Soluble Anhydrite Research Articles

Solubility of anhydrite in supercritical water from 380 ˚C to 625 ˚C and 220 bar to 270 bar

The solubility of anhydrite in deionized water has been determined experimentally from 380 ˚C to 625 ˚C and 220 bar to 270 bar. The experiments were performed using a unique flow-through reactor capable of reaching supercritical conditions for pure water. The results cover the approximate range of temperature and pressure expected to be found in deep geothermal systems where supercritical conditions could be expected. Anhydrite solubility has not been previously well-defined in this region.The new experimental data are used to define empirical parameters for a new thermodynamic model based on Dolejš and Manning (2010), for anhydrite dissolution in silica-bearing aqueous fluids at elevated temperatures and pressures:-RTlnm=a+b+elnρwwhere m is the molar concentration mol∙kg−1, R is the gas constant in J mol−1 K−1, and ρw is the density of pure water in kg m−3. These parameters only apply in solutions containing silica and at fluid densities above 200 kg m−3:a = 14921.46; b = 369.46; e = -48.81.T is temperature in K.

Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

AbstractRecycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H2O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at log fO2 from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite at fO2 conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ34S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke 34S‐rich subducted lithologies.

An accurate model for the solubilities of anhydrite in H2O–NaCl solutions at temperatures up to 1073 K, pressures up to 14,000 bar, and mole fractions of NaCl up to 0.3

An accurate density-based solubility model is established for anhydrite in H2O–NaCl solutions, which is valid for temperatures up to 1073 K, pressures up to 14,000 bar, and mole fractions of NaCl (xNaCl) up to 0.3. The isobaric linear dependence of the mineral solubility logarithm on the solvent density logarithm should be a common phenomenon similar to isothermal linear dependence, particularly at relatively high pressures. The terms about P in the equations of the density-based model are rational and necessary to elevate the accuracy of mineral solubility models. This model reproduces the complicated changing trends of anhydrite solubility with temperature, pressure, and xNaCl. Compared to numerous experimental data, the average relative deviations of calculated anhydrite solubilities in water and the aqueous NaCl solution are 3.52% and 9.64%, respectively, which are within experimental uncertainties. According to this model, the anhydrite deposition would result from any salinity variation departing from the specified xNaCl in skarn- or porphyrite-type iron ore-forming hydrothermal fluids.

Solubility of anhydrite and gypsum at temperatures below 100°C and the gypsum-anhydrite transition temperature in aqueous solutions: a re-assessment

Anhydrite and gypsum are omnipresent in sedimentary rocks of all types. They occur as massive layers or are distributed within other geological formations as in clays. Understanding the conditions of formation and the stability of the hydrated and anhydrous form of calcium sulfate is crucial in an elucidation of the genesis of the geological formations envisaged as potential host rock for radioactive waste disposal. Estimations of the temperature, where gypsum is dehydrated to anhydrite in water vary between 30°C and 60°C. The extremely slow crystallization kinetics of anhydrite at T < 90°C prevents a direct determination of this transition temperature. In the present work the different approaches to fix this temperature are discussed. It is shown that careful assessment of solubility data and calorimetric measurements yields a transition temperature of 42°C ± 1°C. For results essentially deviating from this value methodic deficiencies are revealed and discussed. Thus, a long-standing discussion about the thermodynamic aspect of the gypsum-anhydrite conversion can be closed, not the kinetic part.

Recovery of rare earth elements from phosphogypsum using subcritical water extraction

Phosphogypsum (PG) is generated in massive amount in the production of fertilizer. It has been indicated that PG is a promising secondary source for rare earth elements (REEs). This study examined the leaching of REEs, including yttrium (Y), lanthanum (La), cerium (Ce) and neodymium (Nd) from PG utilizing subcritical water extraction (SWE). Hydrochloric acid (HCl) was selected as the leaching reagent, and the leaching efficiency increased with increasing HCl concentration and decreasing solid to liquid ratio (S/L). Complete leaching of REEs (100.36% of Y, 97.50% of La, 100.06% of Ce and 96.58% of Nd) was achieved under experimental conditions of 0.1 mol/L HCl, 10 g/L, 100 °C, initial pressure of 10 kg/cm2, and 5 min. The total energy consumption in SWE was 4.2 × 106 kJ/kg. Leaching efficiency of Y, La, and Nd slightly increased as temperature increased to 125 °C, and decreased when further increased to 150 °C. The behavior was due to solubility of anhydrite as affected by temperature and pressure under subcritical conditions, and the enhanced mass transfer. From comparisons, SWE could be an efficient and green technology for the recovery of REEs from PG.

