Calcium Sulfate Minerals Research Articles

Anhydrite (Calcium Sulfate) Mineral as a Novel Weighting Material in Drilling Fluids

AbstractDifferent additives such as barite, calcium carbonate, hematite, and ilmenite having high-density and fine solid materials are used to increase the density of drilling fluids. However, some of the weighting additives can cause some serious drilling problems such as barite (particle settling, formation damage, erosion, and insoluble filter cake). In this study and for the first time, anhydrite (calcium sulfate) is used as a weighting additive in the drilling fluids. Several laboratory experiments such as density, rheology, fluid loss, resistivity, and pH were carried out to assess the performance of calcium sulfate as a weighting additive in the drilling fluids. The performance of calcium sulfate as a weighting additive was compared with the commonly used weight enhancing additive calcium carbonate. The results showed that calcium sulfate has higher solubility than calcium carbonate. The fluid loss test showed that both additives lost the same volume of fluid and created the same thickness of filter cake; however, the solubility of calcium sulfate-based filter cake with organic and inorganic acids was higher compared with other weighting materials. Calcium sulfate-based filter cake was completely dissolved using a new formulation that consists of glutamic-diacetic acid (GLDA) chelating agent and potassium carbonate as a convertor. The removal efficiency after 10 h reached 100% in 20 wt% GLDA and 10 wt% potassium carbonate solution at 100 °C.

Investigating the microorganisms-calcium sulfate interaction in reverse osmosis systems using SEM-EDX technique

Abstract Membrane fouling affects the performance of membrane technologies such as reverse osmosis (RO), in water treatments and desalination industries. The membrane fouling is a multifaceted phenomenon and various components of feedwater like organic and inorganic substances, and microorganisms interact with each other to further complicate and intensify the membrane fouling. In this research, the interaction between microorganisms and calcium sulfate (mineral chosen as a model scaling) was investigated. Using scanning electron microscopy – energy dispersive x-ray (SEM-EDX) technique, it was found that the variety of microorganisms present in seawater were able to form calcium sulfate crystals on the RO membrane, although no precipitation occurred under abiotic conditions (i.e. in control samples). The results of SEM-EDX were further confirmed by quantitative analysis of precipitation through ion chromatography (IC) technique, and it was found that the concentration of calcium ions remaining in the bacteria-containing medium was significantly lower than that of controls at 95 % confidence level. Furthermore, the kinetics of biomineralization and the effect of different concentrations of calcium and sulfate ions was investigated. It was found that the bacteria were able to form needle-like crystals even at the lowest concentration (Ca2+=20 mM) and at the lowest incubation times (i.e. 1 day) studied. On the other hand, the effect of calcium ions on bacterial growth was also investigated and it was noted that the presence of ions resulted in a statistically significant increase in growth rates of bacteria, showing their potential to enhance biofouling rates.

Particle-Mediated Nucleation Pathways Are Imprinted in the Internal Structure of Calcium Sulfate Single Crystals

Calcium sulfate minerals are found under the form of three crystalline phases: gypsum (CaSO4.2H2O), bassanite (CaSO4.0.5H2O), and anhydrite (CaSO4). Due to its relevance in natural and industrial processes, the formation pathways of these calcium sulphate phases from aqueous solution have been the subject of intensive research. There is a growing body of literature, that calcium sulfate forms essentially through a non-classical nanoparticle- or cluster-mediated crystallisation process. At the early stages of precipitation calcium sulfate crystals grow through the reorganization and coalescence of aggregates rather than through classical unit addition. Here, we show by using low-dose dark field (DF) transmission electron microscopy (TEM) and electron diffraction, that these re-structuring processes by no means continue until a near-perfectly homogeneous single crystal is obtained. Instead the growth process yields a final imperfect mesocrystal of the overall morphology resembling that of a single crystal, yet composed of smaller nano-domains. Hence, our data indicate that organic-free mesocrystal grown by a particle mediated-pathway may preserve a 'memory or 'imprint' of the non-classical growth process in the final crystal structure, something that has been largely overlooked until now. Furthermore, the nano-scale misalignment of the structural sub-units within crystals might propagate through the length-scales, and potentially is expressed macroscopically as misaligned zones/domains in large single crystals, e.g. as those observed in some of the giant crystals from the Naica Mine, Chihuahua, Mexico.

