Anhydrite Crystals Research Articles

Fluid and solid inclusions in large halite veins in the Kłodawa Salt Dome (Central Poland) – insights into its development

Secondary rocks are often found in salt domes. Among these, large halite crystal (LHC) veins and nests are common, as in the Kłodawa Salt Dome of Poland. Mineralogical tests carried out on LHC here showed the presence of fluid and solid inclusions in the halite crystals. The halite itself commonly exhibits birefringence, suggesting lattice stresses. Among the solid inclusions, anhydrite is the most common. Three types of anhydrite crystals have been distinguished as regards their size and occurrence, while small, rounded inclusions of sylvite appear much less frequently. Fluid inclusions are generally rare and mostly of the secondary type. This type of fluid inclusion assemblage (FIA) comprises various kinds as regards their size and shape, as well as in their liquid-to-gas phase ratio. Primary FIAs are of two types: small, chevron-like inclusions containing sylvite daughter minerals, and large liquid-gas inclusions with carnallite and other daughter minerals. The melting temperature of sylvite ranged from 90 to 278°C, and for carnallite from 68 to 142°C. Complete homogenization of the inclusions took place at temperatures between 260 and 471.2°C. This indicates the high-temperature origin of the LHC and the presence of significant amounts of K+, Mg+2, Ca+2, and SO4-2 ions in the solutions.

Microfabrics impact on rock salt geomechanical diversity – an experimental investigation

Geomechanical studies of rock salt serve as the foundation for designing and ensuring the long-term stability of storage caverns. Research conducted in recent years has shown that rock salt exhibit highly variable mechanical parameters. Even macroscopically similar rock salt samples from the same formation have very different mechanical parameters in standard strength tests. In this paper, the effect of microfabrics on the mechanical behaviour of rock salt was studied using a uniaxial compression device with simultaneous observation under a microscope. The tests were carried out on rock salt samples, previously tested on standard uniaxial compression (UCS). Tested samples showed different strength values despite macroscopic similarity and similar anhydrite content. The tests revealed that the microfabrics, such as fluid inclusions and anhydrite crystals distributed along the halite grain boundaries, play a significant role in the strength of a rock salt sample. Four types of grain boundaries were recognized in both group of samples: (1) dominated by anhydrite crystals with a small number of fluid inclusions (FIs), (2) with FIs prevailing over anhydrite crystals, (3) only with FIs, and (4) without anhydrite crystals or FIs. The experimental uniaxial compression tests in microscale (mUCS) showed that the damage process is initiated by FIs activity at the halite grain boundaries and consists of three main stages. The samples dominated by anhydrite crystals at halite grain boundaries were characterised by the highest mUCS. Conversely, if FIs prevail at the grain boundaries the lowest mUCS values were registered. Therefore, anhydrite crystals located along halite grain boundaries influenced the mechanical behaviour and deformation process. On the contrary, anhydrite crystals dispersed within halite grains had no impact on failure and mechanical parameters. The experiment showed that apart from impurity content, the distribution of these impurities within halite grains should be examined during geomechanical evaluation of rock salt samples. Thus, the overall strength of rock salt samples determined in standard test results from the local variations of petrological features in micro- and macro scale.

The Impact of Hydrothermal Fluids on Porosity Enhancement and Hydrocarbon Migration in Qamchuqa Formation, Lower Cretaceous, Kirkuk Oil Company