Anhydrite solubility enhanced by CaO in silicate melts: Implications for sulfur cycling in subduction zones

The capacity of slab-derived melt in mobilizing sulfur from the slab to the mantle wedge is essential for understanding global S cycling in subduction zones of the present or ancient Earth. The promotive role of the CaO constituent in the silicate melts, provided chiefly by subducting sediments, in anhydrite dissolution in silicate melts has been noted before. Nevertheless, the dependence of SCAS, sulfur concentration in the melt at anhydrite saturation, on melt CaO concentration and the underlying dissolution mechanism have yet to be well constrained. Using a piston-cylinder apparatus, we carried out five series of phase equilibrium experiments between anhydrite and felsic melts to obtain constraints on the effect of melt CaO concentration, water concentration, temperature (T), and Ca/(Ca+Mg) ratios on SCAS at 1–3 GPa and 1050–1350 °C. Electron probe microanalyses of quenched products confirmed that the dissolved sulfur was predominantly present as SO42−. The experimental results show that SCAS increased significantly with increasing melt CaO and Ca/(Ca+Mg) ratios at a fixed degree of silicate melt polymerization (NBO/T = 0.43 ± 0.01). The SCAS also increased with increasing T and dissolved water concentration, and the effect of water was non-linear. Previous SCAS models could not capture well enough our new experimental data, therefore a new SCAS parameterization was developed using previous and our new experimental data. With our new results and model, we find that slab melts with high CaO concentration could efficiently transport S from the slab to the mantle wedge in the Archean. Slab melts in modern hot subduction zones can carry more than 1800 ppm sulfur, raising S concentration in the mantle wedge to the lower end of the observed level by metasomatism. In modern intermediate or colder subduction zones, however, slab melt is not an effective agent of sulfur transfer even at CaO concentration >10 wt%.

Hydration caves and cavities from the recent weathering zone of anhydrites at Dingwall (Nova Scotia, Canada)

Hydration of anhydrite to gypsum (gypsification of anhydrite) in the weathering zone sometimes causes a volume increase. This can lead to local detachments of the surface layer of the weathering anhydrite rocks and creation of the hydration cavities and rare hydration caves (or swelling caves; German: Quellungshöhlen) inside domes formed by the uplifted layer. Currently growing hydration cavities at the anhydrite bottom of the abandoned quarry at Dingwall, Canada, were studied with the use of photogrammetric, statistic, and petrographic methods. Seventy seven cavities were documented including 48 caves (cavities with the height ≥0.30 m large enough to accommodate an adult man). Forty three caves had proper entrances (inlets large enough for an adult to get inside). The cavities contain one flat domed or, less often, tepee-like chamber generally similar in shape to the host hydration landform. The majority of cavities showed the length <4.6 m, the width <3.0 m, and the height <0.85 m. Only 3 caves had the length >8.00 m and the height ≥0.80 m. The largest and the highest cave showed the sizes 10.7 × 6.6 × 1.1 m measured in 2008 which changed into 8.87 × 2.97 × 1.35 m in 2019. The other caves and cavities exhibit also morphological changes with time due to vertical and horizontal movements of rocks slabs and collapse of the roofs depending mainly on the location of expansive gypsification and its intensity. Precipitation (rain- and meltwater) accumulating at the foot of the initial uplift intensifies the hydration in this zone causing a centripetal pressure towards the landform and contributing to its further rising and enlargement of the cavity height. The basic condition required for hydration cave formation is temperature between 0 and 30 °C in which the solubility of gypsum is much lower than that of anhydrite. The meteoric water solution infiltrating in the vadose zone is dissolving the anhydrite and gradually reaches Ca sulphate concentration higher than the level of gypsum and lower than the level of anhydrite solubility. Then, the dissolving anhydrite yields a solution supersaturated with gypsum, capable of its precipitation, and the dissolution of anhydrite is simultaneous with in situ replacive/displacive gypsum crystallization. The crystallization pressure of gypsum is a direct cause of the rock deformations and creation of hydration caves.

