Geochemical studies of northern taiga (gypsum) karst ecosystems and their high vulnerability to natural and anthropogenic hazards

In the taiga gypsum karst ecosystems, gypsum soils formed on the hard gypsum substrates predominate in the soil cover. In these soils, the mineral horizons consist of 95–99% gypsum (CaSO₄·2H₂O) and the litter is the main horizon for nutrient accumulation. For this reason, the ecosystems are vulnerable to fire and erosion by walkers, from which they only recover slowly. Gypsum mining for industrial uses is also leading to the destruction of this unique ecosystem.     

A thermodynamic model for the prediction of phase equilibria and speciation in the H2O-CO2-NaCl-CaCO3-CaSO4 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to halite saturation

A thermodynamic model is developed for the calculation of both phase and speciation equilibrium in the H₂O-CO₂-NaCl-CaCO₃-CaSO₄ system from 0 to 250 °C, and from 1 to 1000 bar with NaCl concentrations up to the saturation of halite. The vapor-liquid-solid (calcite, gypsum, anhydrite and halite) equilibrium together with the chemical equilibrium of H⁺,Na⁺,Ca²⁺, , , and CaSO_4(aq) in the aqueous liquid phase as a function of temperature, pressure and salt concentrations can be calculated with accuracy close to the experimental results.Based on this model validated from experimental data, it can be seen that temperature, pressure and salinity all have significant effects on pH, alkalinity and speciations of aqueous solutions and on the solubility of calcite, halite, anhydrite and gypsum. The solubility of anhydrite and gypsum will decrease as temperature increases (e.g. the solubility will decrease by 90% from 360 K to 460 K). The increase of pressure may increase the solubility of sulphate minerals (e.g. gypsum solubility increases by about 20-40% from vapor pressure to 600 bar). Addition of NaCl to the solution may increase mineral solubility up to about 3 molality of NaCl, adding more NaCl beyond that may slightly decrease its solubility. Dissolved CO₂ in solution may decrease the solubility of minerals. The influence of dissolved calcite on the solubility of gypsum and anhydrite can be ignored, but dissolved gypsum or anhydrite has a big influence on the calcite solubility. Online calculation is made available on www.geochem-model.org/model.     

Mineral equilibria in a six-component seawater system,Na-K-Mg-Ca-SO4-Cl-H2O,at 25°C

Phase relations in the 6-component system Na-K-Mg-Ca-SO₄-Cl-H₂O have been calculated for halite saturation, 25°C and 1 atm pressure. Using a Jänecke projection with the apices Ca-Mg-K₂-SO₄, 27 stable invariant points have been located which are connected by 69 univariant curves. Polyhalite is the only quaternary solid, but anhydrite occupies the bulk of the interior tetrahedral space. Consequently, 24 of the invariant points lie very close to the Ca-free base, Mg-K₂-SO₄. The remaining three points involve tachyhydrite and/or antarcticite. All points but two (20,27) represent peritectic conditions. Metastable equilibria have been calculated for the Ca-free system and yield relations corresponding to the solar diagram.Seawater lies in the subspace anhydrite-halite-carnallite-kieserite-bischofite (point 20) and its evaporation has been discussed for conditions of equilibrium and fractional crystallization. After gypsum is converted to anhydrite, halite precipitates. The next phase, under equilibrium conditions, is glauberite, crystallizing at the expense of anhydrite. Continued evaporation leads to glauberite resorption and eventual replacement by polyhalite. Then follow the magnesium sulfates epsomite, hexahydrite and kieserite, which are joined by carnallite. Polyhalite is replaced by anhydrite and bischoflte is added at the final invariant condition. Kainite does not appear as a primary phase under equilibrium conditions, but it is an important phase during fractional crystallization, where Ca-phases are not allowed to back-react with the brine.Up to the appearance of glauberite, thickness ratios of halite: anhydrite couplets (equilibrium or fractionation) can vary from 0 to 7, the relative amount of halite increasing with more intense evaporation. During evaporation, the activity of H₂O decreases from 0.98 (seawater) to 0.34 (final invariant brine). The data provided can be used to evaluate the effects of mineral precipitation, evaporation and brine mixing for a wide variety of natural brines.     