The Lower Cretaceous reservoir core samples from the upper part of Qamchuqa Formation, Baba Dome, Kirkuk Oil Company, show evidence for multistage episodes of dolomitization and a complex diagenetic history. Optical microscope reveals muti-phase of diagenesis: an early stage of diagenesis and its alteration, later, by evaporated seawater under near-surface setting conditions, followed by different event of dolomitization. The stylolite microstructures postdate anhydrite and early matrix dolomite crystals (DI) and predated the coarse rhombohedral (DII) and saddle dolomite crystals (SD), which were formed under a deep burial realm. High-resolution data from stable isotopes integrated with intensive optical observation, ImageJ software, and litho-log are utilized to establish a qualified methods for mapping a better image of hydrothermal diagenesis under subsurface conditions. These methods revealed different types of dolomites, mostly focused on fractures and void spaces, and the paragenetic sequence shows the complex history of diagenetic carbonate rocks hosted in the limestone of Qamchuqa Formation. The sequence is started from older to younger as follow: Micritization, early anhydrite mineral formation, early dolomite, stylolization, rhombohedral dolomite, and saddle dolomite crystals. The early dolomite phase is usually corroded by hydrocarbon phase, and, geometrically, the hydrocarbon phase is overgrown by the early dolomite. Therefore, the dolomitizing fluids enhanced the porosity system and had positive impact on the hydrocarbon movement. This phase of dolomite and anhydrite formation were associated with the first groups of δ18OVPDB and δ13CVPDB data, a narrow range of oxygen values, and inverse Js of Lohmann curve fits towards the near-surface and shallow diagenetic settings. Detailed optical microscope and supportive data from oxygen-carbon isotopes of saddle dolomite confirm the presence of hot fluids under subsurface condition. The latter data were supported by light δ18OVPDB and constant heavy δ13CVPDB, which indicates a hot fluid possibly circulated in deep burial conditions, and this is channeled along the fracture and pore spaces, consistent with hydrocarbon migration. These pore spaces influenced by leaching were hydrocarbon migrations associated with hot fluids under deep sitting conditions. However, a remarkable part of pristine microfacies of host limestone was preserved. In summary, this study will add a new understanding and insight into the origin, genesis, and timing of these dolomites and their direct connection to hydrocarbon exploration and development in most reservoir oil rocks, which are exposed to hydrothermal fluids. Additionally, the study adds new data on hydrothermal fluids in subsurface conditions, whereas most of the previous reported work has mostly focused on exposed rock.

Late Permian-Triassic sedimentary evolution of the Southern Adriatic area based on wells and cores analysis

Rock samples from two cores within the Triassic interval of the Puglia 1 well, were studied to reveal the sedimentary facies and diagenesis. The Core 1 (5.048–5.056 m deep) is characterized by laminated dolomudstones with thinly microbial laminae alternated to lenticular/tabular shaped anhydrite crystal levels, suggesting an intertidal/sabkha-type environment. The Core 2 (6.067–6.075 m deep), shows coarse-grained crystalline massive dolomite with remnants of oolites and shell fragments, suggesting a marginal shallow-water setting. Dolomitization is ubiquitous and occurred early after the deposition through the circulation of high saline dolomitizing fluids under general reducing conditions. The early-dolomitization preserved the dolomites from major burial diagenetic transformations, permitting only the ordering and the development of xenotopic textures in the dolomite crystals (aging). Primary anhydrite crystals were lately affected at least by one burial hydration-dehydration cycle, whereas fracture-filling anhydrite is a later-stage diagenetic product of circulating sulphate-rich fluids. Moreover, the integration of Puglia 1 well data with other six exploration wells and a seismic line allowed a possible reconstruction of the Permian-Triassic sedimentary evolution of the Southern Adriatic area. During Permian continental/coastal lagoon settings developed, while in the Ladinian, after a generalized subaerial exposure and an erosional phase, the extensional tectonic inputs linked to Tethys rifting brought to a relative sea-level rise and to the formation of carbonate shelves in NW-SE oriented tectonic depressions. During Carnian-early Norian a further extensional tectonic pulse coupled with a relative sea-level drop, led to the formation of NW-SE elongated intrashelf basin with a consequent enhancement of salinity and settlement of evaporative conditions. This triggered a massive evaporite deposition during the Norian, that filled the basinal areas. Lastly, in the late Norian-Rhaetian a marked relative sea-level rise, restored the connections with open-sea and shifted the evaporite deposition up to the shallowest parts of the basins that previously remained under subaerial conditions.