Studies of the kinetics of the reaction of dissolution of gypsum-anhydrite rocks in water of the initial stages

Experimental studies of the kinetics of dissolution of gypsum-anhydrite rocks in water (25 °C; P = 0,1 MPa) made it possible to formulate a process model in the form of a system of equations for the kinetics of dissolution of gypsum and anhydrite (of the first and second order, respectively). The calculated values turned out to be more comparable with the experimental data, taking into account the common-ion effect on the solubility of gypsum and anhydrite, and changes in their values of the specific reaction surface.

Synthesis of robust insoluble anhydrite whiskers from desulphurization gypsum containing organic matter via improved hydrothermal‐calcination process for the removal of lead and zinc ions

AbstractBackgroundAn improved hydrothermal‐calcination method was used to convert desulphurization gypsum containing organic matter (OM‐gypsum) to insoluble anhydrite (II‐CaSO4) whiskers, where OM‐gypsum was directly transformed to soluble anhydrite (γ‐CaSO4) whiskers via the hydrothermal method followed by calcination. The synthesized II‐CaSO4 whiskers were applied for the removal of lead ion (Pb(II)) and zinc ion (Zn(II)).ResultsThe synthesized II‐CaSO4 whiskers possessed a smooth surface, large aspect ratio, and high whiteness of above 90%. The mother liquor after synthesis could be recycled. The synthesized II‐CaSO4 whiskers exhibited maximum adsorption capacities of 641.03 mg/g and 14.00 mg/g for Pb(II) and Zn(II), respectively, under optimal adsorption conditions. The pseudo‐second‐order model and Langmuir isotherm model were appropriate for describing the adsorption process of II‐CaSO4 whiskers. Compared with II‐CaSO4 whiskers calcined from hemihydrate gypsum and the short and columnar II‐CaSO4 crystals, the synthesized II‐CaSO4 whiskers in this work possessed a superior adsorption performance.ConclusionThe improved hydrothermal‐calcination process could completely remove the organic matter from OM‐gypsum, and the direct formation of γ‐CaSO4 intermediate helped avoid the lattice collapse caused by the removal of water molecules during calcination. The excellent adsorption performance of the synthesized II‐CaSO4 whiskers in this work could be attributed to their large negatively charged surface areas, which were formed by the large aspect ratio and excellent stability. This work provides a practical method for the comprehensive utilization of chemical gypsum containing organic impurities. © 2022 Society of Chemical Industry (SCI).

Analysis of Temperature Influence on Precipitation of Secondary Sediments during Water Injection into an Absorptive Well

The extraction of hydrocarbons is associated with obtaining certain amounts of water, which is heavily contaminated with a wide range of chemical compounds that negatively affect the environment. At present, practically the only method of managing extracted reservoir waters is their injection into absorbing horizons. Large changes in parameters (pH, Eh, temperature, etc.) occurring during the extraction and storage of water, as well as the contact of the injected water with reservoir water and rock, may result in the precipitation of secondary sediments. The complexity of the injected water/native water/deposit rock system and the wide range of possible interactions do not always allow for correct interpretation of the processes and their impact on near-well zone permeability. One of the factors which has a decisive influence on dissolution/precipitation is temperature change. Applying analytical data of water with low (W-1) and high (W-2) mineralization, calculations were carried out with the use of PRHEEQC software. Changes in solubility index values were determined at ambient temperature (20 °C) and reservoir temperature (94 °C). The obtained results indicate that with increasing temperature, SI changes for a given chemical compound may run in different directions and take different values, depending on the composition of the injected water. The calculations indicate the possibility of a change in the direction of the reaction from dissolution to precipitation, which may lead to clogging of the near-well zone. Simulations of the injected water’s contact with minerals present in the reservoir rock were also carried out. The obtained data indicate that these minerals, in the entire studied temperature range, dissolve in the injected water, but the solubility of anhydrite and dolomite decreases with increasing temperature. If the water is saturated with minerals at low temperature, after heating in the bed, sedimentation and blockage of rock pores may occur, which means there is a reduction in the efficiency of water injection.