Tufa precipitation from Recycled Concrete Aggregate (RCA) over geotextile: Mechanism, composition, and affecting parameters

One of the limiting factors of using RCA as a construction material has previously been related to the generated tufa despite its acceptable mechanical properties. Studies conducted in the mid-90s seem to overestimate the potential of such formations; whereas, more recent studies tend to underestimate such depositions from RCA. However, a comprehensive study to understand the chemical mechanisms and resulting chemical composition of pure RCA tufa had not been carried out. This paper discusses the results of an analytical study that combines both the findings from experimental and theoretical geochemistry to demonstrate the conditions that result in tufa precipitation, the associated mineral compositions, and what these differences in mineral compositions may mean for the construction.The precipitation experiments were conducted using Plummer et al. reaction vessel. Based on the observations from these experiments, the findings showed that the chemical reactions and the formation of tufa depend on different precipitation mechanisms. The theoretical evaluations showed that Garrels-Mackenzie (progressive evaporation) model can be successfully used to interpret observations from the experiments. The results of this analytical study confirm that in most construction conditions calcium sulfate minerals are more likely to precipitate from RCA than calcium carbonate minerals. This finding would mean that the precipitated deposition from RCA may be seasonal due to the higher solubility constant of calcium sulfate. For practical implications in construction, a one-point leach test and geochemical analysis method is suggested to estimate the characteristics of potential tufa formations from different RCA sources.

Impact of Temperature on Scale Formation in Chalk Reservoirs

Summary Operators are collecting abundant produced-water data that are often underused. Produced-water-composition data provide clues related to the geochemical reactions that are occurring in the subsurface. This information can be useful for monitoring interwell connectivity and predicting and managing oilfield scale resulting from brine supersaturation. Coupling thermodynamic calculations with produced-water analysis helps to identify geochemical effects that could affect oil recovery. This work addresses the difference that reservoir temperature has on geochemical reactions in carbonate reservoirs by comparing data from two offshore fields and identifying the rock/brine and brine/brine reactions that will affect scale management. Two seawater-flooded chalk fields located near each other were selected as candidates for comparison. The temperature of one field is 130°C, whereas for the other field, it is 90°C. Produced-water samples (a total of 6,800) from these two fields were analyzed, and the compositional trends were plotted to identify the deviation from conservative (nonreacting) behavior. The compositional trends were then grouped to identify if there were common features between wells. This analysis was complemented by 1D reactive-transport modeling to identify the reactions that would be consistent with the observed trends. Two groups of wells were identified within each reservoir on the basis of the produced-brine compositional behavior. Each well group exhibits a distinct ion-trend behavior, especially with respect to barium, calcium, strontium, and magnesium concentrations—because these are divalent cations that are abundant in the formation brines. The breakthrough of sulfate, a component primarily introduced during seawater flooding, varies very significantly between the two groups in each case. In one grouping, the sulfate is barely retarded, and it breaks through at seawater fractions lower than 10%. In the other grouping, however, sulfate does not break through until the seawater fraction in the produced brine exceeds 75%. This retardation of sulfate occurs most strongly in the hotter reservoir, and this might be attributed to the lower solubility of the calcium sulfate mineral anhydrite at a higher temperature. The retardation of sulfate then means that barium is produced at higher concentrations because barite precipitation in the reservoir is thus restricted, caused by sulfate being the limiting ion. However, some sulfate stripping does occur in the cooler reservoir, despite the higher solubility of anhydrite. Furthermore, in all cases, magnesium is retarded, with some groupings exhibiting the complete stripping of magnesium from the injected seawater. The magnesium-stripping behavior is reproduced in the reactive-transport models when calcium- and magnesium-replacement reactions are allowed. This phenomenon has been observed elsewhere in coreflood experiments, and it also contributes to the sulfate stripping through the promotion of anhydrite precipitation within the rock. This process, which is beneficial in terms of reducing the scale risk, is more pronounced at higher temperatures. Therefore, higher-temperature chalk reservoirs might act as natural sulfate-reduction plants, reducing scaling, souring risks and, thus, operating costs of the fields.