Exceptional preservation in Quaternary Atacama Desert Tufas: Evidence for increased groundwater and surface water in the Calama Basin, Atacama, Chile

AbstractExceptionally well‐preserved tufas located west of Calama, Atacama Desert, Chile, designated Santa Juana tufas, record episodic wetter conditions, relative to today, over the past 500,000 years. Globally, tufa architecture and depositional details are poorly understood as most described tufas have been degraded by weathering and erosion. In the hyperarid Atacama, post‐depositional alteration is negligible, therefore, the exceptional preservation of Santa Juana tufas documented in this study provides new information about tufa facies and their complex interactions. Santa Juana facies include microbial stromatolites, phytoherms, cascadestone, flowstone and porous limestone. Phytoherms, consisting of former plant stems coated with calcite, developed in channels, within pools, and along spring discharge aprons. Cascadestone, representing former waterfalls, preserves microbial filaments and delicate V‐shaped calcite crystals. Flowstone lines shallow subvertical to subhorizontal channels, representing sites of rapidly sluicing water flow. Porous limestone, containing sparse calcite and/or gypsum and anhydrite cement crystals, represents detrital accumulations. Stable isotope results, coupled with U/Th ages, show that by the Quaternary, relative to the Neogene, groundwater was less supercharged with volcanogenic CO2 so degassing was moderated. The δ18O ratios from Miocene–Pliocene palustrine and lacustrine freshwater carbonates that underlie Santa Juana tufas indicate significant evaporation, but the tufa δ18O signal indicates a less evaporative trend due to shorter atmosphere exposure time. Biological fractionation in δ13C is largely masked by the region's volcanogenic carbon footprint, although tufa petrography shows well‐preserved microbial filaments and laminations. The range of tufa ages in this study shows that there were wetter time periods within the drainage basin headwater area in the Quaternary, but that by the late Pleistocene to early Holocene, aridity to hyperaridity became established. The lack of diagenesis or alteration within the Santa Juana tufas indicates that there has been minimal rainfall since their deposition.

Mechanism and kinetics of the pseudomorphic replacement of anhydrite by calcium phosphate phases at hydrothermal conditions

Abstract Mineral replacement reactions mediated by fluids are common in sedimentary basins, where they influence geochemical cycles. Phosphorous (P) pollution of soils, sediments and water bodies is currently a widespread problem. Some apatite accumulations in sediments may have formed through the interaction of P-bearing aqueous solutions with mineral surfaces that result in mineral replacement reactions. Here, we investigate the pseudomorphic replacement of anhydrite single crystals by aggregates of β-tricalcium phosphate and hydroxyapatite upon interaction with a P-bearing solution at temperatures between 120 and 200 °C. SEM imaging is used to study the texture of the aggregates. Rietveld refinement of the X-ray diffraction patterns and Raman spectra analysis of the reacted samples provide information on the kinetics of the replacement. At all temperatures β-tricalcium phosphate forms alongside hydroxyapatite at early stages of the replacement reaction. At T ≥ 180 °C, hydroxyapatite/β-tricalcium phosphate ratio rapidly increases, and hydroxyapatite is the only phase in fully replaced samples. At T < 180 °C hydroxyapatite/β-tricalcium phosphate ratio increases slowly and fully replaced samples still contain significant amounts of β-tricalcium phosphate. The progress of the replacement is facilitated by the formation of porosity. The evolution of the hydroxyapatite/β-tricalcium phosphate ratio and the crystal habit of both phases strongly influence the arrangement of this porosity. The empirical activation energy Ea (kJ/mol) of the replacement reaction is determined by the Avrami and the iso-conversion methods. Both approaches yield an Ea of ~40 kJ/mol. Anhydrite dissolution appears as the rate-limiting process and the overall kinetics of the replacement reaction is controlled by the rate diffusion of dissolved species through the porosity network. The ripening of the metastable β-tricalcium phosphate into hydroxyapatite affects the characteristics of the porosity network and further modulates the kinetics of the replacement. These results may improve the understanding of the mechanisms of P-sequestration by mineral surfaces through coupled dissolution–precipitation reactions and shed light on the origin of apatite accumulations associated to evaporitic sedimentary rocks.

