Thermodynamics and behavior of γ-Mg2SiO4 at high pressure: Implications for Mg2SiO4 phase equilibrium

Raman spectra of γ-Mg₂SiO₄ taken to 200 kbar were used to calculate entropy and heat capacity at various P-T conditions. These new thermodynamic data on γ-MgSiO₄, similar data on MgSiO₃ perovskite (pv), previous data on β-MgSiO₄ and MgO (mw), and previous volumetric data of all phases were used to calculate the phase boundaries in the Mg₂SiO₄ phase diagram. Our resulting slope for the β→γ transition (50±4 bar K^-1) is in excellent agreement with recent multi-anvil studies. The slopes for the β→pv+MgO and γ→pv+MgO are-7±3 and -25±4 bar K^-1, respectively, and are consistent with our CO₂ laser heated diamond anvil studies. These slopes result in a β-γ-MgO+pv triple point at approximately 229 kbar and 2260 K for the iron free system.     

High pressure Ca2SiO4, the silicate K2NiF4-isotype with crystalchemical and geophysical implications

The first silicate possessing a K₂NiF₄-type structure (Ca₂SiO₄) has been synthesized at loading pressures between 220 and 260 kbar and a temperature of about 1000° C in a diamond-anvil press coupled with a YAG laser heater. The lattice parameters for Ca₂SiO₄ (K₂NiF₄-type) area=3.564±0.002 andc=11.66±0.01 Å at room temperature and 1 bar pressure, and the molar volume is 44.57±0.05 cm³. The lattice parameter for the non-quenchable high-pressure perovskite modification of CaSiO₃ is estimated to be 3.56±0.03 Å at STP conditions. To date, A₂BX₄ compounds possessing the K₂NiF₄-type structure arein all cases less dense than their corresponding mixtures of ABX₃ and AX compounds possessing, respectively, the perovskite (or related structures) and rocksalt structures. Hence the K₂NiF₄ structure is unstable relative to the mixture perovskite plus rocksalt at high pressures. For example, in a preliminary experimental study Ca₂GeO₄ in the K₂NiF₄-type structure has been found to transform to an as-yet-undetermined phase or assemblage at pressures between 200 and 250 kbar and at about 1000° C. It is concluded that a similar phase transformation might also occur in Ca₂SiO₄ (K₂NiF₄ type) but that the K₂NiF₄-type structure would not be adopted by Mg₂SiO₄ in the earth's mantle.     

High pressure polymorphism of dicalcium silicate Ca2SiO4. A transmission electron microscopy study

Ca₂SiO₄ dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca₂SiO₄ (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca₂SiO₄ usually produced with high temperatures, including α-Ca₂SiO₄, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca₂SiO₄ and α-Ca₂SiO₄ polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca₂SiO₄ has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca₂SiO₄ into CaSiO₃ and CaO is reported.     

Forsterite-diopside-spinel-liquid equilibria in the system CaO−MgO−Al2O3−SiO2 at 20 kbar and petrological applications

Liquidus phase relationships in the CaAlAl–SiO₆–Mg₂SiO₄–CaMgSi₂O₆–CaAlSi₂O₈ portion of the simplified basalt tetrahedron in the CaO–MgO–SiO₂–Al₂O₃ system have been experimentally determined at 20 kbar pressure. The fo+di _ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di _ss+ en_ss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di _ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo₁₁Di₅₆An₃CaTs₃₀, very close to and slighthly to the SiO₂-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e _ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di _ss+ sp. Compositions of di _ss in equilibrium with the liquids from the temperature maximum to the fo+di _ss+en_ss+ sp+li invariant point range from Di₆₆En₉CaTs₂₅ to Di₃₆En₄₀CaTs₂₄. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di _ss-sp-li univariant curve onto the sp-di _ss divariant surface. Crystallization of di _ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al₂O₃–CaSiO₃–MgSiO₃ plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO₂-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO₂-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al₂O₃ join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations di CaMgSi₂O₆ - fo Mg₂SiO₄ - an CaAl₂Si₂O₈ - CaTs CaAlAlSiO₆ - sp MgAl₂O₄ - en MgSiO₃ - wo CaSiO₃ - gt Ca₃Al₂Si₃O₁₂–Mg₃Al₂Si₃O₁₂ - qz SiO₂ - li Liquid - gl glass - ss Solid Solution - A An mxn matrix - X A column vector - kbar kilobar     