GEOBIOLOGICAL AND DIAGENETIC INSIGHTS FROM MALVINOKAFFRIC DEVONIAN BIOTA (CHAPADA GROUP, PARANÁ BASIN, BRAZIL): PALEOBIOLOGICAL AND PALEOENVIRONMENTAL IMPLICATIONS

This study tests the presence of differential preservation in the Devonian Malvinokaffric fauna from the Chapada Group (Parana Basin, Brazil). Results of EDXRF, EDS, Raman Spectroscopy, and petrographic analyses show differential preservation of shells that were originally calcite as hematite and goethite fossils, while organisms with original calcium phosphate shells tend to be preserved inside phosphatic concretions. Both preservation types are commonly associated with pseudoframboids, while calcium sulfate minerals are commonly associated with hematized fossils. From this evidence, a diagenetic model for these fossils is proposed. The model includes an early diagenetic phase (characterized by anaerobic sulfate reduction and precipitation of pyrite and carbonate-fluorapatite) and a second, near-surface chemical weathering phase (characterized by the oxidation of pyrite and precipitation of iron oxyhydroxides and calcium sulfates). Acidic conditions in both phases may account for the dissolution of less stable minerals compared to calcium phosphate. It is considered that this model may assist in understanding other similarly preserved biotas, as well as enhancing understanding of the taphonomic overprint that may occur within this important and endemic Devonian biota.

Optimization and Quality Control of Automated Quantitative Mineralogy Analysis for Acid Rock Drainage Prediction

Low ore-grade waste samples from the Codelco Andina mine that were analyzed in an environmental and mineralogical test program for acid rock drainage prediction, revealed inconsistencies between the quantitative mineralogical data (QEMSCAN®) and the results of geochemical characterizations by atomic absorption spectroscopy (AAS), LECO® furnace, and sequential extractions). For the QEMSCAN® results, biases were observed in the proportions of pyrite and calcium sulfate minerals detected. An analysis of the results indicated that the problems observed were likely associated with polished section preparation. Therefore, six different sample preparation protocols were tested and evaluated using three samples from the previous study. One of the methods, which involved particle size reduction and transverse section preparation, was identified as having the greatest potential for correcting the errors observed in the mineralogical analyses. Further, the biases in the quantities of calcium sulfate minerals detected were reduced through the use of ethylene glycol as a polishing lubricant. It is recommended that the sample preparation methodology described in this study be used in order to accurately quantify percentages of pyrite and calcium sulfate minerals in environmental mineralogical studies which use automated mineralogical analysis.

Calcium Sulfate Scaling Risk and Inhibition for a Steamflood Project

Summary Steamflooding is a widely used technique for heavy-oil recovery. Scale control during steamflooding, however, can be challenging because the high temperature of the steamflood can decompose thermally unstable inhibitors and/or lead to the precipitation of metal-inhibitor pseudoscale. In this paper, we present the analysis of the scaling risk and scale inhibition for a pilot steamflood project in a Middle Eastern oil field. The formation of this field is a dolomite formation interbedded with anhydrite (CaSO4) streaks. Anhydrite has been observed to be the predominant scale form. Anhydrite scale was presumably formed by the increased production-system temperature resulting from steamflooding and/or the mixing of steam condensate with connate water at equilibrium with calcium sulfate minerals at lower temperature and higher solubility. Anhydrite is inherently difficult to control because of its high solubility and the high-temperature (HT) conditions under which it forms. Compared with barite and calcite, only limited knowledge has been acquired for anhydrite control. To predict the scaling tendency and inhibitor need in different wells of this field with different supersaturation levels and temperatures, a scaling-risk model has been developed. To build such a model, detailed and revised laboratory procedures have been developed to study nucleation and precipitation kinetics of anhydrite at 125–175°C, different supersaturation, different water composition, and long reaction time. Predictions of this scaling-risk model suggest a saturation index (SI) of 0.8 as a critical SI for anhydrite control at >125°C. For example, when the SI is above 0.8, anhydrite will be difficult to control in the presence of threshold inhibitor. Model predictions were benchmarked with the water-chemistry data from a total of more than 20 wells from this field, and were found to be consistent with field observations of scale occurrence in different wells. With the recommended inhibitor concentrations, anhydrite scale has been controlled in this field, which provides validation that the proposed scaling-risk model is a powerful tool to optimize the scale-treatment plan for anhydrite.