Material characteristics of historic scagliola interiors in Vienna: A view through the microscope.

Composition and microstructure of four Viennese scagliola samples, originating from the early 18th to early 20th centuries, were analysed by different methods of microscopy. Results indicate a similar composition in all samples; only minor differences could be observed in the porosity and grain-size-distribution. While, the calcium sulphate raw material was fired at low temperatures (<200°C), the presumable presence of anhydrite II in two samples may indicate hot spots (200-300°C) during the calcination. To achieve 'marble-like' patterns mineral pigments were used to dye the ground mass. The first results show that scagliola surfaces of this study were produced by using the same or very similar technology between the 18th and early 20th centuries. The present study focuses on scagliola interiors in Vienna, Austria from different stylistic periods between the early 18th and early 20th centuries. Scagliola, also called stucco marble, imitates natural stone. It is produced by a mixture of gypsum (CaSO4 ·2H2 O), different pigments, animal glue and water. In the history of interior design, the material played an important role in the 18th and 19th centuries in Central Europe. The aim of the research was a detailed investigation of four selected samples in order to identify the raw materials and manufacturing technology of the stucco marbles used over time in Vienna. As a first step thin sections (i.e. polished, transparent samples with a thickness of 0.03 mm) were prepared from the samples and analysed by different microscopic techniques. All samples showed similarities in their bulk properties, but detailed investigations revealed also some differences in their mineralogical composition. The gypsum binders contained typical air voids and so-called secondary pores which were formed by dissolving larger binder-related particles during the preparation of the mixture. The macroporosity (i.e. the amount of the pores in the samples which are larger than 0.01 mm) measured on microscopic images varied in a narrow range between 14.1% and 19.3%. The raw materials of all samples were fired at relatively low temperatures (i.e. below 200°C), but in two samples we could also determine a few anhydrite (CaSO4 ) crystals that normally form at higher temperatures in the kiln. This indicates that the distribution of temperature in the kiln was heterogeneous during the firing process. The amount and appearance of some minor mineral constituents such as dolomite (CaMg[CO3 ]2 ) and celestine (SrSO4 ) suggest that they were naturally occurred in the raw material and not deliberately added to the binder as a filling material. To achieve different colour hues and imitate natural stone surfaces, fine-grained mineral pigments and charcoal powder were used. Although we have no distinct information about the gypsum sources of these stucco marbles, several small historical gypsum quarries existed southwest of Vienna. Some of them were already in operation in the 16th century, thus a local gypsum occurrence used as a raw material is very probable. Due to the fact that the main features of the analysed samples were very similar, we assume that the scagliola surfaces of this study were produced by using the same or very similar technology between the 18th and early 20th centuries.

Magnetic Fabrics and Petrography of Rocksalts Reveal Preferred Orientation of Anhydrites within a Halite Matrix

We investigate the magnetic fabrics and microstructures of diamagnetic rocksalt samples from the Sedom salt wall (diapir), Dead Sea Basin, as possible strain markers. A comprehensive study of anisotropy of magnetic susceptibility (AMS), combined with magnetic, microtextural, geochemical and mineralogical analyses allows us to depict the deformation mechanisms and to reveal the mineral sources of the AMS. The rocksalts are composed of halite as the major mineral phase (&gt;80%) and anhydrite as a minor phase (5–20%), and have an average magnetic susceptibility value of −13.4 ± 0.7 × 10−6 SI. Ferromagnetic and paramagnetic minerals make a negligible contribution to the bulk magnetic properties of the samples. The AMS indicates and reveals significant anisotropy with the maximum susceptibility axis (K1) subparallel to the bedding strike, although the cubic halite crystals are isotropic. Polarizing microscope and SEM images show preferred alignment of needle-like anhydrite crystals parallel to the direction of the K1 axis. Petrographic investigation of gamma irradiated thin sections reveals the deformation recorded in the microstructures of the rocksalts and points to a dominant contribution by dislocation creep, although both dislocation creep and pressure solution were active deformation mechanisms. We infer that during dislocation creep, the thin bands of anhydrite crystals deform along with the surrounding halite grains. We suggest that although the shape preferred orientation of halite grains is not indicative of finite strain because of resetting by grain boundary migration, the preferred orientation of the anhydrite crystals may be. These results suggest that the AMS of the rocksalts provides a textural proxy that reflects deformation processes of the rocksalts, despite their very low magnetic susceptibility.

