Uvarovite: Stability of uvarovite-andradite solid solutions at low pressure

Uvarovite (Ca₃Cr₂Si₃O₁₂) forms a complete solid solution series with andradite (Ca₃Fe ₂ ⁺³ Si₃O₁₂) below 1,137±5 ° C at a total pressure of 1 atm. Pure uvarovite decomposes to pseudowollastonite (CaSiO₃)+eskolaite (Cr₂O₃) at 1,385 ± 10 ° C. The incorporation of Ca₃Fe ₂ ⁺³ Si₃O₁₂ component in the uvarovite structure lowers the thermal stability of the garnet. The breakdown assemblage is garnet_ss (Ca₃(Cr,Fe⁺³ ₂)Si₃O₁₂)+pseudowollastonite (CaSiO₃)+hemeskolaite_ss(Cr,Fe⁺³O₃). Pure andradite decomposes to pseudowollastonite (CaSiO₃)+hematite (Fe₂O₃) at 1,137±5 °C. Andradite thermal stability is increased by incorporation of Ca₃Cr₂Si₃O₁₂ component by 248 °C.At 1,264±5 °C pseudowollastonite+hematite react to liquid defining a thermal minimum of the CaSiO₃-Cr₂O₃-Fe₂O₃ ternary system. This minimum is located at about 64.5 wt.-% CaSiO₃, 0.5 wt.-% Cr₂O₃, and 35.0 wt.-% Fe₂O₃. Uvarovite and andradite bulk compositions start to melt at 1,420 °C and 1,265 ±5 °C, respectively.The unit-cell parameter for uvarovite is 11.999 (2) Å, the refractive index 1.866 (2). The substitution of Cr⁺³ by Fe⁺³ increases a and n almost linearly toward the andradite end member which displays a unit-cell parameter of 12.059 (3) Å and a refractive index of 1.887 (2).     

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

Contributions to Mineralogy and Petrology - One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge...     

Stability properties of the CuS2-FeS2 solid solution series of pyrite type

Summary Experimental investigations on the Cu-Fe-substitution and the formation of a solid solution series in the system CuS₂-FeS₂ were carried out under hydrothermal conditions up to 350°C and 3 kb and by means of a piston cylinder apparatus at higher temperatures and pressures up to 900°C and 45 kb. Under dry conditions at 440°C and above 17 kb the system was found to be binary with a miscibility gap between an iron-rich phase near the FeS₂ end-member and a coexisting copper-rich phase being the solvus composition of a homogeneity region from 75 to 100 mole% CuS₂. This solvus of the copper rich phase was found to be almost independent of temperature and pressure up to 45 kb and 700°C. The solubility of CuS₂ in FeS₂ at 45 kb increases from 0.6 mole% at 700°C to 4.5 mole% at 900°C. Under hydrothermal conditions up to 3 kbars the solvus of metastable (Cu, Fe)S₂ is strongly dependent on pressure only in the Cu-rich part of the system.

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Zusammenfassung Stabilität der CuS₂-FeS₂ Mischreihe des Pyrit-Typs Experimentelle Untersuchungen zur Cu-Fe-Substitution und zur Bildung einer festen Lösung im System CuS₂-FeS₂ wurden mit der Hydrothermalsynthese bis 350°C und 3 kb und mit der Stempelzylindermethode bis 900°C und 45 kb durchgeführt. Unter trockenen Bedingungen bei 440°C und oberhalb 17 kb ist dieses System binär und weist eine Mischungslücke zwischen einer eisenreichen Phase nahe dem FeS₂ Endglied und einer koexistierenden kupferreichen Phase mit der Solvuszusammensetzung eines Homogenitätsbereiches zwischen 75 und 100 mol% CuS₂ auf. Dieser Solvus der kupferreichen Phase wurde bis 45 kb und 700°C nahezu druck- und temperaturunabhängig gefunden. Demgegenüber nimmt die Löslichkeit von CuS₂ in FeS₂ bei 45 kb von 0.6 mol% bei 700°C auf 4.5 mol% bei 900°C zu. Der Solvus der metastabilen (Cu, Fe)S₂-Phasen, die bislang nur unter hydrothermalen Bedingungen synthetisiert werden können, zeigte bis 3 kbar nur im kupferreichen Teil des Systems eine starke Druckabhängigkeit.