Modeling calcium sulfate chemistries with applications to Mars

On Mars, evidence indicates widespread calcium sulfate minerals. Gypsum (CaSO4⋅2H2O) seems to be the dominant calcium sulfate mineral in the north polar region of Mars. On the other hand, anhydrite (CaSO4) and bassanite (CaSO4⋅0.5H2O) appear to be more common in large sedimentary deposits in the lower latitudes. The tropics are generally warmer and drier, and at least locally show evidence of acidic environments in the past. FREZCHEM is a thermodynamic modeling tool used for assessment of equilibrium involving high salinity solutions and salts, designed especially for low temperatures below 298K (with one version adapted for temperatures up to 373K), and we have used it to investigate many Earth, Mars, and other planetary science problems. Gypsum and anhydrite were included in earlier versions of FREZCHEM and our model Mars applications, but bassanite (the CaSO4 hemihydrate) has not previously been included. The objectives of this work are to (1) add bassanite to the FREZCHEM model, (2) examine the environments in which thermodynamic equilibrium precipitation of calcium sulfate minerals would be favored on Mars, and (3) use FREZCHEM to model situations where metastable equilibrium might be favored and promote the formation or persistence of one of these phases over the others in violation of an idealized equilibrium state.We added a bassanite equation based on high temperatures (343–373K). A Mars simulation was based on a previously published NaCaMgClSO4 system over the temperature range of 273 to 373K. With declining temperatures, the first solid phase under equilibrium precipitation is anhydrite at 373K, then gypsum forms at 319K (46°C), and epsomite (MgSO4⋅7H2O) at 277K. This sequence could reflect, for example, the precipitation sequence in a saturated solution that is slowly cooled in a deep, warm aquifer.Because FREZCHEM is based on thermodynamic equilibrium, a crude approach to problems involving metastable equilibria is available by removing phases that may have kinetically inhibited formation. Removing anhydrite allows bassanite to precipitate at 373K, followed by gypsum at 351K (78°C), and epsomite at 277K. Removing anhydrite and gypsum allows bassanite to form from 373 to 273K. But bassanite formation from warm to cold temperatures does not seem appropriate for Mars and Earth.An explanation for spatial patterns of gypsum, anhydrite, and bassanite on Mars and Earth could be past environmental differences. Anhydrite and bassanite are favored near Mars' equator with higher temperatures, along with drier, more saline, and more acidic environments. Gypsum would be favored at the lower temperatures in the Mars polar region with wetter, lower salinity, and less acidic environments. On Earth, Ca-sulfate would likely over time largely finish re-precipitating as the more insoluble gypsum. But Mars was not in long-term moderate climates compared to Earth that strongly influenced the dominance of gypsum on Earth. So while temperature and water/acid environments for CaSO4 minerals on Mars may have been a major factor for these precipitations, the short-term moderate climates on Mars may also have influenced the prevalence of higher soluble CaSO4 species in the lower Mars latitudes.

Phase Analysis of Portland Cement by Combined Quantitative X-Ray Powder Diffraction and Scanning Electron Microscopy.