Original and alteration mineral phases in the NWA 10628 Martian shergottite determined by micro‐Raman spectroscopy assisted with micro‐energy dispersive X‐ray fluorescence imaging

AbstractNorthWest Africa 10628 (NWA 10628) is a shergottite found in Tindouf (Algeria) in 2014. Little is known on this meteorite apart from the Meteoritical Bulletin report. In this work, original and altered mineral phases of NWA 10628 were characterised by micro‐Raman spectroscopy assisted with micro‐energy dispersive X‐ray fluorescence imaging. The matrix of the meteorite is dominated by high‐Ca pyroxenes, augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) and diopside (CaMgSi2O6). Olivine and feldspars appeared as small crystals all along the matrix. Merrillite (Ca9NaMg(PO4)7) was found as big crystals in inclusions. As whitlockite (Ca9NaMg(PO4)6(HPO4)) can be transformed into merrillite at high temperature and pressure, we suggest that merrillite in the meteorite was originally whitlockite, before its ejection from Mars. Gypsum (CaSO4·2H2O) was found as small crystals and in fractures; this last one could come from terrestrial weathering because it was associated with calcite (CaCO3). Small anhydrite (CaSO4) crystals are shown in the bulk together with gypsum, probably from the partial dehydration of original Martian gypsum by the high‐pressure/temperature of ejection. Pyrite and marcasite (FeS2 polymorphs) minerals were detected, as well as their oxidation products, goethite (α‐FeO (OH)) and rozenite (FeSO4·4H2O), due to their terrestrial weathering. The identification of hematite (Fe2O3), magnetite (Fe3O4), ilmenite (FeTiO3) and anatase (TiO2) suggested that ulvospinel (Fe2TiO4) was the precursor on Mars, because ulvospinel is transformed into ilmenite by pressure and later ilmenite can be altered to magnetite/hematite and anatase by oxidative weathering on Earth. This non‐destructive analytical methodology provides deep mineralogical characterisation adequate for scarce samples like those returning from the Mars Sample Return mission.

Petrographic Record and Conditions of Expansive Hydration of Anhydrite in the Recent Weathering Zone at the Abandoned Dingwall Gypsum Quarry, Nova Scotia, Canada

In the Dingwall gypsum quarry in Nova Scotia, Canada, operating in 1933–1955, the bedrock anhydrite deposits of the Carboniferous Windsor Group have been uncovered from beneath the secondary gypsum beds of the extracted raw material. The anhydrite has been subjected to weathering undergoing hydration (gypsification), transforming into secondary gypsum due to contact with water of meteoric derivation. The ongoing gypsification is associated with a volume increase and deformation of the quarry bottom. The surface layer of the rocks is locally split from the substrate and raised, forming spectacular hydration relief. It shows numerous domes, ridges and tepee structures with empty internal chambers, some of which represent unique hydration caves (swelling caves, Quellungshöhlen). The petrographic structure of the weathering zone has been revealed by macro- and microscopic observations. It was recognized that gypsification commonly starts from a developing network of tiny fractures penetrating massive anhydrite. The gypsification advances from the fractures towards the interior of the anhydrite rocks, which are subdivided into blocks or nodules similar to corestones. Characteristic zones can be recognized at the contact of the anhydrite and the secondary gypsum: (1) massive and/or microporous anhydrite, (2) anhydrite penetrated by tiny gypsum veinlets separating the disturbed crystals and their fragments (commonly along cleavage planes), (3) gypsum with scattered anhydrite relics, and (4) secondary gypsum. The secondary gypsum crystals grow both by replacement and displacement, and also as cement. Displacive growth, evidenced by abundant deformation of the fragmented anhydrite crystals, is the direct cause of the volume increase. Crystallization pressure exerted by gypsum growth is thought to be the main factor generating volume increase and, consequently, also the formation of new fractures allowing water access to “fresh” massive anhydrite and thus accelerating its further hydration. The expansive hydration is taking place within temperature range from 0 to ~30 °C in which the solubility of gypsum is lower than that of anhydrite. In such conditions, dissolving anhydrite yields a solution supersaturated with gypsum and the dissolution of anhydrite is simultaneous with in situ replacive gypsum crystallization. Accompanying displacive growth leads to volume increase in the poorly confined environment of the weathering zone that is susceptible to upward expansion.