The crystal structure of Ca5(PO4)2SiO4 (Silico-Carnotite)

Summary The crystal structure of Ca₅(PO₄)₂SiO₄ (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR _w =0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) Å,b=15.508 (2) Å andc=10.132 (1) Å at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm^–3. The crystal contains about 2.5% V₂O₅ as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO₄ ^3–, SiO₄ ^4– and possibly VO₄ ^3– occurs among the anion sites. The structure has some relationship to that of Ca₅(PO₄)₃OH, the predominant inorganic phase in the human body, but suggests that the Ca₅(PO₄)₃OH type structure may not be stable without some of the OH positions being filled. Ca₅(PO₄)₂SiO₄ is more closely related to K₃Na(SO₄)₂ (glaserite) if it is considered that there are systematic cation vacancies in Ca₅(PO₄)₂SiO₄.This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca₂SiO₄ and Ca₃(PO₄)₂ and between Ca₃(PO₄)₂ and CaNaPO₄.

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Die Kristallstruktur vonCa ₅(PO ₄)₂ SiO ₄ (Silicocarnotit)

Zusammenfassung Die Kristallstruktur von Ca₅(PO₄)₂SiO₄ (Silicocarnotit) wurde aus 3358 Röntgendiffraktometer-Daten bestimmt und in Raumgruppe Pnma aufR _w =0,038,R=0,045 verfeinert. Die Gitterkonstanten (bei 24° C) sind:a=6,737 (1) Å,b=15,508 (2) Å undc=10,132 (1) Å,Z=4; D_obs.=3,06 g · cm^–3, D_exp.=3,03 g · cm^–3. Der Kristall enthält etwa 2,5% V₂O₅ als Verunreinigung. Die Bindungslängen in den tetraedrischen Anionen legen nahe, daß unter den Anionenplätzen gegenseitige Vertretung oder Unordnung von PO₄ ^3–, SiO₄ ^4– und möglicherweise VO₄ ^3– auftritt. Die Struktur zeigt einige Verwandtschaft zu der von Ca₅(PO₄)₃OH, der wichtigsten anorganischen Substanz im menschlichen Körper, weist aber darauf hin, daß eine Struktur vom Ca₅(PO₄)₃OH-Typ ohne Besetzung eines Teiles der OH-Position nicht stabil ist. Ca₅(PO₄)₂SiO₄ zeigt engere Beziehungen zu K₃Na(SO₄)₂ (Glaserit), wenn man berücksichtigt, daß in Ca₅(PO₄)₃SiO₄ systematische Kationen-Leerstellen sind. Dieser Strukturtyp ist mit derAuffassung in Übereinstimmung, daß Kationenleerstellen für die festen Lösungen zwischen Ca₂SiO₄ und Ca₃(PO₄)₂ und zwischen Ca₃(PO₄)₂ und CaNaPO₄ verantwortlich sind.

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With 9 Figures     

Spinelloid phases in the system MgGa2O4-Mg2GeO4

Spinelloid phases have been observed and characterized by powder X-ray diffraction and high-resolution electron microscopy. Mg₃Ga₂GeO₃(III), with a narrow composition range of approximately 3 mole percent Mg₂GeO₄, is stable at atmospheric pressure up to about 1,420° C, and is isostructural with β-Mg₂SiO₄ and the spinelloid Phase III of the NiAl₂O₄-Ni₂SiO₄ system. This represents the first occurrence of a β-phase structure stable at 1 atm pressure. Above 1,420° C (1 atm) Mg₃Ga₂GeO₈ (III) decomposes reversibly into a mixture of spinel and olivine. At high pressure (around 30 kbar at 1,100° C) it transforms into another spinelloid phase, Mg₃Ga₂GeO₈ (IV), isostructural with Phase IV of the NiAl₂O₄-Ni₂SiO₄ system. In terms of crystal structures and phase relations therefore there exists a close analogy between the magnesium gallo-germanate and nickel alumino-silicate systems, the former being a lower-pressure analogue of the latter. Our investigation of a number of other pseudo-binary spinel-olivine oxide systems suggests that the formation of spinelloid phases can be associated with the inverse character of the spinel component.     