X-ray powder diffraction (XRD) has been used for several decades to identify and measure the mass fractions of various crystalline phases in portland cement. More recently, a combination of scanning electron microscopy with X-ray microanalysis (SEM/XMA) and image processing has been shown to enable the quantitative characterization of microstructural features in these materials. Each technique can furnish some information that is not accessible from the other. For example, SEM/XMA can identify the microstructural location and morphology of calcium sulfate minerals, while only XRD can determine the relative abundance of the different forms of calcium sulfate, such as gypsum (CaSO4 · 2H2O), bassanite , and anhydrite (CaSO4). This document describes how XRD and SEM/XMA can be used together to establish and validate the portland cement phase composition and microstructure. Particular emphasis is laid on step-by-step procedures and best practices for XRD specimen preparation, data collection, and intepretation. Similar detail has been given recently for SEM/XMA [Stutzman et al., NIST Tech Note 1877, U.S. Department of Commerce, April 2015]. The methods are demonstrated for three portland cement powders, through which apparent discrepancies between the results of the two methods are identified and procedures are described for resolving the discrepancies and quantifying uncertainty.

Microbiologically induced concrete corrosion: A case study from a combined sewer network

In this study, a strongly deteriorated concrete-based sewer system was investigated by using a multi proxy approach based on gaseous, hydro-geochemical, microbiological, mineralogical and mechanical analyses. Therefore, gas, liquid, and solid samples were taken throughout the entire sewer system. Long term measurements of gaseous hydrogen sulfide (H2S) within the sewer atmosphere yielded concentrations up to 367ppm. Interstitial fluids, extracted from deteriorated concrete by squeezing, contained sulfate (SO42−) concentrations of up to 104gl−1 at strong acidic conditions (0.7>pH>3.1) and are close to the saturation state of gypsum. This sulfuric acid attack is indicative for a well-established biofilm containing sulfide oxidizing bacteria (SOB), which was analyzed to consist mainly of Acidithiobacillus thiooxidans. The micro-structure of the attacked concrete displays a progressing alteration zone, which is caused by microbially induced concrete corrosion (MICC), with a suggested pH gradient from about 13 to <1, from the intact inner concrete zone to the outermost heavily deteriorated concrete. Calcium sulfate minerals such as gypsum (CaSO4·2H2O), bassanite (CaSO4·1/2H2O) and anhydrite (CaSO4) are abundant in the altered concrete, which were formed from the dissolution of the cement phases and Ca-bearing aggregates. Remarkably high corrosion rates of different precast concrete manholes were quantified to reach values greater than 1cmyr−1, despite the fact that C3A-free cement, fly ash and a w/c of ~0.35 was used.

Reusable gypsum based catalyst for synthesis of glycerol carbonate from glycerol and urea

In this study, the catalytic carbonylation of glycerol with urea in the presence of gypsum based heterogeneous catalyst is reported for the first time. Gypsum (CaSO4·2H2O) is one of the two calcium sulphate minerals found in nature and also one of the waste materials produced from advanced material industrial processing plant. The effect of physical and chemical pre-treatment procedures on gypsum was investigated. To obtain the catalyst structure-activity relationship, the treated catalysts were characterized by means of several characterization techniques (i.e. XRD, TGA, BET surface area, SEM, FTIR, CO2–TPD, NH3–TPD and Hammett test). Tuneable physico-chemical properties of gypsum based catalysts were successfully prepared by varying the pre-treatment techniques, which later on contributed to the variation of catalytic activity toward glycerol carbonate formation from glycerol. The highest catalytic activity obtained was for catalyst consisting β-CaSO4 phase where it produced 92.8% conversion of glycerol, 90.1% selectivity and 83.6% yield of glycerol carbonate, respectively. The gypsum catalyst is easily recoverable and reusable for subsequent cycles of reaction. Similar physico-chemical properties of fresh and used catalyst were confirmed through XRD, FTIR and Hammett test analysis. Besides, the mechanistic pathway of glycerol carbonate was confirmed through the formation of glycerol carbamate as intermediate compound which was further established through time online analysis study using 13C NMR and ATR–FTIR, respectively. The study also clearly supports conversion of waste into wealth while promising proper disposal of waste to produce value added product.