High Sulfur in Primitive Arc Magmas, Its Origin and Implications

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

DIVARENT EQUILIBRIUM IN MULTI-COMPONENT SYSTEMS

The structure of the phase complex of the six-component reciprocal system Na, K, Mg, Ca || SO4, Cl-H2O at 500C in the region of crystallization of anhydrite (CaSO4) was studied by the translation method, and variants of the formation of divariant fields during the transition of the system from the five-component to the six-component state were shown

Simple superphosphate by two-stage acid treatment of phosphate raw materials

The distinguishing feature of the proposed flow method before the classical (chamber) method is that the entire production cycle of natural phosphate processing is carried out in two stages. At the first stage, the phosphorite is treated with a stoichiometric flow rate of concentrated sulphuric acid (at least 93%), under conditions of complete decomposition of phosphorite to form phosphoric acid and crystals of anhydrite (calcium sulfate). The reaction temperature is 122 °C. In the second stage, the resulting concentrated solution of phosphoric acid in a mixture with sulphur is involved in a reaction with an additional input of phosphorite, which is the basis for the mechanism of chemical formation of monocalciumphosphate and granulation of superphosphate mass. The processes for neutralizing phosphoric acid on monocalciumphosphate and for granulating the product by coagulation are combined in one apparatus. The drying stage of the product is excluded from the scheme.

Seeds of imperfection rule the mesocrystalline disorder in natural anhydrite single crystals

In recent years, we have come to appreciate the astounding intricacies associated with the formation of minerals from ions in aqueous solutions. In this context, a number of studies have revealed that the nucleation of calcium sulfate systems occurs nonclassically, involving the aggregation and reorganization of nanosized prenucleation species. In recent work, we have shown that this particle-mediated nucleation pathway is actually imprinted in the resultant micrometer-sized CaSO4 crystals. This property of CaSO4 minerals provides us with the unique opportunity to search for evidence of nonclassical nucleation pathways in geological environments. In particular, we focused on large anhydrite crystals extracted from the Naica Mine in Mexico. We were able to shed light on this mineral's growth history by mapping defects at different length scales. Based on this, we argue that the nanoscale misalignment of the structural subunits, observed in the initial calcium sulfate crystal seeds, propagates through different length scales both in morphological, as well as in strictly crystallographic aspects, eventually causing the formation of large mesostructured single crystals of anhydrite. Hence, the nonclassical nucleation mechanism introduces a "seed of imperfection," which leads to a macroscopic "single" crystal whose fragments do not fit together at different length scales in a self-similar manner. Consequently, anisotropic voids of various sizes are formed with very well-defined walls/edges. However, at the same time, the material retains in part its single crystal nature.

