Anhydrite Samples Research Articles

Synthesis and morphological characterization of CaSO$$_{4}$$: low-temperature Raman spectroscopy analysis of vibrational modes

AbstractThis study delves into the synthesis of CaSO$$_{4}$$ 4 materials using the Pechini and sol-gel methods, followed by heat treatment up to 900 $$^{\circ }$$ ∘ C. X-ray analysis discerned distinct phases, with one method yielding pure orthorhombic anhydrite and the other predominantly producing a mixture of orthorhombic anhydrite and bassanite. Scanning electron microscopy (SEM) showcased homogeneous columnar morphologies for the anhydrite samples and irregularly sized particles for the alternative method. Photoluminescence (PL) measurements demonstrated emissions in the visible range (approximately 400 nm to 700 nm) with differences noted between synthesis methods. Low-temperature Raman spectroscopy (20 K to room temperature) unveiled unique vibrational modes, notably with the orthorhombic phase of anhydrite exhibiting the most intense peak at 1017 cm$$^{-1}$$ - 1 . These powders, intended for thin film applications, bear significance in electronics, optoelectronics, and nanotechnology. A thorough comprehension of their structural and vibrational properties at the microscopic level is paramount for material and device design and optimization.

A Practical Swelling Constitutive Model of Anhydrite and Its Application on Tunnel Engineering

Swelling of anhydrite rock causes serious damage to the tunnel and generates high additional costs in the process of tunnel construction and operation and has gradually become one of the main factors that threaten the safety of the tunnel. It is extremely difficult to predict swelling pressures and deformations accurately based on conventional swelling constitutive models. Thus, a new practical swelling constitutive model of anhydrite for tunnel engineering has been developed. First, swelling tests of natural anhydrite samples focusing on the time effect have been designed and conducted, whose test results show that swelling strain-time can be described by the S-curve model and that swelling stress-strain can be described by the quadratic model. Second, a swelling constitutive model with considering the time effect has been developed to reproduce the swelling behavior of anhydrite observed in swelling tests. This model can track the evolution of swelling activity in tunneling, which has practical significance for process simulation and process control of swelling disaster. Then, this model has been implemented within ANSYS for numerical simulation of the Lirang tunnel. Based on simulation results, useful measures have been proposed. Satisfactory results have been achieved according to the feedback from the site.

Ekonomik Açıdan Sivas Havzası Doğal Anhidrit Potansiyeli, Rezerv Tahmini ve Jeokimyasal Özellikleri, Orta Anadolu, Türkiye

This study focuses on the economization of natural anhydrite, which has a wide range of uses and is environmentally friendly, in Türkiye. Anhydrite, which is one of the most common evaporitic minerals in the world, has almost no usage area in Türkiye. Evaporite deposits consisting of gypsum, anhydrite, halite, bassanite and selenite are prominent in the country, especially in Tertiary period sedimentary basins. The Sivas basin is one of these important Tertiary basins. Mining operates in the basin are carried out only on gypsum and anhydrite is not utilized. This study is carried out on the geological and geochemical characteristics of anhydrite, which is prominent with its usage areas in the world, its potential in the basin and its economic importance. The geochemical and mineralogical analyses are carried out on anhydrite samples taken from the basin. Anhydrite formations up to 300 meters in thickness are detected in the evaporite deposits that outcrop in most of the basin. In addition, a block model is extracted from the drilling data of a mining company operating in the basin and the reserve is calculated. As a result, it is concluded that the Sivas basin has a high potential in terms of anhydrite and should be brought to the national economy.