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With 4 Figures     

The elastic solid solution model for minerals at high pressures and temperatures

Non-ideality in mineral solid solutions affects their elastic and thermodynamic properties, their thermobaric stability, and the equilibrium phase relations in multiphase assemblages. At a given composition and state of order, non-ideality in minerals is typically modelled via excesses in Gibbs free energy which are either constant or linear with respect to pressure and temperature. This approach has been extremely successful when modelling near-ideal solutions. However, when the lattice parameters of the solution endmembers differ significantly, extrapolations of thermodynamic properties to high pressures using these models may result in significant errors. In this paper, I investigate the effect of parameterising solution models in terms of the Helmholtz free energy, treating volume (or lattice parameters) rather than pressure as an independent variable. This approach has been previously applied to models of order–disorder, but the implications for the thermodynamics and elasticity of solid solutions have not been fully explored. Solid solution models based on the Helmholtz free energy are intuitive at a microscopic level, as they automatically include the energetic contribution from elastic deformation of the endmember lattices. A chemical contribution must also be included in such models, which arises from atomic exchange within the solution. Derivations are provided for the thermodynamic properties of n-endmember solutions. Examples of the use of the elastic model are presented for the alkali halides, pyroxene, garnet, and bridgmanite solid solutions. Elastic theory provides insights into the microscopic origins of non-ideality in a range of solutions, and can make accurate predictions of excess enthalpies, entropies, and volumes as a function of volume and temperature. In solutions where experimental data are sparse or contradictory, the Helmholtz free energy approach can be used to assess the magnitude of excess properties and their variation as a function of pressure and temperature. The formulation is expected to be useful for geochemical and geophysical studies of the Earth and other planetary bodies.     

Mechanical characteristics of a lunar regolith simulant at low confining pressure

Manned lunar exploration has recently attracted renewed interest. This includes the NASA Constellation program to return humans to the Moon by 2020, the ESA Aurora program which may use the Moon as a way station to prepare for major interplanetary exploration by 2025, and the PRC program to send a human to the Moon by 2030 and build a temporary manned lunar base by 2040. One of the problems demanding a solution is the stresses on the mechanical characteristics of the lunar regolith under the microgravity environment. The gravity on the Moon is about 1/6 that on Earth. The regolith is subject to very low confining stresses under a microgravity environment and the mechanical properties can change correspondingly. Because of the limited amount of lunar regolith brought back to Earth by the Apollo missions, a lunar regolith simulant was developed using silicon carbide to investigate the properties of the lunar regolith. Based on triaxial tests, this study analyzed the mechanical properties of the lunar regolith simulant at low stresses including the shear strength, peak strength and dilatation angle. The research results provide useful information on lunar regolith characteristics for astronauts returning to the Moon and for building a temporary manned lunar base.     

The solid solution chemistry of vesuvianite

Solid solution in vesuvianite is elucidated by examining chemical trends and cation abundances in 22 microprobe analyses of samples from the Big Maria Mountains, southeastern California. Two recent structure refinements indicate 50 filled cation sites per formula, providing the basis for data normalization. Previous optical absorption and Mössbauer studies help clarify site occupancies. Stoichiometric abundances of Si and Ca + Na indicate 18 and 19 per formula, filling all 4- and 8-fold sites respectively. The four 6-fold A-sites are filled with Al. The solid solution occurs mainly within the eight 6-fold AlFe-sites (Al, Mg, Fe²⁺, Fe³⁺, Ti) and one 5-fold B-site (Mg, Fe²⁺, Fe³⁺). Chemical trends and crystal chemical constraints delineate eight independent substitutions.An extensive solid solution in the elements Mg, Fe, Al, and Ti suggests considerable potential as a petrogenetic indicator. In order to treat equilibria involving vesuvianite thermodynamically, a reference composition must be chosen and activity-composition relations modeled. For a reference composition, Mg-vesuvianite (Fe, Ti, Na-free) was chosen because of its chemical simplicity, but problems in ascertaining its stoichiometry have led previous workers to propose at least six different formulas. In this study, its formula is determined from the microprobe analyses by applying exchange vectors to substitute components of pure Mg-vesuvianite for Fe and Ti. This yields Ca₁₉Mg₂Al₁₁-Si₁₈ O₆₉(OH)₉, with AlFe-sites=MgAl₇, and B-sites=Mg. Subdivision of the AlFe-sites into at least two distinct sites is suggested by observed chemical trends which are explanable only when different substitutions are considered to operate within different AlFe-sites.A thermodynamic mole fraction is formulated for Mg-vesuvianite based on an ideal mixing-on-sites solution model. A method is provided for estimating the distribution of Fe between the AlFe- and B-sites. Thermodynamic mole fractions calculated using Fe site distributions estimated from microprobe data yield results similar to those calulated using Fe site distributions determined from Mössbauer analysis.     