New experimental data for the system CaO-MgO-SiO2-H2O and a synthesis of inferred phase relations

Subsolidus and vapor-saturated liquidus phase relations for a portion of the system CaO-MgO-SiO₂-H₂O, as inferred from experimental data for the composition regions CaMgSi₂O₆-Mg₂SiO₄-SiO₂-H₂O and CaMgSi₂O₆-Mg₂SiO₄-Ca₃MgSi₂O₈ (merwinite)-H₂O, are presented in pressure-temperature projection. Sixteen invariant points and 39 univariant reactions are defined on the basis of the 1 atm and 10 kbar (vapor-saturated) liquidus diagrams. Lack of experimental control over many of the reactions makes the depicted relations schematic in part.An invariant point involving orthoenstatite, protoenstatite, pigeonite, and diopside (all solid solutions) occurs at low pressure (probably between 1 and 2 kbar). At pressures below this invariant point, orthoenstatite breaks down at high temperature to the assemblage diopside + protoenstatite; with increasing temperature, the latter assemblage reacts to form pigeonite. At pressures above the invariant point, pigeonite forms according to the reaction diopside + orthoenstatite = pigeonite, and the assemblage diopside + protoenstatite is not stable. At 1 atm, both pigeonite and protoenstatite occur as primary liquidus phases, but at pressures above 6–7 kbar orthoenstatite is the only Ca-poor pyroxene polymorph which appears on the vapor-saturated liquidus surface.At pressures above approximately 10.8 kbar, only diopside, forsterite, and merwinite occur as primary liquidus phases in the system CaMgSi₂O₆-Mg₂SiO₄-Ca₃MgSi₂O₈-H₂O, in the presence of an aqueous vapor phase. At pressures between 1 atm and 10.2 kbar, both akermanite and monticellite also occur as primary liquidus phases. Comparison of the 1 atm and 10 kbar vapor-saturated liquidus diagrams suggests that melilite basalt bears a low pressure, or shallow depth, relationship to monticellite-bearing ultrabasites.     

Calorimetric study of olivine solid solutions in the system Mg2SiO4-Fe2SiO4

Solution enthalpies of synthetic olivine solid solutions in the system Mg₂SiO₄-Fe₂SiO₄ have been measured in molten 2PbO·B₂O₃ at 979 K. The enthalpy data show that olivine solid solutions have a positive enthalpy of mixing and the deviation from ideality is approximated as symmetric with respect to composition, in contrast to the previous study. Applying the symmetric regular solution model to the present enthalpy data, the interaction parameter of ethalpy (W_H) is estimated to be 5.3±1.7 kJ/mol (one cation site basis). Using this W_h and the published data on excess free energy of mixing, the nonideal parameter of entropy (W_s) of olivine solid solutions is estimated as 0.6±1.5 J/mol·K.     

Single crystal hydrostatic compression of (Mg,Mn,Fe,Co)2SiO4 olivines

Four synthetic endmember olivines (Mg,Mn, Fe,Co)₂SiO₄ with space group Pbnm were loaded together in one diamond cell mount. Their unit-cell parameters were determined by single crystal X-ray diffraction to 10 GPa. The linear compressibilities β_a, β_b, β_c were 1.53, 2.90, 2.32; 1.45, 3.48, 1.98; 1.35, 3.29, 1.76; and 1.25, 2.82, 2.01×10⁻³ GPa⁻¹ for Mg₂SiO₄, Mn₂SiO₄, Fe₂SiO₄ and Co₂SiO₄, respectively. The b axis is the most compressible direction in all crystals studied. Bulk modulus K_T0 and its first pressure derivative were simultaneously determined for Mg₂SiO₄, Fe₂SiO₄ and Co₂SiO₄ crystals respectively by fitting volume data to a third order Birch-Murnaghan equation of state. They are 127(4) and 4.2(8), 136(3) and 4.1(7), and 144(2) and 4.1(5). The K_T0 and could not simultaneously be determined unambiguously for Mn₂SiO₄. Direct comparisons of unit-cell volumes at high pressure among pairs of olivines reveal anomalous compression behavior of the Mg₂SiO₄ crystal regarding the bulk modulus-volume relationship. This behavior, however, could not be observed in the transition metal olivines (Mn,Fe,Co)₂SiO₄. The distinct electronic configurations of Mg²⁺ and the transition metal cations Mn²⁺, Fe²⁺, and Co²⁺ result in the different compression behaviors of Mg₂SiO₄ and (Mn,Fe,Co)₂SiO₄. Received: 14 April 1997 / Revised, accepted: 29 July 1997     