Micrometallogeny and hydrothermal fluid evolution of the Iju porphyry Cu deposit, NW Kerman, Iran: Evidence from fluid inclusions, Laser Raman spectroscopy, and S[sbnd]O isotope systematics

The Iju Cu porphyry is located in the NW part of the Kerman Magmatic Copper Belt (KMCB). It is related to a ~ 9 Ma granodiorite porphyry intrusion, with three main stages of hydrothermal activity. The homogenization temperatures for the fluid inclusions are in the ranges of 200–494 °C, and their salinities vary from 4.0 to 42.8 wt% NaCl equiv., which are typical magmatic-hydrothermal fluids. The δ34S values of sulfides range from −0.4 to +3.2 ‰ (V-CDT), and the δ34S values of anhydrite samples range from +11.6 to +16.8 ‰. The δ34S values of sulfides show a narrow range, implying a homogeneous sulfur source. The oxygen isotopic composition of hydrothermal water in equilibrium with quartz samples ranges from +3.4 to +6.0 ‰ (V-SMOW) consistent with the hydrothermal fluids having a magmatic signature, but diluted with meteoric waters in the main mineralizing stage. The most important factors responsible for metal precipitation in the Iju porphyry deposit are fluid boiling, oxygen fugacity decrease and cooling followed by dilution with meteoric water. The primary fluids of the Iju Cu deposit are characterized by relatively high temperature and moderate salinity, and are CO2-rich, indicating a typical post-collisional porphyry system.

Reservoir characterisation of unconventional rock units in the middle jurassic Sargelu Formation, atrush field, zagros folded belt

The heterogeneous lithological components of the Middle Jurassic Sargelu Formation in the Atrush field of the high folded zone were examined using rock samples, conventional wireline logs, mud logs and well test results throughout two wells. The Sargelu Formation is 45 m thick and has been divided into three units based on lithological characteristics and shale volume. The upper unit of the Sargelu Formation is composed of argillaceous limestone with intercalation of dolostone and anhydrite beds. The middle unit of the Sargelu Formation is composed of organic rich laminated shales, which contain residual bitumen, whilst the lower unit of the formation is dominated by argillaceous limestones. Lithological variation, mineralogy and organic matter contents are shown to be essential factors in controlling the reservoir matrix porosity and permeability. The maximum matrix porosity and permeability were observed within the dolomite plug samples (13.40%, 10.40 mD) and the lowest values were recorded in the anhydrite samples, both found in the upper unit of the Sargelu Formation. The fracture permeability is three orders of magnitude higher than the matrix permeability and the hydrocarbon production is predominately derived from the fractured intervals throughout the Sargelu Formation. The interconnected fracture pathways are influenced by the lithological variation and mineralogical components of the formation along with the tectonic position of the field. It was found that as the confining pressure increased the fracture permeability reduced. The different lithologies were also found to be sensitive to increases in the confining pressure. The clastic shale rich rocks are most sensitive to increases in confining pressure, whilst the more carbonate rich rocks showed the lowest variation as a function of confining pressure.

In situ LA-ICPMS U–Pb dating of sulfates: applicability of carbonate reference materials as matrix-matched standards

Abstract. Recent developments in analytical capabilities in the field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded the applications of U–Pb geochronometers in low-U minerals such as carbonates or garnets. The rapid evolution of the technique relies on well-characterized matrix-matched reference materials. In this article, we explore the suitability of using carbonate as an “almost-matrix-matched reference material” for in situ U–Pb dating of sulfates. For such purpose, we have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97–5.33 Ma) and compared these dates with the U–Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common Pb content and low 238U/204Pb ratios, five of the samples showed a higher dispersion on U/Pb ratios. The obtained dates in four of these samples are comparable with the expected ages, while another gave an unexpected younger age, each of them with 6 %–11 % of uncertainty. The pit depth of the spots showed that the sulfates ablate similar to carbonates, so the offset due to the crater geometry mismatch or downhole fractionation can be assumed to be negligible. To sum up, the bias between the U–Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty, and thus the results obtained here suggest that carbonate reference material is currently the best option for standardization of in situ U–Pb sulfate analyses.