Modeling Solubility of Anhydrite and Gypsum in Aqueous Solutions: Implications for Swelling of Clay-Sulfate Rocks

The swelling of clay-sulfate rocks is a well-known phenomenon often causing threats to the success of various geotechnical projects, including tunneling, road and bridge construction, and geothermal drilling. The origin of clay-sulfate swelling is usually explained by physical swelling due to clay expansion combined with chemical swelling associated with the transformation of anhydrite (CaSO4) into gypsum (CaSO4∙2H2O). The latter occurs through anhydrite dissolution and subsequent gypsum precipitation. Numerical models that simulate rock swelling must consider hydraulic, mechanical, and chemical processes. The simulation of the chemical processes is performed by solving thermodynamic equations, which usually contribute a significant portion of the overall computation time. This paper employs feed-forward neural network (FFNN) and cascade-forward neural network (CFNN) models trained with a Bayesian regularization (BR) algorithm as an alternative approach to determine the solubility of anhydrite and gypsum in the aqueous phase. The network models are developed using calcium sulfate experimental data collected from the literature. Our results indicate that the FFNN-BR is the most accurate model for the regression task. The comparison analysis with the Pitzer ion interaction model as well as previously published data-driven models shows that the FFNN-BR model is highly accurate in determining the solubility of sulfate minerals in acid and salt-containing solutions. We conclude from our results that the FFNN-BR model can be used to determine the solubility of anhydrite and gypsum needed to address typical subsurface engineering problems such as swelling of clay-sulfate rocks.

A model for the solubility of anhydrite in H2O-NaCl fluids from 25 to 800 °C, 0.1 to 1400 MPa, and 0 to 60 wt% NaCl: Applications to hydrothermal ore-forming systems

Anhydrite (CaSO4) is a common gangue mineral in hydrothermal ore-forming systems, but currently there is no comprehensive numerical model for the solubility of anhydrite in H2O-NaCl hydrothermal fluids over wide ranges of pressure, temperature and fluid salinity (P, T and X). Here, we present a new thermodynamic model for the solubility of anhydrite in saline aqueous fluids throughout a wide range of P-T-X conditions relevant to crustal hydrothermal settings. A database of experimentally determined solubility of anhydrite was used to calibrate the model, and the experimental data covers P-T-X ranges of 0.1–1400 MPa, 25–800 °C, and 0–60 wt% NaCl. The model is successful at reproducing solubility values and overall trends from the experimental dataset within error, and also reproduces results for experimental solubility data that were not included in the calibration. We demonstrate that the model can perform well over a wide-range of P-T-X conditions, including conditions of liquid + vapor coexistence. We also provide examples of applications of the solubility model to predict the precipitation and/or dissolution of anhydrite along fluid flow paths representative of hydrothermal systems.

Evidence of gas plume model in porphyry copper deposits based on anatomy, fluid inclusions and H-O isotopes: Insight from Kahang deposit, Iran

The Kahang deposit (100 Mt at 0.6% Cu) is one of the important porphyry copper deposits in the central part of Urumieh-Dokhtar Magmatic Arc (UDMA), Iran. The mineralization occurs as chalcopyrite and molybdenite in quartz and anhydrite -bearing veinlets or breccias and is closely associated with sericitic and phyllic alterations. Through comprehensive studies of the deposit's anatomy, some previously overlooked evidence raised new questions about the ore-forming process in the Kahang deposit. The coexistence of sulfate and sulfide minerals as veinlets and breccias, pore space breccias, could not be justified solely based on a previously introduced concept (i.e., mixing high temperature-high salinity magmatic water with low temperature-low salinity meteoric water). Fluid inclusions were examined in quartz veinlets of the A1, A2, and B types, anhydrite veinlets, and sphalerite of the L type veinlets. There are three types of fluid inclusions: liquid-rich (L-type), vapor-rich (V-type), and hypersaline (H-type). According to microthermometry of fluid inclusions, the fluids associated with anhydrite veinlets have relatively high temperatures (390–495 °C, mean = 455 °C) and low salinities (12.8–20.9 wt% NaCl equiv.) On the contrary, temperatures below 400 °C justified the coexistence of sulfate and sulfide minerals. The low solubility of anhydrite in high-temperature waters, on the other hand, demonstrates the gas–solid reaction of SO2 (g) and plagioclase at such temperatures, as evidenced by fluid inclusions. Additionally, the fluid inclusions in quartz veinlets generated temperatures (290 to >600 °C) higher than those in the majority of other porphyry copper deposits. H-O isotopes indicated that magmatic fluids dominated, and meteoric waters played an insignificant role. These findings and observations supported the formation of the Kahang porphyry copper deposit as a result of a gas plume and the reactions of gas species with plagioclase to form anhydrite and sulfide minerals, as well as the formation of sericitic alteration in the host rock as a novel scenario.