The system Mg2SiO4-Ca2SiO4-CaAl2O4-NaAlSiO4-SiO2: one atmosphere liquidus equilibria of analogs of alkaline mafic lavas

One atmosphere liquidus relationships in the system Mg₂SiO₄ (Fo)–Ca₂SiO₄ (La)–NaAlSiO₄ (Ne)–CaAl₂O₄ (CA)–SiO₂ (Sil) are presented as analogs for alkaline mafic lavas. Liquidus diagrams are constructed from electron microprobe analyses of quenched liquids and coexisting mineral phases produced in melting experiments and they are depicted in terms of sub-projections within the pseudo-quaternary system Fo–La–[Ne,CA]–Sil. The Ne and CA components are combined to create a normative feldspathoid component defined as Ne#=Ne/[Ne+CA]. Liquidus relations at Ne#=0.5 from this study are compared to relations at Ne#=0.0 and 1.0 from previous studies. In general, liquidus temperatures decrease and positions of liquidus boundaries involving feldspathic phases shift toward the [Ne, CA] component as Ne# increases. The pseudoinvariant points fo+di+pl+mel+l and fo+pl+mel+sp+l exist at Ne#=0.5. These equilibria between forsterite-plagioclase-melilite-liquid are not present in the system when Ne#=1.0 because a boundary curve (fo+di+ne+l) separates the plagioclase and melilite liquidus fields. The liquidus diagrams provide useful analogs for the crystallization sequences of natural primary alkali olivine basalts, basanitoids, basanites, olivine nephelinites, olivine-melilite nephelinites and olivine melilitites.     

Molecular dynamics simulations of pressure and temperature effects on MgSiO3 and Mg2SiO4 melts and glasses

Ab-initio interionic potentials for Mg²⁺, Si⁴⁺, and O^2– have been used in molecular dynamics (MD) simulations to investigate diffusivity changes, pressure-induced structural transitions, and temperature effects on polymerization in MgSiO₃ and Mg₂SiO₄ melts and glasses. The potential gives reasonable agreement with the 0.1 MPa radial distribution function of MgSiO₃ glass. Maxima in the diffusion coefficients of Si⁴⁺ and O^2– occur as pressure is increased on the MgSiO₃ melt. The controlling structural mechanism for this behavior is the Q¹ species of SiO₄ tetrahedra. Mg²⁺ diffusion coefficients decrease monotonically with pressure in both melt compositions. Increasing Mg²⁺ coordination number and population of 3- and 4-membered SiO₄ rings with pressure combine to hinder translation of the Mg²⁺ ions. The dominant changes in structure with pressure are a decrease in the intertetrahedral (Si-O--Si) angle up to approximately 4 g/cm³ and coordination changes of the ions above this density. Temperature effects on viscosity in these simulated melts are indirectly studied by analyzing polymerization changes with temperature. Polymerization and coordination numbers increase with decreasing temperature and a small quench rate effect is observed. Fair agreement is found between the MD simulations and experimental equation of state for Mg₂SiO₄, but the equation of state predictions for MgSiO₃ melts are much less accurate. The zero pressure volume, V ₀, is significantly higher and K ₀ is lower in the simulations than empirical values. The inadequacies reflect error in using the ionic approximation for polymerized systems and a need to collect more data for a variety of molecular configurations in the development of ab-initio potentials.     