230Th dating of gypsum from lacustrine, brackish-marine and terrestrial environments

Gypsum is an important environmental archive in hyperarid settings, but its application in earth sciences has been hampered by the limited availability of suitable dating methods. Here we re-evaluate the potential of 230Th/U dating for sedimentary gypsum as well as anhydrite samples in different depositional environments. We provide a robust analytical protocol based on a simple dissolution using nitric acid followed by a two-column separation procedure using anion exchange resin. Isotope analyses were performed using MC-ICPMS and a mixed 229Th-233U-236U tracer. We applied the method to a suite of samples that likely reached secular equilibrium as well as to three suites of younger (<300 ka old) samples, two from the Atacama Desert in Chile and one from a drill core of a saline playa lake in southern Spain. We employed a selective sampling approach targeting the least visually altered and clearest parts of the samples largely devoid of detritus and analysed multiple subsamples.The results demonstrate the general applicability of the 230Th/U method to gypsum as well as anhydrite, and recrystallization of gypsum to anhydrite does not significantly affect the U-series isotope system. However, there is evidence for open-system behaviour, which needs to be carefully assessed. Our results suggest that α-recoil processes as well as uptake or loss of uranium are important processes biasing the results of gypsum dating. Furthermore, we see signs of multiple-phase contamination in our case studies involving detrital components and authigenic phases. Based on these results we propose geochemical criteria in to identify biased 230Th/U gypsum ages.The application of our protocol to a suite of selenite samples deposited by brine outflows on the lowest terrace of Cerro Soledad (Atacama Desert, Chile) provides a mean age of 212 ± 8 ka, indicating a relatively humid climate interval with local rainfall in this area that is coinciding with marine isotope stage 7C. Selenites deposited in the paleo-lake Soledad system (Atacama Desert, Chile) yield 230Th/U ages ranging from 250 to 320 ka, in agreement with previous age estimates for the drainage of this paleo-lake obtained by cosmogenic nuclide dating. Application to a suite of gypsum crystals recovered from a drill core in the Laguna de Fuente de Piedra (South Spain) shows good agreement with their stratigraphic context, however, multiple-phase contamination produces larger age uncertainties for these ages.

Experimental studies on the pore structure and mechanical properties of anhydrite rock under freeze-thaw cycles

To study the deterioration mechanisms of anhydrite rock under the freeze-thaw weathering process, the physico-mechanical characteristics and microstructure evolutions of anhydrite samples were determined by a series of laboratory tests. Then, a descriptive-behavioral model was used to measure the integrity loss in anhydrite samples caused by cyclic freeze-thaw. Finally, the freeze-thaw damage mechanisms of anhydrite rock were revealed from the macro and micro aspects. The results show that the pore size of the anhydrite rock is mainly concentrated in the range of 0.001–10 μm. As the number of freeze-thaw cycles increases, there is a growth in the proportion of macropores and mesopores. However, the proportion of micropores shows a declining trend. The relations of the uniaxial compressive strength, triaxial compressive strength, cohesion, and elastic modulus versus freeze-thaw cycles can be fitted by a decreasing exponential function, while the internal friction angle is basically unchanged with freeze-thaw cycles. With the increase of confining pressure, the disintegration rates of the compressive strength and the elastic modulus decrease, and the corresponding half-life values increase, which reveals that the increase of confining pressures could inhibit freeze-thaw damage to rocks. Moreover, it has been proven that the water chemical softening mechanism plays an essential role in the freeze-thaw damage to anhydrite rock. Furtherly, it is concluded that the freeze-thaw weathering process significantly influences the macroscopic and microscopic damages of anhydrite rock.

A critical evaluation of the chlorine quantification method based on molecular emission detection in LIBS

The entire process involving the determination of Cl by molecular emission detection in Laser-Induced Breakdown Spectroscopy (LIBS) is thoroughly studied in this paper. This critical evaluation considers how spectra are normalized, how interferences from other molecular species signals are removed, and how signal integration is applied. Moreover, a data treatment protocol is proposed to achieve reliable and accurate Cl determination from the CaCl molecular spectral signal, not requiring the use of more complex numerical approaches. Calcium chloride dihydrate (CaCl2·2H2O) and high purity anhydrite samples (CaSO4) are used to optimize the acquisition conditions and data treatment of CaCl emission signal. Using the developed protocol, calibration curves for Cl, covering the concentration range from 0 μg/g to 60,000 μg/g of Cl, are successfully achieved. Finally, the suitability of the proposed methodology for Cl determination is successfully applied in industrial gypsum waste samples, where the results obtained by LIBS are validated using high-resolution molecular absorption spectroscopy (HR-CS-MAS) and potentiometric titration.