Evolution of the Astonishing Naica Giant Crystals in Chihuahua, Mexico

Calcium sulfate (CaSO4) is one of the most common evaporites found in the earth’s crust. It can be found as four main variations: gypsum (CaSO4∙2H2O), bassanite (CaSO4∙0.5H2O), soluble anhydrite, and insoluble anhydrite (CaSO4), being the key difference the hydration state of the sulfate mineral. Naica giant crystals’ growth starts from a supersaturated solution in a delicate thermodynamic balance close to equilibrium, where gypsum can form nanocrystals able to grow up to 11–12 m long. The growth rates are reported to be as slow as (1.4 ± 0.2) × 10−5 nm/s, taking thousands of years to form crystals with a unique smoothness and diaphaneity, which may or may not include solid or liquid inclusions. Conservation efforts can be traced back to other gypsum structures found prior to Naica’s. Furthermore, in the last two decades, several authors have explored the unique requirements in which these crystals grow, the characterization of their environment and microclimatic conditions, and the prediction of deterioration scenarios. We present a state-of-the-art review on the mentioned topics. Beyond the findings on the origin, in this work we present the current state and the foreseeable future of these astounding crystals.

Thermodynamic Modeling of Calcium Sulfate Hydrates in a CaSO4-H2SO4-H2O System from 273.15 to 473.15 K up to 5 m Sulfuric Acid.

To prevent scaling and to recycle aqueous solutions in industrial processes, the thermodynamic properties of the CaSO4–H2SO4–H2O system are studied by thermodynamic modeling with the Pitzer model. The published solubility data of calcium sulfate hydrates in sulfuric acid solutions were collected and reviewed critically. Then, the CaSO4–H2SO4–H2O system was modeled using the Pitzer activity coefficient approach from critically selected experimental data to obtain optimized parameters. The model reproduces the solubility data with good accuracy up to 5 m sulfuric acid at temperatures of 283.15–368.15, 283.15–473.15, and 298.15–398.15 K for gypsum (CaSO4·2H2O), anhydrite (CaSO4), and hemihydrate (CaSO4·0.5H2O), respectively. However, at temperatures above 398.15 K and sulfuric acid concentration above 0.5 mol/kg, the solubility of anhydrite predicted by our model deviates significantly from the literature data. Our model predicts that the solubility of anhydrite would first increase but then decrease in more concentrated sulfuric acid solutions, which is in disagreement with the experimental data showing constantly increasing solubilities as a function of increasing sulfuric acid concentration. This discrepancy has been discussed. The transformations of gypsum to anhydrite and hemihydrate were predicted in sulfuric acid solutions. With increasing H2SO4 concentration, the transformation temperatures of gypsum to anhydrite and hemihydrate will decrease. Thus, gypsum is stable at low temperatures in solutions of low H2SO4 concentrations and transforms to anhydrite at high temperatures and in concentrated H2SO4 solutions, while hemihydrate is always a metastable phase. Furthermore, the predicted results were compared with previous experimental studies to verify the accuracy of the model.

An accurate model to predict sulfur concentration at anhydrite saturation in silicate melts

Magmatic sulfur is an essential constituent for the genesis of porphyry-type ore deposits and also significantly affects the Earth’s climate when emitted into the atmosphere. As an increasing number of studies indicate that anhydrite commonly occurs as primary igneous mineral in arc magmas, any quantitative model aiming to assess the efficiency of sulfur degassing from magmas must take the potential presence of anhydrite into account. To facilitate this, we present a new empirical model to predict the solubility of anhydrite in silicate melts as a function of pressure (P), temperature (T) and silicate melt composition. The model is based on 189 experimental data points, encompassing T from 750 to 1325 °C, P from 30 to 3000 MPa and basaltic to rhyolitic melt compositions. Fifteen of these experiments were conducted as part of this study in rapid-quench Molybdenum – Hafnium Carbide pressure vessel assemblies to obtain tighter constraints on the effect of water concentration, P and T on anhydrite solubility. These experiments show that anhydrite solubility in the silicate melt rapidly increases with increasing T following Arrhenius relationship. The solubility of anhydrite also increases with increasing dissolved water concentration, with the relative increase being the most significant for felsic melt compositions. The new model predicts sulfur concentration at anhydrite saturation (SCAS) in the silicate melt with median and mean absolute percentage errors of 19 and 25 relative%, respectively. The model is equally successful over the entire P, T and compositional space included in its calibration. This is a significant improvement over most previously published models, which were calibrated based on a more limited P, T and compositional range, and predict SCAS with significantly larger errors when applied to the entire dataset utilized in this study. The implementation of the new anhydrite saturation model in an example scenario of the crystallization of a hydrous andesite magma at P = 200 MPa suggests that anhydrite saturation may severely limit the amount of S that can be transferred into the exsolving magmatic volatile phase during crystallization-driven degassing of upper crustal magma reservoirs.