An assessment of selenate co-precipitation with gypsum

In this study, we assessed the co-precipitation of selenate (SeO₄²⁻) with gypsum (CaSO₄·2H₂O) in controlled laboratory experiments. Batch testing was used to quantify the ability of CaSO₄·2H₂O to co-precipitate dissolved SeO₄²⁻ over a range of dissolved SeO₄²⁻-Se concentrations (0–50 mg/L) and under slightly acidic (pH ∼5.5–6.1) and oxic (Eh ∼416−501 mV) conditions. Aqueous samples were analyzed using inductively coupled plasma optical emission spectrometry, solid samples using X-ray diffraction and Raman spectroscopy, and digests of selected CaSO₄·2H₂O precipitates using inductively coupled plasma-mass spectrometry. The concentration of Se co-precipitated in CaSO₄·2H₂O increased linearly with dissolved SeO₄²⁻-Se concentration. The aqueous analyses and calculations based on the CaSO₄·2H₂O digest data show between 14–19 % of the dissolved Se was removed during the co-precipitation experiments. The strong linear relationship between SeO₄²⁻-Se added to the test solutions and Se co-precipitated in CaSO₄·2H₂O can be used to estimate the concentration of co-precipitated SeO₄^2- if the concentration of SeO₄^2- in the associated porewater is known, and vice versa. Results indicate that <1% of SeO₄²-Se was removed from the test solutions during co-precipitation and the mass of Se in CaSO₄·2H₂O solids was low, ranging between 0−120 μg/g. These results were used in conjunction with field- and model-derived data to show co-precipitation of SeO₄^2- with CaSO₄·2H₂O should be a minor SeO₄^2- sequestration mechanism. The findings of this study should be applicable to mined rock dumps in North America and elsewhere.     

Weathering of the black limestone of historical monuments (Ljubljana,Slovenia): Oxygen and sulfur isotope composition of sulfate salts

The black limestone widely used in Slovenian monuments, particularly in the baroque architecture, is deteriorating extensively due to salt crystallization. Samples of soluble salts from two important historical monuments (in Ljubljana, Slovenia) were investigated in terms of their mineral and isotopic (S and O) compositions. Results revealed the presence of gypsum and soluble salts of the MgSO₄·nH₂O series, such as starkeyite (MgSO₄·4H₂O), pentahydrite (MgSO₄·5H₂O) and hexahydrite (MgSO₄·6H₂O). Whereas black crusts and subflorescences consisted of gypsum, efflorescences appeared to be an assemblage of gypsum and MgSO₄ hydrates. Sample δ¹⁸O_sulfate values varied from ?1.9‰ to +5.5‰ vs. V-SMOW and δ³⁴S_sulfate values from ?19.8‰ to +3.2‰ vs. V-CDT. The respective isotopic composition of analysed outdoor and indoor monument samples indicated different sources of contamination.     

Thermodynamic and experimental data on stability of gypsum,hennihydrate, and anhydrite under hydrothermal conditions

Experimentally, gypsum remains stable at temperatures up to 110°C, whereupon it is replaced by hemihydrate [plaster of Paris], which in turn is replaced by anhydrite at 140°C and over. However, the hemihydrate proves to be metastable by thermodynamic calculations. Anhydrite, by the calculations 6 is a highly stable mineral; its decomposition begins at temperatures exceeding 1385°C. — Author.     

The measurement of sulfate mineral solubilities in the Na-K-Ca-Cl-SO4-H2O system at temperatures of 100, 150 and 200°C

At T > 100°C development of thermodynamic models suffers from missing experimental data, particularly for solubilities of sulfate minerals in mixed solutions. Solubilities in Na⁺-K⁺-Ca²⁺-Cl⁻-SO₄²⁻/H₂O subsystems were investigated at 150, 200°C and at selected compositions at 100°C. The apparatus used to examine solid-liquid phase equilibria under hydrothermal conditions has been described.In the system NaCl-CaSO₄-H₂O the missing anhydrite (CaSO₄) solubilities at high NaCl concentrations up to halite saturation have been determined. In the system Na₂SO₄-CaSO₄-H₂O the observed glauberite (Na₂SO₄ · CaSO₄) solubility is higher than that predicted by the high temperature model of Greenberg and Møller (1989), especially at 200°C. At high salt concentrations, solubilities of both anhydrite and glauberite increase with increasing temperature. Stability fields of the minerals syngenite (K₂SO₄ · CaSO₄ · H₂O) and goergeyite (K₂SO₄ · 5 CaSO₄ · H₂O) were determined, and a new phase was found at 200°C in the K₂SO₄-CaSO₄-H₂O system. Chemical and single crystal structure analysis give the formula K₂SO₄ · CaSO₄. The structure is isostructural with palmierite (K₂SO₄ · PbSO₄). The glaserite (“3 K₂SO₄ · Na₂SO₄”) appears as solid solution in the system Na₂SO₄-K₂SO₄-H₂O. Its solubility and stoichiometry was determined as a function of solution composition.     