Assessing hydrodynamic effects on jarosite dissolution rates, reaction products, and preservation on Mars

AbstractJarosite flow‐through dissolution experiments were conducted in ultrapure water (UPW), pH 2 sulfuric acid, and saturated NaCl and CaCl2 brines at 295–298 K to investigate how hydrologic variables may affect jarosite preservation and reaction products on Mars. K+‐based dissolution rates in flowing UPW did not vary significantly with flow rate, indicating that mineral surface reactions control dissolution rates over the range of flow rates investigated. In all of the solutions tested, hydrologic variables do not significantly affect extent of jarosite alteration; therefore, jarosite is equally likely to be preserved in flowing or stagnant waters on Mars. However, increasing flow rate did affect the mineralogy and accumulation of secondary reaction products. Iron release rates in dilute solutions increased as the flow rate increased, likely due to nanoscale iron (hydr)oxide transport in flowing water. Anhydrite formed in CaCl2 brine flow‐through experiments despite low temperatures, while metastable gypsum and bassanite were observed in batch experiments. Therefore, observations of the hydration state of calcium sulfate minerals on Mars may provide clues to unravel past salinity and hydrologic conditions as well as temperatures and vapor pressures.

The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation

Calcium sulfate minerals such as gypsum play important roles in natural and industrial processes, but their precipitation mechanisms remain largely unexplored. We used time-resolved sample quenching and high-resolution microscopy to demonstrate that gypsum forms via a three-stage process: (i) homogeneous precipitation of nanocrystalline hemihydrate bassanite below its predicted solubility, (ii) self-assembly of bassanite into elongated aggregates co-oriented along their c axis, and (iii) transformation into dihydrate gypsum. These findings indicate that a stable nanocrystalline precursor phase can form below its bulk solubility and that in the CaSO(4) system, the self-assembly of nanoparticles plays a crucial role. Understanding why bassanite forms prior to gypsum can lead to more efficient anti-scaling strategies for water desalination and may help to explain the persistence of CaSO(4) phases in regions of low water activity on Mars.

Geochemical and isotopic (oxygen, hydrogen, carbon, strontium) constraints for the origin, salinity, and residence time of groundwater from a carbonate aquifer in the Western Anti-Atlas Mountains, Morocco

Summary Groundwater in many arid basins, particularly in developing countries, is the only available water resource that sustains local communities. Yet, information on the basic hydrological parameters and the sustainability of the groundwater exploitation are often lacking. This study investigates the origin of groundwater from the Lower Cambrian carbonate aquifer of the Lakhssas Plateau in the Anti-Atlas Mountains of southwestern Morocco. The study aims to reveal the origin of the groundwater, salinity sources, and the residence time of the water. The study is based on a comprehensive geochemical and isotopic (oxygen, hydrogen, carbon, and strontium) investigation of groundwater from different parts of the basin. The hydrochemical and isotopes results indicated three types of groundwater in the Lakhssas Plateau: (1) thermal water in the southern part of the basin with solute composition that reflects dissolution of calcium–sulfate and calcium carbonate minerals; (2) low-temperature groundwater at the southern margin of the basin with low salinity (chloride content up to 100 mg/L) and chemical composition that is expected from equilibrium with limestone–dolomite rocks; and (3) low-temperature groundwater in the northern, western, and eastern margins of the basin with a wide range of salinity (chloride up to 800 mg/L). The different water types had also different stable isotope composition; the thermal water was depleted in 18 O and 2 H (δ 18 O as low as −7.6‰) relative to the southern (−5.9 to −5.3‰) and northern waters (−5.7 to −3.8‰). The differences in δ 18 O and δ 2 H between the southern and northern waters are related to elevation that induced fractionation of oxygen and hydrogen isotopes in recharge water originated from coastal moisture. The data suggest that the high salinity in groundwater from the northern, western and eastern margins of the Lakhssas Plateau is related to the presence of schist rocks in these areas. The distinctive low Na/Cl and Br/Cl ratios, coupled with high silica contents and high 87 Sr/ 86 Sr ratios (up to 0.713) in the saline groundwater provide additional evidences for the link between salinity and the schist rocks. In contrast, the thermal water had relatively low 87 Sr/ 86 Sr ratio (0.7089), which is identical to the secular seawater Sr isotope ratio during the Early Cambrian period and presumably reflects interaction with the Early Cambrian carbonate and sulfate aquifer rocks. In the northern and southern groundwater, the 87 Sr/ 86 Sr ratios were higher and correlated with the Mg/Ca ratios, inferring mixing between the Early Cambrian limestone and other rocks with higher 87 Sr/ 86 Sr, such as the schist rocks. The radiocarbon data showed 14 C activities ranging from 6 pmC in the thermal water to 62 pmC in the southern water. Age-modeling, corrected for dissolution of the carbonate rocks with dead carbon, simulated mean residence time of 10–15 ka BP for the thermal water and a range of 0 to 3 ka BP for the northern and southern waters, depending on the used models. The integration of the data enables us to establish a conceptual model for the origin of groundwater in the Lakhssas Plateau: (1) recharge to the aquifer from relatively heavy-isotope depleted recharge water, presumably during wetter conditions about 10–15 ka BP. The recharge water interacted at high depth with limestone and calcium sulfate minerals and emerged to the surface as thermal water at the southern part of the basin; (2) more recent recharge from coastal moisture originated from the Atlantic Ocean. The stable-isotope composition of groundwater was controlled by the elevation of their recharge areas: recharge at higher elevation, particularly in the southern margin resulted in lower δ 18 O and δ 2 H values; (3) the recharge water interacted with the carbonate aquifer rocks, particularly with calcite and dolomite minerals. In areas of exposure of schist rocks, the water–rock interaction induced salinization of the groundwater. Overall, our data reveal that the limited water resources in this semi-arid zone of Morocco could be in some parts less renewable and also saline. Future exploitation of this basin will have to account the salinity factor and the suggested contribution of water recharged some thousands years ago.