How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

Abstract Most of the world's Zn and Pb is extracted from sediment-hosted Zn-Pb deposits. The Zn-Pb deposits hosted in carbonate rocks are hypothesized to form by mixing of acidic metal-bearing brines with reduced sulfur-bearing fluids while dissolving sedimentary carbonate. To test the role of carbonate in this process, we conducted hydrothermal experiments simulating ore formation by reacting Zn ± Pb ± Ba–bearing brines with H2S and SO42– produced by native sulfur, with and without carbonate minerals (calcite or dolomite crystals), at 200 °C and water-saturated pressure. Sphalerite, galena, and barite (or anhydrite) crystals formed only when carbonate was present in the experiment, accompanied by carbonate dissolution. The textures of sphalerite clusters are similar to those observed in ancient and modern hydrothermal deposits. Thermodynamic modeling at 150 °C and 250 °C demonstrates that mixing of metal-rich brines and H2S causes most of the Zn in solution to precipitate as sphalerite only when carbonate dissolution occurs to buffer the pH, consistent with the experimental observations. The need for a pH buffer increases with increasing temperature, and different pH buffers may play a role for different deposit types. We propose that carbonate-buffered fluid mixing is a critical process for forming post-sedimentary Zn ± Pb ± Ba deposits in sedimentary carbonate rocks.

The stability and composition of sulfate melts in arc magmas

The stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures, allowing an assessment of their possible role in transporting sulfur into the sub-volcanic arc and porphyry copper deposit systems. Experimental starting materials were based on natural trachyandesite and trachydacite compositions, with 3.5–7 wt% H2O and 3.5–5.5 wt% sulfur added to produce large, analyzable amounts of sulfate phases. Conditions ranged over 800–1200 °C, 0.2-1GPa and ƒO2 > NNO + 2.5. Sulfate melts formed at temperatures above 1000 °C at 0.75 and 1 GPa and above 900 °C at 0.2 GPa, suggesting some pressure dependence on their stability. At temperatures below 1100 °C sulfate melts and anhydrite crystals commonly coexist. Sulfate melts quenched to an intergrowth that was difficult to prepare for analysis. However, the composition was approximated by EPMA and further constrained by mass balance calculations. Sulfate melts were dominated by CaO and SO3, but also contained, in order of decreasing abundance, Na2O, K2O, MgO, FeO, Cl and P2O5. Chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt, and calculated partition coefficients for sulfate/silicate melts were 5–13 at 1200 °C, 0.75–1 GPa. Experimental data show that, in the absence of an exsolved, hydrous fluid phase, sulfate melts can form in natural arc magmas at near-liquidus temperatures ≥ 1000 °C, assuming that magmas are oxidized and contain sufficient sulfur (> 2000–3000 ppm S). These results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions. Both the non-quenchable and water-soluble nature of sulfate melts (and anhydrite) make them difficult to identify, unless trapped as mineral inclusions similar to the “wormy anhydrite” trapped in high-temperature amphiboles from Yanacocha, Peru.

Research on Mechanical Properties of High-Pressure Anhydrite Based on First Principles

This article focuses on the elucidation of a three-dimensional model of the structure of anhydrite crystal (CaSO4). The structure parameters of anhydrite crystal were obtained by means of first principles after structure optimization at 0~120 MPa. In comparison with previous experimental and theoretical calculation values, the results we obtained are strikingly similar to the previous data. The elastic constants and physical parameters of anhydrite crystal were also studied by the first-principles method. Based on this, we further studied the Young’s modulus and Poisson’s ratio of anhydrite crystal, the anisotropy factor, the speed of sound, the minimum thermal conductivity and the hardness of the material. It was shown that the bulk modulus and Poisson’s ratio of anhydrite crystal rose slowly with increasing pressure. The anisotropy characteristics of the Young’s modulus and shear modulus of anhydrite crystal were consistent under various pressure levels, while the difference in the anisotropy characteristics of the bulk modulus appeared. The acoustic velocities of anhydrite crystal tended to be stable with increasing pressure. The minimum thermal conductivity remained relatively unchanged with increasing pressure. However, the material hardness declined gradually with increasing pressure.