The thermal behaviour of γ-CaSO<Subscript>4</Subscript>

Thermal behaviour of γ-anhydrite (γ-CaSO₄, soluble anhydrite) has been investigated in situ real-time using laboratory parallel-beam X-ray powder diffraction data. Thermal expansion has been analysed from 303 to 569 K with temperature steps of 4 K. Lattice parameters and volume were fitted with a second-order polynomial to calculate thermal expansion coefficients. Thermal expansion of γ-anhydrite is anisotropic being larger along the c axis. Within the 343–383 K thermal range, γ-anhydrite has been found to partially re-hydrate to bassanite CaSO₄·0.5H₂O. At 455 K the transformation γ-CaSO₄ → β-CaSO₄, insoluble anhydrite, starts reaching completion at 653 K.     

Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4-H2O and the generation of stress

Mechanical disintegration by crystal growth of salts in pores is generally considered as an important mechanism of rock breakdown both on Earth and on Mars. Crystal growth is also a major cause of damage in porous building materials. Sodium sulfate is the most widely used salt in accelerated weathering tests of natural rocks and building materials. This paper provides an updated phase diagram of the Na₂SO₄-H₂O system based on a careful review of the available thermodynamic data of aqueous sodium sulfate and the crystalline phases. The phase diagram includes both the stable phases thenardite, Na₂SO₄(V), and mirabilite, Na₂SO₄·10H₂O, and, the metastable phases Na₂SO₄(III) and Na₂SO₄·7H₂O. The phase diagram is used to discuss the crystallization pathways and the crystallization pressures generated by these solids in common laboratory weathering experiments and under field conditions. New crystallization experiments carried out at different temperatures are presented. A dilatometric technique is used to study the mechanical response of sandstone samples in typical wetting-drying experiments as in the standard salt crystallization test. Additional experiments with continuous immersion and evaporation were carried out with the same type of sandstone. Both, the theoretical treatment and the results of the crystallization experiments confirm that the crystallization of mirabilite from highly supersaturated solutions is the most important cause of damage of sodium sulfate in porous materials.     

Chemical and isotopic effects of retrogression in metamorphic fluid inclusions

Fluid inclusions may provide compositional and isotopic information about fluids from which the host mineral precipitated as long as the host mineral does not react with the fluid. Our transmission electron microscope (TEM) investigation of grain boundaries and of fluid inclusions in zoisite and quartz of high-pressure metamorphic rocks from Dabie Shan (eastern China) demonstrates daughter minerals, such as margarite, muscovite, calcite, and anhydrite. Their precipitation changes (1) the composition of the fluid by selective and mineral-specific removal of CO₂ (carbonates), H₂O (sheet silicates, hydration of the walls), or S (gypsum, anhydrite, sulfides), (2) the concentrations and proportions of ions dissolved in the fluid, and (3) the isotopic composition of the fluid because of isotopic fractionation between mineral precipitate and fluid and between unmixed fluids. Fluid leakage from overpressurized fluid inclusions with daughter minerals changes the overall chemical and isotopic composition of the inclusion irreversibly, even when the daughter crystals later redissolve. Such fluid loss yields a wide range of compositionally and isotopically different fluids from a single starting fluid. Depending on the relation between mineral reactions in and fluid loss from the inclusion, the fluid remaining in the inclusion and the fluid lost from the inclusion may appear entirely unrelated.     

Spatial relationships of salt distribution and related physical changes of underlying rocks on naturally weathered sandstone exposures (Bohemian Switzerland National Park,Czech Republic)

Efflorescence, case hardening, and granular disintegration represent common weathering features of Upper Cretaceous quartz sandstones exposed in the Bohemian Switzerland National Park (NW Bohemia, Czech Republic). Salt species (sulphates: gypsum (CaSO₄·2H₂O), potassium alum (KAl(SO₄)₂·12H₂O), tschermigite (NH₄Al(SO₄)₂·12H₂O), alunite (K(Al₃(SO₄)₂(OH)₆), and alunogen (Al₂(SO₄)₃·17H₂O), minor nitrates: nitrammite (NH₄NO₃)) determined by X-ray diffraction exhibit vertical and geographic zoning. More soluble salts (chlorides, nitrates, tschermigite) crystallize preferentially on the cliffs exposed to the south, whereas the north face is characterized by the presence of less soluble phases: gypsum and K(Al₃(SO₄)₂(OH)₆. Vertical zoning of salt distribution on natural outcrops differs from the salt distribution in masonry. Salt distribution near the base of the cliff (profile to about 2–2.5 m above the ground) is affected by capillary rise from the ground level (first maximum of water-soluble salts at the level of 1–1.5 m above the ground) and by percolation of precipitation through the overhanging rock sequence (second maximum of 2–2.5 m above the ground). Percolation of salt solution from higher parts is affected by the asperity of the rock surface. The concentration of salts (determined by ion exchange chromatography) correlates to the changes of physical properties: bulk porosity, microporosity and water absorption. The porosity, microporosity, moisture content and absorption generally increase with the increasing volume of sulphates and nitrates.     