Sulfate Sources of Thermal Sulfate Reduction (TSR) in the Permian Changxing and Triassic Feixianguan Formations, Northeastern Sichuan Basin, China

Thermal sulfate reduction (TSR) occurred throughout the Permian Changxing (P2c) and Triassic Feixianguan (T1f) dolostone reservoirs in the western and eastern parts of the Kaijiang-Liangping (K-L) trough in the northeastern part of the Sichuan Basin. To determine the sulfate sources of this TSR, fourteen solid bitumen samples and eight anhydrite samples were collected from the northeastern part of the Sichuan Basin. These samples were analyzed to determine their sulfur isotopes. In addition, untreated, HNO3-treated, and CrCl2-treated solid bitumen samples were analyzed to determine their sulfur isotopes in order to obtain reliable δ34S data for the TSR solid bitumen. The results show that the HNO3 method is more effective at removing pyrite from solid bitumen than the method using CrCl2 thrice because the HNO3-treated solid bitumen has lower sulfur contents and higher δ34S. The δ34S of the T1f solid bitumen samples from the Puguang gas field (in the eastern part of the K-L trough, 12.0-24.0‰) is significantly lower than that of the samples from the Yuanba gas field (in the western part of the K-L trough, 24.1-34.2‰). The δ34S of the T1f1–2 anhydrite is 18.1-26.6‰, which is lower than that of the T1f3–4 anhydrite samples (29.9-39.6‰). The TSR sulfates from the Puguang gas field were most likely from the coeval T1f1–2 evaporating seawater and were enriched during the reflux-seepage dolomitization process. The TSR sulfates from the Yuanba gas field were primarily caused by the evaporation of seawater during the T1f4. First, the evaporating seawater would flow vertically into the P2c reservoirs in the adjacent area, and then, it would flow laterally into the P2c reservoirs in the Yuanba gas field. Considering the fact that the sulfate sources of TSR and the δ34S values of the TSR sulfates are different in the Puguang and Yuanba gas fields, the δ34S of TSR solid bitumen cannot be simply used to show the extent of TSR.

Balancing the amount and composition of gas contained in the pore space of cupriferous rocks

From the perspective of economy and safety, balancing gas contained in the pore space of rocks is extremely essential, predominantly to establishments dealing with extraction of mineral resources. One of the main methods of evaluating the amount and composition of gas contained in the pore space of a rock is releasing the gas as a result of comminuting the investigated rock material. In the case of cupriferous rocks, effective comminution is a very difficult task, due to the strength properties of these rocks. The present paper discusses the results of studies into the gas content of cupriferous rocks, obtained by means of an original device named the GPR analyzer. The research was done on 41 samples of dolomites and anhydrites from various areas of two copper mines located in Poland: “Rudna” and “Polkowice-Sieroszowice.” For all samples, on the basis of microscope analyses performed on cuts and polished sections, the open porosity, closed porosity, and total porosity were determined. In the case of the dolomite samples, the total porosity variability fell in the range of 4.75–23.05%, and in the case of the anhydrite samples—in the range of 3.87–16.60%. The maximum gas content of the dolomite samples was 166.67 cm3/kg, and of the anhydrite samples—84.66 cm3/kg. In some of the studied samples, the presence of methane was confirmed. Toxic gases, such as H2S, were not found. The main gas in the pore space of the investigated rocks was nitrogen. Knowing the amount of the released gas and the value of the closed porosity in the investigated samples, the authors were able to estimate the pore pressure of the gas, whose maximum value was 0.583 MPa.