An experimental study of glaucophanic amphiboles in the system Na2O-MgO-Al2O3-SiO2-SiF4 (NMASF): some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures

The phase relations of glaucophanic amphiboles have been studied at 18–31 kbar/680–950°C in the synthetic system Na₂O–MgO–Al₂O₃–SiO₂–SiF₄ (NMASF) using the bulk composition of fluor-glaucophane, Na₂Mg₃Al₂Si₈O₂₂F₂. Previous experimental studies of glaucophane in the water-bearing system (NMASH) have been hampered by problems of fine grain size (electron microprobe analyses with low oxide totals and contamination by other phases), and consequently good compositional data are lacking. Fluor-amphiboles, on the other hand, generally have much higher thermal stabilities than their hydrous counterparts. By using the fluorine-analogue system NMASF, amphibole crystals sufficiently coarse for electron microprobe analysis have been obtained. Furthermore, NMASH amphibole phase relations are directly analogous to those of the NMASF system because SiF₄ fills the role of H₂O as the fluid species. High-pressure NMASF amphibole parageneses are comparable to those obtained for NMASH amphiboles under similar pressure-temperature conditions, except that the NMASF solidus was not encountered. In the pressure-temperature range of the NMASF experiments, fluor-glaucophane is unstable relative to glaucophanenyböite-Mg-magnesio-katophorite amphiboles. Variations in synthetic fluor-amphibole composition with P and T are discussed in terms of changes in the thermodynamic activities of the principal amphibole end-members, such as glaucophane (a_Gp) and nyböite (a_Ny) using an ideal-mixing-on-sites model. The most glaucophanic amphiboles analysed have a_Gp=0.50–0.60 and coexist with jadeite and coesite at 30 kbar/800°C. Amphiboles become increasingly nyböitic with decreasing pressure through the NaAlSi_-1 exchange, which is the principal variation observed. The most nyböitic amphiboles have a_Ny =0.65–0.70 and coexist with fluor-sodium-phlogopite and quartz at 21–24 kbar/800–850°C. At 800°C amphiboles are essentially glaucophane-nyböite solid solutions. At 850°C there is some minor displacement along MgMgSi_-1, but Mg-magnesio-katophorite activities are very low (<0.06). Activities of the eight other NMASF amphibole end-members are <0.001, except for eckermannite activity which varies from 0.01–0.11. Our results indicate that: (a) synthetic amphiboles mimic the essential stoichiometries observed in blueschist amphiboles; (b) synthetic studies should be relevant to petrologically important high-pressure parageneses and reactions involving glaucophanicamphiboles, sodic pyroxenes, albite and talc; (c) the high-pressure stability limit of fluorglaucophane lies at pressures higher than those reached in this study (31 kbar); (d) in natural systems an approach to glaucophane stoichiometry should be favoured by high water activities as well as high pressures.Abbreviations and formulae used in this paper Glaucophane (Gp) oNa₂(Mg₃Al₂)Si₈O₂₂(OH,F)₂ - Nyböite (Ny) NaNa₂(Mg₃Al₂)Si₇AlO₂₂(OH,F)₂ - Eckermannite (Ek) NaNa₂(Mg₄Al)Si₈O₂₂(OH,F)₂ - Magnesio-cummingtonite (MC) oMg₂(Mg₅)Si₈O₂₂(OH,F)₂ - Sodium-magnesio-cummingtonite (SMC) NaNaMg(Mg₅)Si₈O₂₂(OH,F)₂ - Sodium-anthophyllite (SAn) NaMg₂(Mg₅)Si₇AlO₂₂(OH,F)₂ - Gedrite (Gd) oMg₂(Mg₃Al₂)Si₆Al₂O₂₂(OH,F)₂ - Sodium-gedrite (SGd) NaMg₂(Mg₄Al)Si₆Al₂O₂₂(OH,F)₂ - Mg-magnesio-aluminotaramite (MAT) NaNaMg(Mg₃Al₂)Si₆Al₂O₂₂(OH,F)₂ - Mg-magnesio-katophorite (MKt) NaNaMg(Mg₄Al)Si₇AlO₂₂(OH,F)₂ - Mg-magnesio-barroisite (MBa) oNaMg(Mg₄Al)Si₇AlO₂₂(OH,F)₂ - Jadeite (Jd) NaAlSi₂O₆ - Enstatite (En) Mg₂Si₂O₆ - Forsterite (Fo) Mg₂SiO₄ - Nepheline (Ne) NaAlSiO₄ - Albite (Ab) NaAlSi₃O₈ - Quartz/Coesite (Qz/Co) SiO₂ - Sodium-phlogopite (Sphl) NaMg₃Si₃AlO₁₀(OH,F)₂ - Talc (Tc) oMg₃Si₄O₁₀(OH,F)₂ - o vacant A-site in amphiboles and interlayer site in talc. Octahedral cations in amphiboles are bracketted     