Sulfide inclusions in diamonds: not monosulfide solid solution

Sulfide inclusions in diamonds, the most common of all inclusions, contain critical evidence about the timing and physical/chemical conditions prevailing during diamond formation. Typically, sulfide inclusions are encapsulated as a monosulfide solid solution (Mss) in the Fe-Ni-S system, with a minor amount of Cu. This Mss and the enclosing diamond have sufficiently different thermal expansion properties, so that, after encapsulation, the Mss creates a series of cracks in the diamond radiating from the sulfide. On cooling, this increase in volume permits the Mss to undergo exsolution to an assemblage of pyrrhotite + pentlandite + chalcopyrite + pyrite. The kinetics of this exsolution is so rapid that practically no Mss remains in nature. Instead, in recovered diamonds, all sulfides that originally were Mss now consist of this fine-grained assemblage. Chalcopyrite prefers to form around the edges of the inclusions and also migrates into the minute cracks in the diamonds. It is the bulk composition of the Mss as encapsulated that is important for interpretation of diamond petrogenesis (P- versus E-type diamonds) and to the commonly used Re-Os dating technique. However, this bulk composition is definitely not attainable with polished sections cut through the inclusions. The assumption that the kernel of the sulfide inclusion for Re-Os age dating represents the entire original Mss may also be incorrect, depending what has been lost, mostly chalcopyrite, which has migrated into the surrounding cracks within the diamond host.     

Pyroxmangite: Stability in H2O-CO2 mixtures at a total pressure of 2000 bars

The curve of the reaction: 1 Rhodochrosite + 1 Quartz Pyroxmangite was determined experimentally using gas mixtures with different CO₂H₂O ratios at a total pressure of 2 kb. In pure CO₂, the equilibrium temperature is 508±2° C. By plotting the equilibrium data in terms of InX_CO2 versus reciprocal temperature, a H of reaction of 51.7 kcal was obtained. This plot shows a straight-line relationship, indicating internal consistency of the data, and is in good agreement with previous findings that CO₂ and H₂O mix ideally within the investigated temperature range (400–600° C). When applied to pyroxmangites formed during moderate greenschist facies metamorphism, these data indicate a high acitivity of H₂O.     

Experimental test of the calcite strain gage technique at low confining pressure

A series of experiments were conducted on Indiana limestone to test the applicability of calcite strain gage technique to deformations at low confining pressures. Experiments were conducted dry, at 13.8 MPa (138 bars, 2000 psi) confining pressure. These experiments indicate that both the least squares strain gage technique and the numerical dynamic analysis (NDA) technique locate the principal strain axes to within 5.0 degrees. Strains calculated by the least squares strain gage technique are accurate at low strains (less than 5%), and increasingly inaccurate at higher strains. At low strains, the calculated strain reflects the maximum strain (including elastic strain) experienced by the core. At higher strains, the increasing inaccuracy of calculated strain was accompanied by increasing negative expected values. In these experiments, microcracking played a significant part in deformation and effected twinning in calcite, and therefore the recorded strain.     