The Kinetics of Gypso Anhydrites Dissolution in Water: Experimental Studies

Experiments on the kinetics of gypso anhydrites dissolution in water (25°С, 0.1 MPa) resulted in the development of a model of the process in the form of balance equations of dissolution kinetics for gypsum and anhydrite (first and second order, respectively), as well as of the kinetics of gypsum precipitation (the second order). The data from the experiments were processed based on the Riccati equation. The calculated values were closer to the experimental data when the common-ion effect on the solubility of gypsum and anhydrite was included.

Kinetics of hypso anhydrites dissolution in water. Experimental studies

Experimental study of kinetics of dissolution of hypso anhydrites at 25 ᵒC made it possible to formulate model of the process in the form of a balance equation for the kinetics of dissolution of gypsum, anhydrite (first and second orders, respectively) and kinetics of precipitation of gypsum (second order). The processing of the experimental data were carried out on the basis of the solution of the Riccati equation. When taking into account the common-ion effect on the solubility of gypsum and anhydrite, the calculated values turned out to be more comparable with the experimental ones.

Anhydrite solubility in low-density hydrothermal fluids: Experimental measurements and thermodynamic calculations

Thermodynamic predictions of fluid-mineral equilibria significantly expand our ability to test hypotheses regarding mass and heat transfer in geological systems. In many cases, successful development of thermodynamic models relies heavily on experimental measurements. A dearth of experimental data in low-density hydrothermal fluids has prevented calculation of fluid-mineral equilibria at the most extreme temperature-pressure conditions associated with mid-ocean ridge hydrothermal systems. This study compares newly attained anhydrite solubility measurements in low-density NaCl-H2O fluids (410 °C, 25–32 MPa and 450 °C, 36.3–50 MPa) with novel thermodynamic calculations. To address the lack of equilibrium constants at the precise temperature-pressure conditions of the experiments, data are extrapolated from reference conditions as a linear function of the logarithm of water density. Several solubility calculations are performed that vary in the choice of aqueous speciation and activity coefficient model. There is overall good agreement between experimental observations and thermodynamic predictions, indicating the potential for accurate assessment of fluid-mineral equilibria in low-density fluids. However, speciation calculations are non-unique, highlighting uncertainties in aqueous speciation and calculation of activity coefficients that will require further examination. This study represents an important first step in increasing the temperature-pressure conditions over which thermodynamic calculations of fluid-mineral equilibria are possible, with important implications for understanding mass and heat transfer in submarine hydrothermal systems.

Dehydration Pathways of Gypsum and the Rehydration Mechanism of Soluble Anhydrite γ-CaSO4.

The dehydration products of gypsum under different temperature and water vapor pressure were investigated by thermodynamic theory. Additionally, the rehydration mechanism of soluble anhydrite was also studied by Monte Carlo (MC) simulations. The thermodynamic calculation results reveal that the dehydration mechanism of gypsum significantly depended on ambient temperature and water vapor pressure. In the high-temperature and low water vapor pressure region, gypsum dehydrates to form γ-CaSO4 in a single-step process (CaSO4·2H2O → γ-CaSO4); with increasing water vapor pressure, gypsum undergoes the CaSO4·2H2O → γ-CaSO4 → β-CaSO4·0.5H2O reaction path and as water vapor pressure increases further, the occurrence of a two-step conversion path CaSO4·2H2O → β-CaSO4·0.5H2O → γ-CaSO4 was observed. It was also found that gypsum is stable in the low-temperature and high water vapor pressure region and does not dehydrate to form any calcium sulfate hemihydrate. Finally, the rehydration mechanism of soluble anhydrite was studied by MC simulations. The simulation results are in agreement with the experimental data and support the finding that γ-CaSO4 rehydration forms CaSO4·0.67H2O in high relative humidity. Another important result revealed by the MC simulation is that γ-CaSO4 has an extraordinary ability to capture water molecules from an extremely dry atmosphere, which is very useful in some fields, such as in drying processes and even for extracting liquid water from extremely dry atmosphere.