Geochemical behavior of rare elements in Paleogene saline lake sediments of the Qaidam Basin, NE Tibetan Plateau

The Paleogene paleo-lake in the Qaidam Basin on NE Tibetan Plateau is one of the largest perennial inland saline lakes in China. Forty-nine samples collected from four depositional environments (shallow lake, lakeshore, sandy mudflat and gypsiferous mudflat) have been tested for major, trace and rare earth elements to study the geochemical behavior of rare elements (including trace and rare earth elements) in sediments of saline lakes. Based on correlation degrees and relationships with major elements, six categories of rare elements in the Qaidam saline lake have been recognized: (1) REE-type (REEs, Nb, Ta, Ge, and Y), (2) nutrient-type (Zn), (3) scavenged-type (Cr, Ga, Sc, and V), (4) conservative-type (Be, Rb, Th, Ni, Cs, and Li), (5) grained-type (Zr and Hf), and (6) independent-type (U, Pb, Co, Cu, Sr, and Ba). Contents of rare elements in saline lakes are comprehensively controlled by detritus inputs, grain size, adsorption by or co-precipitation with clay particles, and biological activities. However, these factors influence the behavior of rare elements of different categories to different degrees. The presence of salts has a diluting effect to all the rare elements except Sr and Ba. The ratio Sr/Ba is proved to be a good indicator of paleosalinity in saline sediments. More depletion of HREEs and relative enrichment of Co, Ni, and Cu have been observed in pure anhydrite samples. The conservative- and independent-type elements are more likely to be enriched in saline lake centers during the process of evaporation.

Analyzer for measuring gas contained in the pore space of rocks

In the present paper, the authors discussed the functioning of their own analyzer for measuring gas contained in the pore space of high strength rocks. A sample is placed inside a hermetic measuring chamber, and then undergoes impact milling as a result of colliding with the vibrating blade of a knife which is rotationally driven by a high-speed brushless electric motor. The measuring chamber is equipped with all the necessary sensors, i.e. gas, pressure, and temperature sensors. Trial tests involving the comminution of dolomite and anhydrite samples demonstrated that the constructed device is able to break up rocks into grains so fine that they are measured in single microns, and the sensors used in the construction ensure balancing of the released gas. The tests of the analyzer showed that the metrological concept behind it, together with the way it was built, make it fit for measurements of the content and composition of selected gases from the rock pore space. On the basis of the conducted tests of balancing the gases contained in the two samples, it was stated that the gas content of Sample no. 1 was (0.055 ± 0.002) cm3 g−1, and Sample no. 2 contained gas at atmospheric pressure, composed mostly of air.

Mapping of carnallite along with semi-quantitative estimation of potassium content of drill cores using hyperspectral imagery

ABSTRACTReflectance spectra and hyperspectral images of carnallite, halite, and anhydrite samples from the Mboukoumassi sylvite deposit, Congo, were obtained using an ASDTM spectrometer and a HySpexTM imager, respectively. The spectral absorption features of the ore mineral, carnallite, at 999, 1076, 1206, 1232, 1763, and 1970 nm are highly reproducible and distinct from absorption features of the gangue minerals, halite and anhydrite. The intensity of the approximately 1200 nm absorption feature can be used to map the spatial distribution of carnallite in drill core. We also established an empirical relationship between potassium content estimated through X-ray fluorescence (XRF) measurements and absorption intensity at approximately 1200 nm. Based on this empirical relationship, the potassium content of a core surface was mapped. This technique provides more detailed estimates of potassium content and its spatial distribution in core samples than possible by point sampling, and this is beneficial in assessing the resource potential sylvite deposits.

Geomechanical characteristics of common reservoir caprock in Iran (Gachsaran Formation), experimental and statistical analysis

The evaporitic Gachsaran Formation consists of seven members with sequence of anhydrite, marl and salt layers. The first member (the lowest one) which is made up of anhydrite is known as the caprock of the Asmari carbonate reservoir, the most famous hydrocarbon reservoir in Iran. The geomechanical behavior of the Gachsaran Formation is the main concern of drilling and development operations in Iranian oil and gas fields. However, the chief problem is lack of deep core samples from Gachsaran sequences due to the challenging and costly coring process in the non-reservoir Gachsaran formation. In a practical approach of obtaining samples for essential geomechanical laboratory tests, caprock (anhydrite) core samples of Gachsaran Formation have been taken from depths of 80–300 m, at three dam sites. The rock physical and geomechanical properties were examined by analysis of variance (ANOVA) and Tukey's honestly significant difference (HSD) tests. The results from analysis of the properties of samples collected from Ghotvand and Chamshir dam sites showed no significant differences, but some of the characteristics of samples of Khersan dam site were considerably different from those of the other samples. Moreover, the physical properties of anhydrite samples were compared with the properties inferred from the petrophysical well logs from a depth of about 2000 m. It was demonstrated that the observed discrepancies between the measured values from Ghotvand and Chamshir samples and those from deep logs are not significant at the 95% confidence level. The investigation results illustrated that anhydrite samples taken from shallow depths can be used for evaluating the geomechanical properties of deeper anhydrites.

