite mineralization needed to be reevaluated. Our study provides significant insight into the long-standing "dolomite problem" in geochemistry and mineralogy and may promote a better understanding of the fundamental chemistry in biomineralization and mineral-carbonation processes.

Resistance of blended cement pastes subjected to organic acids: Quantification of anhydrous and hydrated phases

Concrete for agricultural construction is often subject to aggressive environmental conditions. Ground granulated blast furnace slag (GGBFS) or metakaolin (MK) largely improve the chemical resistance of the binder. Anhydrous particles seem particularly resistant to the acid solution. The purpose of this study is to quantify anhydrous particles in blended cement pastes as a function of acid exposition time in order to evaluate their acid resistance.Cement pastes were moist cured for 28days and then immersed in an acetic acid solution for 2months. The quantification of the anhydrous phases was carried out using 29Si MAS NMR, selective dissolution and back-scattered electron (BSE) images analysis, while the hydrated phases content was evaluated by TGA. After 28days of hydration, 60% of OPC, 44% of GGBFS and 76% of MK particles were hydrated. The amount of anhydrous particles drops for all materials during acid immersion. After 2months of immersion, the amount of anhydrous particles drops by 49%, 23% and 15% for OPC, GGBFS, and MK respectively. This study confirms that GGBFS and MK anhydrous and hydrates phases present higher acid resistance than OPC.

High-temperature induced dehydration, phase transition and exsolution in amicite: A single-crystal X-ray study

Temperature dependent single-crystal X-ray data were collected on amicite K4Na4(Al8Si8O32)·11H2O from Kola Peninsula (Russia) in steps of 25°C from room temperature to 175°C and of 50°C up to 425°C. At room temperature amicite has space group I2 with a=10.2112(1), b=10.4154(1), c=9.8802(1)Å, β=88.458(1)°, V=1050.416(18) Å3. Its crystal structure is based on a Si–Al ordered tetrahedral framework of the GIS type with two systems of eight-membered channels running along the a and c axes. Extraframework K and Na cations are ordered at two fully occupied sites.Above 75°C amicite was found to partly dehydrate into two separate but coherently intergrown phases, both of space group I2/a, one K-rich ∼K8(Al8Si8O32) ·4H2O (at 75°C: a=10.038(2), b=9.6805(19), c=9.843(2) Å, β=89.93(3)°, V=956.5(3)Å3) and the other Na-rich ∼Na8(Al8Si8O32)·2H2O (at 75°C: a=9.759(2), b=8.9078(18), c=9.5270(19) Å, β=89.98(3)°, V=828.2(3)Å3).Upon further heating above 75°C the Na- and K-phases lost remaining H2O with only minor influence on the framework structure and became anhydrous at 175°C and 375°C, respectively. The two anhydrous phases persisted up to 425°C.Backscattered electron images of a heated crystal displayed lamellar intergrowth of the K- and Na-rich phases. Exposed to ambient humid conditions K- and Na-rich phases rehydrated and conjoined to the original one phase I2 structure.