Dual-thermodynamic estimation of stoichiometry and stability of solid solution end members in aqueous–solid solution systems

The dual-thermodynamic (DualTh) approach is shown to provide a useful alternative to other methods in: (i) forward modelling of equilibrium speciation, activities, and element partitioning in a heterogeneous system involving several variable-composition phases, such as the aqueous–solid solution system; (ii) estimation of interaction parameters of a non-ideal mixing model from known bulk compositions of coexisting aqueous and solid-solution phases; and (iii) retrieval of unknown stoichiometries and apparent standard chemical potentials of trace solid-solution end-members. Inverse-modelling tasks (ii) and (iii) can be performed when the solid solution of interest is shown experimentally to co-exist with the aqueous phase either in the equilibrium or at the minimum stoichiometric saturation state.DualTh calculations exploit the ability of Gibbs energy minimisation (GEM) algorithms to find simultaneously two numerical solutions of the isobaric–isothermal chemical equilibrium speciation problem: (1) primal solution x — a vector of amounts of components (species) in phases; and (2) dual solution u — a vector of chemical potentials of stoichiometry units (usually chemical elements and charge). Conversely, the chemical potential of a phase component can be found in two complementary ways: (i) primal via its standard-state potential, concentration and activity coefficient (the latter two are functions of the x vector); and (ii) dual through its formula stoichiometry multiplied by the u vector. The DualTh methods compare primal and dual values of the chemical potential in simple and straightforward equations that can be easily computed in a spreadsheet, or implemented in GEM geochemical modelling codes.     

The energetics of natural garnet solid solution

Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)₃Al₂Si₃O₁₂, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C.     

The energetics of natural garnet solid solution

Approximate mixing properties of the dominant calcium silicate end-member components of natural garnets, namely grossularite, andradite and uvarovite, have been derived through theoretical thermodynamic and crystal chemical analysis, and appropriate reduction of the available experimental data. The stability of the solid solution with respect to phase separation in the ternary system has been analyzed. Finally, a general model is presented as to the approximate mixing properties of multicomponent natural garnet solid solution involving substitutions in both eight and six coordinated sites.     

低温盐侵蚀环境下砂浆孔隙水相变特性研究北大核心CSCD

砂浆孔隙溶液水盐相变特性是研究其水-热-盐耦合模型以及水泥基材料损伤机理的关键理论基础。为探究河西走廊盐渍土地区砂浆孔隙溶液水、盐相变规律,首先利用差示扫描量热法测定水盐相变过程中热流、相变温度等热参数;其次基于热量守恒和水分质量平衡方程,初步实现水盐分离;然后分别计算不同温度下的冰、盐晶和未冻水含量,揭示了低温下砂浆孔隙溶液水盐相变机理;最后结合微观扫描和物质能谱图,明晰了砂浆内部劣化机制。研究结果表明:随着盐浓度增大,砂浆孔隙溶液冻结温度降低、相变时间延后、水盐相变顺序调换及未冻水含量向低温方向移动;相同盐浓度下,氯化钠对砂浆孔隙溶液冻结温度的降幅最大,混合盐次之,硫酸钠最小;砂浆在低温盐侵蚀下遭受物理化学耦合作用,其中氯化钠对砂浆破坏性最大,混合盐次之,硫酸钠劣化相对较轻。     

Investigation on the effective remediation of quinoline at solid/solution interface using modified agricultural waste: an inclusive study

This research work explores the potential of modified agricultural waste for the sorption of quinoline from aqueous media. A quinoline removal efficiency of around 97 % and sorption capacities of ~20 (batch) and ~35 mg g^?1 (fixed-bed) were achieved. Pseudo-second-order kinetics and Temkin isotherm best represented the equilibrium sorption data. The sorption of quinoline is exothermic and spontaneous in nature with a slight increase in the system entropy. The quinoline sorption mechanism is controlled by H-bonding, π–π dispersive interactions, boundary layer, and intraparticle diffusion. Microwave–chemical integrated regeneration technique retrieves the sorption capacity of the exhausted sorbent with 99.15, 97.64, and 95.55 % of the original, in three sorption–regeneration cycles. Energy recovery (19.365 MJ kg^?1) from the quinoline-loaded sorbent and the potential utilization of left-over ash materials enhanced the prospective of the sorbent for the remediation of pollutants for a clean and green environment.     