Petrographic study and geochemical investigation of the evaporites associated with the Germik Formation (Siirt Basin, Turkey)

Evaporites of the Oligocene Germik Formation are represented by massive, laminated, laminated-banded, banded, nodular and/or brecciated nodular and nodular-banded lithofacies in the Siirt–Kurtalan area. Observations of massive and laminated-banded gypsums with undulated-shaped stromatolitic algal mats, corrugated folded structures and clay-carbonate and interlayered marl in laminated gypsum levels are very important in terms of their mechanisms of occurrences. Petrographic and mineralogic examinations of secondary gypsums in the Germik Formation generally display alabastrine and porphyroblastic textures with corroded anhydrite. Structures as satin-spar, chickenwire, enterolithic and minerals as dolomite, calcite and celestite are also observed. Geochemical evaluations of gypsum and anhydrite samples belong to the lithofacies have provided an approach of their original formation and understandings the environmental impacts (such as pH, paleotemperature, paleosalinity, surface–groundwater activity and bacterial activity). Therefore, a number of analyses were conducted on the different samples for major and trace elements and the gypsum–anhydrite samples were analyzed for isotope compositions. Trace elements of these evaporites were found to be in the range of the geochemical values of the depositional environment of shallow marine evaporites. These values show also occasional influx of terrestrial waters to the environment. Moreover, the differences in the isotopic values of the lithofacies identified by the stable isotopic studies indicate the effects of temperature, salinity, biogenic activity and diagenetic processes for the formation of evaporites, under the influence of climate. Geochemical evolution of gypsum and anhydrite lithofacies of the Germik Formation leads up to muddy evaporites indicating that they were deposited in the coastal sabkha or shallow marine environments within effectively developed paleoclimatic conditions, tectonic activities, diagenetic processes and depositional system.

The Miocene Gachsaran Formation evaporite cap rock, Zeloi oilfield, SW Iran

Petroleum at the Zeloi oilfield (SW Iran) is produced from the Oligocene/Miocene Asmari Formation. Evaporites in the overlying lowermost member of the Gachsaran Formation form an efficient cap rock seal. In the present study, this cap rock was studied and interpreted on the basis of gamma-ray and sonic logs data from eleven boreholes. Geochemical data obtained from X-ray fluorescence analyses of the anhydrite samples and scanning electron microscope analyses of the halite samples have been interpreted in terms of depositional environment. Chemostratigraphy based on the use of Na2O/Al2O3, Na2O/Fe2O3, Na2O/TiO2, and Na2O/K2O ratios allowed division of the cap rock into three marker units: (1) a unit of weathering-resistant elements; (2) an index key unit; and (3) a unit of highly soluble elements. The stratigraphy of the cap rock suggested the occurrence of cyclic regression and transgression. Geochemical data in combination with textural analyses in scanning electron microscopy indicate arid depositional conditions for this part of the Gachsaran Formation. Cycles of marine transgression–regression taking place in a barred coastal sabkha-lagoon basin, must have been responsible for the formation of the evaporite sequences of the Gachsaran Formation.