Hydrogeological investigations of thermal waters in the Sfax Basin (Tunisia)

The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23–36°C, and electrical conductivities of 3,200 and 14,980 μS/cm. Most of the thermal waters are characterized as Na–Cl type although there are a few Na–SO₄–Cl waters. They issue from Miocene units which are made up sands and sandstones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gradient which are determined downhole temperature measurements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120°C. According to δ¹⁸O and δ²H values, all thermal and cold groundwater is of meteoric origin.     

Miscibility in the CaSO4·2H2O–CaSeO4·2H2O system: Implications for the crystallisation and dehydration behaviour

SeO₄²⁻ ions can substitute for sulphate in the gypsum structure. In this work crystals of different Ca(SO₄,SeO₄)·2H₂O solid solutions were precipitated by mixing a CaCl₂ solution with solutions containing different ratios of Na₂SO₄ and Na₂SeO₄. The compositions of the precipitates were analysed by EDS and the cell parameters were determined by X-ray powder diffraction. Moreover, a comparative study on dehydration behaviour of selenate rich and sulfate rich Ca(SO₄,SeO₄)·2H₂O solid solutions was carried out by thermogravimetry.The experimental results show that the Ca(SO₄,SeO₄)·2H₂O solid solution presents a symmetric miscibility gap for compositions ranging from X_CaSO4·2H2O = 0.23 to X_CaSO4·2H2O = 0.77. By considering a regular solution model a Guggenheim parameter a₀ = 2.238 was calculated. The solid phase activity coefficients obtained with this parameter were used to calculate a Lippmann diagram for the system Ca(SO₄,SeO₄)·2H₂O–H₂O.     

Formation and chemical evolution of magnesium chloride brines by evaporite dissolution processes—Implications for evaporite geochemistry

The evolution of magnesium chloride brines with high bromide contents via a multistage reaction and dissolution process has been studied in brine seeps of a German potash mine. The observed chemical trends and phase equilibria can be modeled and interpreted in terms of a NaCl solution (cap rock brine) infiltrating into a potash zone characterized by the metamorphic mineral assemblage kieserite + sylvite + halite + anhydrite. Establishment of a persistent, stable equilibrium assemblage and constant fluid composition in the invariant point IP1 of the six component (Na-K-Mg-Ca-Cl-SO₄-H₂O) system of oceanic salts is prevented by the perpetually renewed input of NaCl-brine and by the intermittent exposure of incompatible kieserite. Instead, the solutions develop towards the metastable invariant point IP1(gy), with the mineral assemblage carnallite + polyhalite + sylvite + halite + gypsum, where gypsum takes the place of anhydrite (stage I). The temporary exposure of kieserite and the ensuing formation of polyhalite effectively buffer the solutions along the metastable polyhalite phase boundary during stages II and III. Eventually, in stage IV, polyhalite becomes depleted and admixture of more NaCl brine leads to low sulfate solution compositions, which are now only constrained by carnallite + sylvite + halite, and the once hexary system degenerates to a quaternary one (Na-K-Mg-Cl-H₂O) in point E. Bromide in brines shows equilibrium partitioning with respect to the wall rock minerals. The pattern of evolving brine compositions may serve as a model for similar brine occurrences, which in some cases may have been misinterpreted as remains of fossil, highly concentrated and chemically modified seawater. Similar magnesium chloride brines of salt lakes (e.g., Dead Sea, Dabusun Lake) show subtle differences and are constrained by fewer mineral equilibria (more degrees of freedom), and their low sulfate contents are due to gypsum precipitation, driven by calcium chloride input from dolomitization reactions. Finally, the observed reaction sequence is generalized, and a model for the formation of magnesium sulfate depleted, chloride-type potash salts and bischofite deposits by leaching of sulfate-type evaporites is proposed.     