The enigmatic ascent of Ca-sulphate rocks from a deep dense source layer: evidences of hydration diapirism in the Lesina Marina area (Apulia, southern Italy)

In the Lesina Marina village area, the cropping out gypsum rocks, that rose up from the deep and thick Upper Trias Burano Fm anhydrite layer, have been analyzed to investigate the trigger mechanism for their ascent. In this regard, we focus on the anhydrite-to-gypsum transformation starting from the deep source layer. Indeed, petrographic observations of the widespread corroded anhydrite crystals embedded in these gypsum rocks revealed unequivocal evidences of the anhydrite-to-gypsum transformation. In addition, and referring to gypsum, microstructural features indicate that ductile deformation mechanisms initially operated under higher temperature conditions. This temperature should be close to 107 °C, namely the value of the upper temperature limit of stability of gypsum from Lesina Marina, above which it starts to transform and dehydrate, as revealed by microthermometry heating/dehydration experiments combined with micro-Raman analyses. All these evidences are in favor of the dramatic density decrease and volume increase due to anhydrite-to-gypsum transformation by hydration in the deep source layer; these variations of physical conditions, triggered by hydration, promoted diapirism of the gypsum mass, in other words “hydration diapirism”. As revealed by seismic lines interpretation, the diapirism, which gave rise to the Lesina diapir, occurred during Plio-Pleistocene and was genetically related to the Lesina graben-type structure. Hydration at depth was favored by the downward circulation of water-rich fluids channeled in faults, and the gypsum mass used the weakened zone of the southern fault to pierce the overlying Mesozoic and Tertiary sediments.

ВЛИЯНИЕ ДОБАВКИ ГИПСА НА КАЧЕСТВО КОМПОЗИЦИОННОГО ВЯЖУЩЕГО ДЛЯ ИЗДЕЛИЙ ЯЧЕИСТОЙ СТРУКТУРЫ

The effect of gypsum additives on the process of lime slaking and the quality of the composite binder is discussed. It is established that a different water-solid ratio, the water temperature and the use of an additive of natural gypsum can regulate the rate of the lime slaking process. The effect of this additive on the kinetics of the process, the temperature factor, the dispersion of the product and the quality of the products obtained by slaking are considered. During the hydration of highly active lime, the slaking temperature reaches high values, at which the process of gypsum dehydration begins. The combination of the exothermic process of lime slaking and the formation of a medium of saturated water vapor with the addition of natural gypsum to lime forms the semi-aquatic gypsum СаSО4∙0,5Н2О. It does not dehydrate, does not become insoluble and retains its chemical properties. There is no exothermic effect of crystallization of anhydrite. Strength results of composite binder cellular structure demonstrate that lime-based binder samples with the addition of natural gypsum under autoclave conditions are more solid than binder-free samples.

Phase composition and burning history of high-fired medieval gypsum mortars studied by Raman microspectroscopy

The use of high-fired gypsum as binder for masonry and joint mortars or stuccowork in Central Europe in the Early and High Middle Ages was a regional specific as it depended on local gypsum deposits. The calcination technology possible at the time resulted in an assemblage of calcium sulphate phases dehydrated to different degrees and partly thermally damaged accessory minerals of the raw gypsum. Not hydrated clusters of firing products preserved in the binder matrix are a typical feature of such mortars. A novel Raman microspectroscopic approach, providing access to the burning history of individual anhydrite grains, was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for tracing and visualising pyrometamorphic reactions in natural impurities of the kiln run. In the discussed examples mineral thermometry indicates process temperatures above 800°C: the breakdown of magnesium-rich chlorite led to the formation of forsterite Mg2SiO4, while the thermal decomposition of dolomite CaMg(CO3)2 to periclase MgO and lime CaO yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Hydration of periclase in the mixed gypsum paste containing sulphate ions also resulted in magnesium sulphate hydrates, here identified in the form of hexahydrite MgSO4·6H2O. Lower burning temperatures left the accessory minerals in their pristine form, but can be traced by measuring the spectra of individual anhydrite crystals in grains of firing products and evaluating Raman band widths. Throughout the present study, calcination temperatures ranging from approx. 600°C to 900°C were determined.