The worm turned, and the ocean followed

Unless you hang drywall or travel in certain biogeochemical circles, the calcium sulfate mineral gypsum is probably not on your radar—nor is the amount of sulfate (SO42−) dissolved in seawater, which determines how much gypsum is left behind when that seawater evaporates. All the same, sulfate is the second most abundant negatively charged ion dissolved in seawater today, and tracking its changing concentration in the ocean over the earth's 4.5-billion-year history is one of a geochemist's best windows to the chemical and biological evolution of the early ocean and atmosphere. Now, in a novel slant on the sulfate tracer, Canfield and Farquhar in this issue of PNAS (1) link dramatic increases in seawater sulfate in the early Paleozoic Era, 540–460 million years ago, to a major biological innovation: the invasion of marine sediments by burrowing, mud-churning organisms (Fig. 1). And the net result, the authors argue, was the world's first massive deposits of gypsum.

Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile

We measured Ca stable isotope ratios (δ 44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m −2) is present as sulfates (2.5 kmol m −2), and to a lesser extent carbonates (0.4 kmol m −2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ 44/40Ca E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ 44/40Ca in the top six horizons resembles that of sulfate-δ 44/40Ca, but with values 0.1–0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ 44/40Ca values observed on Earth. Linear correlation among δ 44/40Ca, δ 34S and δ 18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ 34S values are positively correlated with sulfate-δ 18O values ( R 2 = 0.78) and negatively correlated with sulfate δ 44/40Ca E values ( R 2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ 44/40Ca fractionation factor of −0.4‰ in CaSO 4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO 4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.

Polyhalite occurrence in the Werra (Zechstein, upper Permian) peribaltic basin of Poland and Russia: Evaporite facies constraints

Polyhalite is a common constituent of many ancient evaporite sequences, especially Permian and Neogene ones, that is related to the Na−K−Mg−Cl−SO4 type of marine brines in those time intervals. There are four polyhalite deposits in the Zechstein of northern Poland, and more than ten polyhalite-bearing areas in the adjacent part of Russia, and they are commonly accompanied by K−Mg chlorides. Most polyhalite occurrences are related to the upper part of the Lower Werra Anhydrite and in most cases, polyhalite deposits are concentrated at the sulfate platform close to its boundary with platform slope, where they can pass horizontally into polyhalite beds occurring in the Oldest Halite. The bromine content in samples of the Oldest Halite range from 40–120 ppm and the composition of fluid inclusions in halite are characteristic of halite precipitated from seawater concentrated to the early and middle stages of halite precipitation. The δ18O and δ34S values for sulfates are 10.03‰–13.50‰ and 10.03‰–12.14‰, respectively, and the δ37Cl values for halites from −0.1‰ to +0.4‰ support their marine origin. Bromine distribution in the Oldest Halite and the occurrence of anhydrite intercalations indicate fluctuations of the brine density during the Oldest Halite deposition. The formation of polyhalite was preceded by the syndepositional dehydration of the original gypsum deposit and it appears that the anhydrite was then transformed to polyhalite by reaction with marine brines more evolved than those from which precipitated precursor calcium sulfate minerals. These concentrated brines could have been derived from the evaporation of marine brines and/or inflow of K- and Mg-rich brines that were formed in nearby shallow salt pans occurring in sulfate platform areas and thus sulfate platform areas and adjacent slopes of those platforms were predestined for polyhalite formation.