Phononspectroscopy and lattice dynamical calculations of anhydrite and gypsum

Detailed experimental and theoretical studies of the k=0 vibrational spectra of anhydrite and gypsum are reported. The dielectric constants and the infrared reflection and Raman spectra of the single crystal have been measured. The frequencies of the phonon spectrum, the contribution of potential terms to potential energy and detailed mode assignments have been determined based on a polarizable-ion model. The relative experimental intensity of the spectra, the observed crystal field effects, the rotatory lattice modes of the H₂O molecule and the mixed character of translatory and rotatory modes of the Ca, SO₄, and H₂O groups are discussed based on the theoretical vibrational modes and the potential energy distribution. The principal moments of inertia of the water molecule and the Lennard-Jones potential constants for nonbonded oxygen-oxygen interactions are presented.     

Gypsum treatment as a restoration method for sediments of eutrophied lakes – experiments from southern Finland

The internal load of phosphorus can be treated with chemicals improving a sediment's phosphorus binding capacity. Gypsum (CaSO₄?2H₂O) is a novel material to be applied for this purpose. In the hypereutrophic Lake Enäjärvi, southern Finland, sediments were treated with gypsum in test basins. The basins were isolated from open water to create artificial anoxia, which allowed the effects of the treatment on the internal load to be measured. The results indicate that gypsum treatment decreases the sediment's release of nutrients under anoxic conditions. This was demonstrated as less increased total-P and total-N concentrations and decreased chlorophyll-a concentrations in the water column of treated basins as compared with the control basins. Because the gypsum treatment prevents internal load, it has potential in lake restoration in cases where the internal load of phosphorus is accelerated by anoxic conditions at the sediment/water interface.     

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine deposits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine. High homogenization temperatures (T _h ) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147?C351°C) and calcite (108?C352°C) indicate high temperatures after precipitation and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism. Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO₂-H₂O fluid. Progressive heating and hydrothermal pulses, involving a CO₂-H₂O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling.     

Aquifer disposal of acid gases: modelling of water–rock reactions for trapping of acid wastes

The pore space of deep saline aquifers in the Alberta (sedimentary) Basin offers a significant volume for waste storage by “hydrodynamic trapping”. Furthermore, given the slow regional fluid flow in these deep saline aquifers, ample time exists for waste-water/rock chemical reactions to take place. A geochemical computer model (PATHARC) was used to compute the interaction of industrial waste streams comprising CO₂, H₂SO₄ and H₂S with the minerals in typical carbonate and sandstone aquifers from the Alberta Basin. The results support the idea that these acids can be neutralized by such reactions and that new mineral products are formed, such as calcite, siderite, anhydrite/gypsum and pyrrhotite, thereby trapping the CO₃, SO₄ and S ions that are formed when the acid gases dissolve in the formation water. Siliciclastic aquifers appear to be a better host for “mineral trapping” than carbonate aquifers, especially with regard to CO₂. Carbonate aquifers may be more prone to leakage due to high CO₂ pressures generated by reaction with H₂SO₄ and H₂S. Even though permeability decreases are expected due to this “mineral trapping”, they can be partially controlled so that plugging of the aquifer does not occur.     

On the origin of the livingstonite deposits at Huitzuco,Guerrero, Mexico

Livingstonite is the principal ore mineral in the deposits of the Huitzuco District in the State of Guerrero, Mexico. The ore is found in the lower part of the Morelos Formation, which consists of a thick bed of sedimentary anhydrite containing lenses of dolomite and dolomite breccia. In the unweathered ore practically all the mercury is in the livingstonite, whereas the antimony occurs partly in the livingstonite and partly in stibnite. Native sulfur forms pockets as much as 30 centimeters in diameter in the ore and is also found in gypsum on the surface away from the ore.It appears that the deposition of livingstonite, rather than of the combination of cinnabar and stibnite that is more usual in other districts, was caused by the native sulfur present in considerable quantity scattered through the sedimentary dolomite and anhydrite above, below, and in the ore. Since the formula of livingstonite is actually HgSb₄S₈ (not HgSb₄S₇ as was previously supposed), it is not stable in solutions containing only HgS, Sb₂S₃, Na₂S, and H₂O. It has been proved by one of us, experimentally, that in order to form livingstonite, the solutions must contain elemental sulfur in addition to HgS, Sb₂S₃, Na₂S, and H₂O. In such solutions the solubility of mercuric sulfide is extremely low. However, the problem of transport is overcome if the elemental sulfur is already present in the wall rock. In that case, the reaction of the elemental sulfur with a solution containing mercuric sulfide and antimony sulfide, but not saturated with either, would precipitate livingstonite, as was proved by our experimental work.