Dynamic properties of anhydrites, marls and salts of the Gachsaran evaporitic formation, Iran

A large carbonate oil field in Iran is suffering from severe casing collapses and related operational problems in anhydrite, marl and salt sequences of the Gachsaran cap rock formation. To investigate the causes and cures of operational problems, specifically casing collapse, knowing geomechanical properties of anhydrite, marl and salt of this formation is a prerequisite. However, taking cores in this formation is virtually impossible due to high solubility and weakness of the rocks. Moreover, there are insufficient well log data in this formation and the only available running well log is Vp. In this paper, in order to obtain the dynamic parameters of the Gachsaran formation, Vp, Vs and ρb in anhydrite, marl and salt cores, which had been taken from depths up to 300 m, were measured. Moreover, Vp and Vs in salt cores under different triaxial and hydrostatic stress conditions were obtained. The Vp‐Vs, (Vp/Vs)‐Vp and Vp‐ρb relations in anhydrite, marl and salt were investigated. The established relations in these anhydrite samples were verified by the data derived from limited cores which were taken from 3600 m depth. The relations between dynamic properties of salt with lateral and hydrostatic stresses were investigated. In conclusion, Vs, ρb and the ratio of Vp/Vs in anhydrite and marl can be estimated through the established relations and having Vp logs in the Gachsaran formation. As a result, the dynamic properties of these rocks can be calculated in different depths of this evaporitic formation. Furthermore, the dynamic properties of salt rock seem to be constant in various depths and under differing triaxial and hydrostatic stress conditions.

Crystal-structure analysis of four mineral samples of anhydrite, CaSO4, using synchrotron high-resolution powder X-ray diffraction data

The crystal structures of four samples of anhydrite, CaSO4, were obtained by Rietveld refinements using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and space group Amma. As an example, for one sample of anhydrite from Hants County, Nova Scotia, the unit-cell parameters are a = 7.00032(2), b = 6.99234(1), c = 6.24097(1) Å, and V = 305.487(1) Å3 with a &gt; b. The eight-coordinated Ca atom has an average &lt;Ca-O&gt; distance of 2.4667(4) Å. The tetrahedral SO4 group has two independent S-O distances of 1.484(1) to O1 and 1.478(1) Å to O2 and an average &lt;S-O&gt; distance of 1.4810(5) Å. The three independent O-S-O angles [108.99(8) × 1, 110.38(3) × 4, 106.34(9)° × 1; average &lt;O-S-O&gt; [6] = 109.47(2)°] and S-O distances indicate that the geometry of the SO4 group is quite distorted in anhydrite. The four anhydrite samples have structural trends where the a, b, and c unit-cell parameters increase linearly with increasing unit-cell volume, V, and their average &lt;Ca-O&gt; and &lt;S-O&gt; distances are nearly constant. The grand mean &lt;Ca-O&gt; = 2.4660(2) Å, and grand mean &lt;S-O&gt; = 1.4848(3) Å, the latter is longer than 1.480(1) Å in celestite, SrSO4, as expected.

Shock experiments on anhydrite and new constraints on the impact‐induced SOx release at the K‐Pg boundary

Abstract– We have performed six shock experiments at nominal peak‐shock pressures of 12.5, 20, 33, 46.5, 64, and 85 GPa using polycrystalline anhydrite discs embedded in ARMCO‐Fe sample containers and the shock reverberation technique. The recovered samples were analyzed using X‐ray powder diffraction and transmission electron microscopy (TEM). The X‐ray diffraction patterns recorded on all samples are compatible with the anhydrite structure; extra‐peaks have not been observed. Peak intensities decrease and peak broadening increases progressively in the pressure range from 0 to 46.5 GPa. At higher pressures, peak broadening diminishes and the X‐ray diffraction pattern of the 85 GPa sample resembles essentially that of unshocked, well‐crystallized anhydrite. Related structural changes at the nanoscale include in the pressure regime up to 20 GPa “cold” deformation phenomena such as cracks and deformation twins. Dislocation density increases up to 33 GPa and the strain increases up to 46.5 GPa. In the pressure range from 46.5 to 85 GPa, high postshock temperatures caused annealing of the deformation features. Increasing density and size of voids in the anhydrite samples shocked at 64 and 85 GPa indicate partial decomposition of anhydrite. Recalculation of the peak‐shock pressure in the experiments to a more realistic natural loading path indicates the onset of degassing of anhydrite in the pressure range of 30–41 GPa.