A Model for the Origin of Large Carbonate- and Evaporite-Hosted Celestine (SrSO4) Deposits

ABSTRACT Celestine (SrSO4) is a common minor diagenetic mineral phase in marine carbonate sediments, where it is thought to form during the transformation of Sr-bearing aragonite or biogenic calcite to low-Sr calcite or dolomite. The largest known sedimentary celestine occurrences, however, contain several million metric tons of SrSO4 and cannot be as readily explained. These large deposits occur as epigenetic replacements and cavity-fill in coastal marine carbonate and evaporite sequences. Host rocks range in age from Silurian to Pliocene. There is variable evidence of associated hydrothermal activity, and the Sr typically has a radiogenic signature. One of the keys to explaining the origin of these massive celestine deposits is determining why celestine and not barite (BaSO4), which is orders of magnitude less soluble, is the replacement phase. Although precipitation of massive celestine beds during the early stages of evaporation of sea water has been invoked by some, this hypothesis is not supported by either experimental or field observations, which indicate most sea-water Sr is removed as dispersed sulfate phases during the halite stage of evaporation. The association of large celestine deposits with coastal marine carbonate-evaporite sequences more likely reflects in part the significant difference in the geochemical behavior of Sr and Ba during burial diagenesis. The concentration of Sr in subsurface sedimentary fluids is buffered by silicate-carbonate mineral assemblages, and the concentration of Sr increases significantly with increasing salinity. The concentration of Ba, in contrast, is controlled by equilibrium with barite and is inversely related to the concentration of dissolved sulfate. Basinal fluids having the highest Sr/Ba ratios, and thus those most likely to form celestine rather than barite, have high salinities and moderately high levels of sulfate. The precursors to such waters can be produced in coastal marine settings by the evaporation of sea water. As these brines reflux into underlying or laterally adjacent sediments, they can leach substantial amounts of Sr as they attempt to achieve chemical equilibrium with their host sediments. Barium concentrations are kept low by the presence of dissolved sulfate. If these diagenetically altered fluids are then discharged into beds containing calcium sulfate minerals and/or sulfate-rich waters, the precipitation of celestine would result. Porosity produced by uplift and karstification of associated carbonates would help focus fluid flow.

Pseudomorphs after evaporitic minerals interbedded with 2.2 Ga stromatolites of the Yerrida basin, Western Australia: Origin and significance

Widespread quartz pseudomorphs after evaporitic minerals are interbedded with stromatolites in 2.2 Ga sedimentary rocks in the Yerrida rift basin of Western Australia. These deposits preserve diverse original crystal morphologies that grew displacively either as individuals or as clusters within stromatolitic horizons and associated fine-grained siliciclastic beds. The stromatolites were deposited in shallow-marine, restricted environments. Although evaporitic minerals are largely replaced by quartz, their crystal shapes include lenticular, rosette, needle, and nodular forms, suggesting that they formed from calcium sulfate–rich brines. Textures suggest that replacement by quartz occurred during early alteration of the sediments. Original calcium sulfate minerals such as gypsum or anhydrite are indicated by the morphologies of the pseudomorphs; this concept is supported by anhydrite inclusions in the quartz crystals that form the pseudomorphs, and this finding indicates that calcium sulfate existed in Early Proterozoic sedimentary environments.