The effect of crystal field stabilization on the olivine→spinel transition in the system Mg2SiO4 - Fe2SiO4

Crystal field stabilization (CFS) plays a significant role in determining equilibrium phase boundaries in olivine→spinel transformations involving transition-metal cations, including Fe²⁺ which is a major constituent of the upper mantle. Previous calculations for Fe₂SiO₄ ignored pressure and temperature dependencies of crystal field stabilization enthalpies (CFSE) and the electronic configurational entropy (S _CFS). We have calculated free energy changes (ΔG _CFS) due to differences of crystal field splittings between Fe₂SiO₄ spinel and fayalite from: ΔG _CFS=?ΔCFSE?TΔS _CFS, as functions of P and T, for different energy splittings of t _2g orbital levels of Fe²⁺ in spinel. The results indicate that ΔG _CFS is always negative, suggesting that CFS always promotes the olivine→spinel transition in Fe₂SiO₄, and expands the stability field of spinel at the expense of olivine. Because of crystal field effects, transition pressures for olivine→spinel transformations in compositions (Mg_1?x Fe _x )₂SiO₄ are lowered by approximately 50x kbar, which is equivalent to having raised the olivine→spinel boundary in the upper mantle by about 15 km.     

Tetrahedral compression in (Mg,Fe)SiO3 orthopyroxenes

Single-crystal structure determinations at pressure have shown that the structural response of synthetic (Mg_0.6Fe_0.4)SiO₃ orthopyroxene to compression is the same as that previously observed in MgSiO₃ orthoenstatite. At pressure below ～4?GPa there is no significant compression of the SiO₄ tetrahedra, while above ～4?GPa the tetrahedra decrease in volume as a result of Si?O bond shortening. A study of the compressional behaviour of synthetic FeSiO₃ orthoferrosilite also shows the same behaviour below 4?GPa, but studies at higher pressures are precluded due to the transformation of the sample to the higher density C2/c high-clinoferrosilite polymorph. A further single-crystal study to 6?GPa of a Ca²⁺-containing natural orthopyroxene shows that chemical substitution of, primarily, Al³⁺ and Ca²⁺ into the structure of orthopyroxene inhibits the initial rapid compression of the M2?O3 bonds observed in the synthetic samples, and no significant tetrahedral compression is observed in this sample. Raman data collected from synthetic MgSiO₃ orthoenstatite show that there is a change in the rate of change of frequency with pressure, δν/δP, between 3.5 and 6.0?GPa, but no changes in the number of observed bands. These observations indicate a non-symmetry-breaking change in the properties of the orthoenstatite, which is associated with the change in compression mechanism observed using X-ray diffraction techniques at this pressure.     

Importance of considerations of mixing properties in establishing an internally consistent thermodynamic database: thermochemistry of minerals in the system Mg2SiO4-Fe2SiO4-SiO2

A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg₂SiO₄-Fe₂SiO₄ and Mg₂Si₂O₆-Fe₂Si₂O₆. The formulation makes explicit provision for nonconvergent ordering of Fe²⁺ and Mg²⁺ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mössbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phases-quartz,-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg₂SiO₄-Fe₂SiO₄-SiO₂ over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)₂SiO₄ olivines, (Fe, Mg)₃Al₂Si₃O₁₂ garnets, (Fe, Mg)Al₂O₄ and (Fe, Mg)Fe₂O₄ spinels, and K(Mg, Fe)₃AlSi₃O₁₀(OH)₂ biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of trace elements (Al³⁺ and Fe³⁺) to the thermodynamic formulation for orthopyroxenes.     