Metamorphic incongruent solution,diffusion and pressure solution stripes

The formation of pressure solution stripes is discussed on the basis of previosly published observations and of local chemical equilibria between solution and solid phases. Chemically driven mineral reactions can be localized at early sites of nucleation and be enhanced by stresses to create patterns of cleavage stripes. These reactions cause transport of material over distances greater than single grains. They provide suitable sites and suitable chemistry for later incongruent pressure solution, potentially a much faster process than congruent pressure solution discussed in previous literature. Neither reaction nor straightforward incongruent pressure solution explains the solution of quartz, commonly observed at cleavage stripes. Quartz pressure solution is explicable on certain assumptions about diffusivities and the effects of stresses. This involves interaction between quartz and the established incongruent solution of other minerals, an overall process termed ‘interdependent pressure solution’. Electrical potential differences will be greatest during early mineral reactions, a feature which may provide a means to corroborate the chemical role in initiating ‘pressure solution stripes’.     

Experimental knife-edge pressure solution of halite

Pressure solution experiments were carried out, using a quartz knife-edge 0.26 mm wide on halite single crystals in halite saturated solutions, to observe the detailed development of pressure solution contacts and the rates of pressure solution. A rate of about 3 μm/day was observed for initial knife-edge stresses ranging from 4.5 to 15 MPa. Close examination of the contact leads to the conclusion that the mechanism of pressure solution is a combination of plastic deformation at the contact and free surface pressure dissolution near its periphery. Free surface pressure dissolution increases the contact stress to about 18 MPa, high enough to cause plastic deformation, by changing the area of contact. This mechanism differs from a water film diffusion mechanism, previously suggested by many authors, but is similar in some ways to the undercutting hypothesis of Bathurst (1958). We infer a steady state plastic deformation instead of catastrophic grain crushing at the contact. Free surface dissolution plus the plastic deformation mechanism may be primarily responsible for pressure solution in relatively porous rocks.     

酸性环境下生物成因施氏矿物稳定性研究

施氏矿物是酸性矿山废水中广泛存在的次生矿物,其形成和转化受环境pH值、温度和共存离子等条件影响。文中研究了酸性环境中生物成因施氏矿物和吸附了三价砷的生物成因施氏矿物,在不同温度和钾离子浓度条件下的稳定性。结果表明,老化温度的增加促进施氏矿物相的转变:4℃条件下,在15周的老化时间里,无砷及含砷施氏矿物均未发生相转变;而在40℃条件下,经过15周的老化,则无砷和含砷施氏矿物均发生了部分相转变。此外,钾离子浓度变化可以导致施氏矿物老化产物不同:生物成因施氏矿物在0.01mM钾离子条件下老化15周后的转化产物主要为针铁矿,在100mM钾离子条件下老化产物为黄钾铁矾和针铁矿。含As(Ⅲ)施氏矿物在0.01mM钾离子条件下老化15周后没有发生相转变,在100mM钾离子条件下发生了部分相转变,产物为黄钾铁矾。生物成因施氏矿物中的As(Ⅲ)使得矿物在环境中更加稳定。     

Melt inclusions in pegmatite quartz: complete miscibility between silicate melts and hydrous fluids at low pressure

Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth: a silicate-rich H₂O-poor melt and a silicate-poor H₂O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H₂O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions define a solvus boundary in an X_H2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H₂O, and the conjugate water-rich melt about 47 wt% H₂O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively. Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H₂O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable to an aqueous fluid. Received: 15 September 1999 / Accepted: 10 December 1999     

Strength of solid pressure media and implications for high pressure apparatus

The stress-strain properties of talc, pyrophyllite, silver chloride, sodium chloride, boron nitride and graphite have been measured under confining pressures up to 8 or 10 kb at room temperature, and, in the case of talc, also at temperatures up to 900° C. The extrapolation and application of these results to solid medium high pressure apparatus of pistoncylinder type is discussed and a calculation made of the correction to nominal pressure (friction correction), taking into account the stress gradients in the medium and the shearing between the medium and the cylinder wall. Correction to the nominal differential stress measured in solid medium stress-strain apparatus is also discussed.