Stability relations in the system CaSiO3-CaMnSi2O6-CaFeSi2O6

In the system CaSiO₃-CaMnSi₂O₆-CaFeSi₂O₆ extensive miscibility gaps between pyroxenoids and clinopyroxenes are observed. The miscibility gap between Mn-bustamite and Mn-wollastonite has been determined experimentally by a hydrothermal technique between 400° and 1200° C at P _f= 2 kbar. Further experiments have been performed at P _f=9 kbar, which revealed a shifting of the miscibility gap towards more Ca-rich compositions. The bustamite phase is stabilized by high pressures and the wollastonite structure is the stable phase at high temperatures.Similar phase relations as along the join CaSiO₃-CaMnSi₂O₆ exist along the join CaSiO₃-CaFeSi₂O₆ but with a more extensive two-phase field of bustamite-clinopyroxene.Possible phase relations along the joins CaSiO₃-CaMnSi₂O₆, CaSiO₃-CaFeSi₂O₆ and CaFeSi₂O₆-CaMnSi₂O₆ are given in temperature-composition diagrams for low pressures, based on natural and experimental data.     

Structure of Cr2SiO4 and possible metal-metal interactions in crystal and melt

Cr₂SiO₄ has been prepared both as quench crystals and as an apparently stable subsolidus phase, at T=1600 and 1650 °C, P=37 kbar on Cr₂SiO₄ and Cr-SiO₃ bulk compositions. Crystal structure determination by Rietveld analysis of X-ray powder data from 3 samples show the structure, in space group Fddd, to be related to that of Cd₂SiO₄ (thenardite Na₂SO₄-V structure) but with approximately “square planar” coordination of the Cr²⁺ (d ⁴) atom. In one of the axial ligand positions, normal to the strongly bonded CrO₄ equatorial plane, is found a Cr atom (Cr-Cr=2.75 Å). Stereochemistry permits this contact to be a weak metal-metal bond, though it cannot be ruled out to be a non-bonded result of polyhedral connectivity.     

A computer simulation study of OH defects in Mg2SiO4 and Mg2GeO4 spinels

Classical atomistic simulation techniques have been used to investigate the energies of hydrogen defects in Mg₂SiO₄ and Mg₂GeO₄ spinels. Ringwoodite (γ-Mg₂SiO₄) is considered to be the most abundant mineral in the lower part of the transition zone and can incorporate large amounts of water in the form of hydroxyls, whereas the germanate spinel (γ-Mg₂GeO₄) corresponds to a low-pressure structural analogue for ringwoodite. The calculated defect energies indicate that the most favourable mechanisms for hydrogen incorporation are coupled either with the reduction of ferric iron or with the creation of tetrahedral vacancies. Hydrogen will go preferentially into tetrahedral vacancies, eventually leading to the formation of the hydrogarnet defect, before associating with other negatively charged point defects. The presence of isolated hydroxyls is not expected. The same trend is observed for germanate, and thus γ-Mg₂GeO₄ could be used as a low-pressure analogue for ringwoodite in studies of water-related defects and their effect on physical properties.     

The disproportionation of andalusite (Al2SiO5) to Al2O3 and SiO2 under shock compression

Shock-recovery experiments have been carried out on andalusite single crystals of gem quality in a pressure range from 300 up to 575 kbar. Infrared spectroscopic investigations indicate a progressive shock-induced transformation of andalusite into short-range-ordered Al₂O₃ and SiO₂ phases within a pressure interval from ～360 to ～575 kbar. Exposure to dynamic pressures of about 575 kbar results in andalusite breaking down into incoherently crystallized γ-Al₂O₃, well-crystallized α-Al₂O₃ and X-ray amorphous SiO₂. The shock disproportionation of andalusite is presumed to take place in three separate stages of reaction. The comparison of shock-induced reactions with results from static experiments on kyanite indicates significant differences in the transformation pressures and in the mechanism of the high pressure decomposition.