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.     

Calorimetric study of the stability of spinelloids in the system NiAl2O4-Ni2SiO4

Enthalpies of solution in molten 2 PbO · B₂O₃ at 974 K were measured for four spinelloids, phases I (0.75 NiAl₂O₄ · 0.25 Ni₂SiO₄), II (0.60 NiAl₂O₄ · 0.40 Ni₂SiO₄), III and IV (0.50 NiAl₂O₄ · 0.50 Ni₂SiO₄) in the system NiAl₂O₄ · Ni₂SiO₄. The enthalpies (in cal per 4-oxygen mol) of formation from NiAl₂O₄ and Ni₂SiO₄ spinels are: phase I, 945±366; phase II, 1072±360; phase III, 2253±390; phase IV, 3565±544. Using these enthalpy data in combination with phase relations at high pressure at 1373 K, positive entropies of formation of the spinelloids from NiAl₂O₄ and Ni₂SiO₄ spinels were estimated (in cal mol^?1 K^?1): phase I, 1.2; phase II, 1.5; phase III, 2.0–2.3; phase IV, 3.0–3.1. The thermochemical data obtained above suggest that the spinelloids are “entropy-stabilized” phases with partially disordered cation distributions. The configurational entropies of the spinelloids were calculated based on the observed cation distribution in each spinelloid phase. The positive entropies of formation of the spinelloids from the spinel endmembers are due primarily to the configurational entropies although small positive vibrational entropy changes may also exist.     

Subsolidus Phase Relations in the System MgO-SiO_2-Cr-O and Thermodynamic Properties of Related Cr~(2+)-Containing End-members

Subsolidus phase relations have been determined in the systems SiO2-Cr-O and MgO-SiO2-Cr-O in equilibrium with metallic Cr, at 1100 to 1500℃ and 0 to 2.88 GPa. The results show that there are no ternary phases in the SiO2-Cr-O system at these conditions, i.e., only the assemblage eskolaite-Cr-metal-quartz (or tridymite) is found. In the MgO-containing system, however, extensive substitution of Cr2+ for Mg is observed in (Mg, Cr2+)2SiO4 olivine, (Mg, Cr2+)2Si2O6 pyroxene, and (Mg, Cr2+)Cr2O4 spinel. Cr3+ levels in olivine and pyroxene are below detection limits. The pyroxene is orthohombic at XCrPx2+ < 0.2, monoclinic at higher XCrPx2+ . Thestructure of the spinels becomes tetragonally distorted at XCr2+Sp >0.2. The experimental datahave been fitted to a thermodynamic model, and the authors obtained the mixing parameter (W) of Mg-Cr2+ in olivine, pyroxene and spinel, and the relation between temperatures and free energies of formation for the end-members: Cr2+-olivine (Cr2SiO4), Cr2+-pyroxene (Cr2Si2O6)     

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.     

Crystal structures of Ni2SiO4 and Fe2SiO4 as a function of temperature and heating duration

X-ray structure refinements of Ni₂SiO₄ and Fe₂SiO₄ spinels have been made as a function of temperature and heating duration by intensity measurements at high temperatures and room pressure. The lattice parameters of Ni₂SiO₄ spinel linearly increased with temperature up to 1,000° C. However, Fe₂SiO₄ spinel exhibited a nonlinear thermal expansion and was converted to a polycrystalline mixture of spinel and olivine by heating of less than one-hour at 800° C. The ratios between the octahedral and tetrahedral bond lengths D _oct/D _tetr and between the shared and unshared edge distances (O-O)_sh/(O-O)_unsh in Fe₂SiO₄ spinel were both much larger than those in Ni₂SiO₄. These ratios increase with temperature. The Fe₂SiO₄ spinel more readily approached a activation state which facilitated the transition to the olivine structure than the Ni₂SiO₄ spinel. The lattice parameter of Ni₂SiO₄ spinel decreased with heating period at constant temperatures of 700° C and 800° C. The parameter of the quenched sample after heating for 52 h at 700° C was smaller than that of the nonheated sample. The refinements of the site occupancies at each heating duration indicated an increase in the cation deficiency in both tetrahedral and octahedral sites. Electron microprobe analysis, however, proved no significant difference in the chemical compositions between the quenched and nonheated samples. Si and Ni atoms displaced from normally occupied spinel lattice sites are assumed to settle in vacant sites defined by the cubic close packed oxygen sublattice in a manner which preserves the electric neutrality of the bulk crystal.     

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.     

Calorimetric measurements of fusion enthalpies for Ni2SiO4 and Co2SiO4 olivines and application to olivine-liquid partitioning

Calorimetric measurements of fusion enthalpies for Ni₂SiO₄ and Co₂SiO₄ olivines were carried out using a high-temperature calorimeter, and Ni and Co partitioning between olivine and silicate liquid was analyzed using the measured heats of fusion. The fusion enthalpy of Co₂SiO₄ olivine measured by transposed-temperature drop calorimetry was 103 ± 15 kJ/mol at melting point (1688 K). The fusion enthalpy of Ni₂SiO₄ olivine was calculated based on the enthalpies of liquids in the system An₅₀Di₅₀-Ni₂SiO₄ measured by transposed-temperature drop calorimetry at 1773 K, and was 221 ± 26 kJ/mol at its metastable melting point (1923 K). The fusion enthalpy of Ni₂SiO₄ is the largest among those of olivine group, this is caused by the large crystal field stabilization energy of six-coordinated Ni²⁺ in olivine. The larger fusion enthalpy of Ni₂SiO₄ can account for the large and variable partition coefficient of Ni between olivine and silicate liquid. Based on the comparison between partition coefficients calculated from thermodynamic data and those observed in partition experiments, it is considered that the magnitude of partition coefficients is primarily dependent on the heats of fusion of the components. Furthermore, the activity coefficients for Ni-, Co- and Mn-bearing components in magmatic liquid are nearly of the same magnitude.     

Cation ordering in NiAl2O4 spinel by a 111 systematic row CBED technique

A focussed probe wide angle systematic row CBED technique has been used to determine the degree of cation inversion, i.e. the magnitude of the cation ordering parameter x, in (Ni_{1− x} Al _x ) [Ni _{x /2}Al_{1− x /2}]₂O₄ spinel to an estimated accuracy of ±0.1. For comparison purposes, the same technique has also been applied to an MgCr₂O₄ spinel (where very little cation inversion is expected). The simplicity of this CBED technique in conjunction with the fact that the experimental data can be obtained from small illuminated areas several tens to 100 nm in diameter, suggests that it may be a very useful technique for estimating the extent of cation inversion in multi-phase mineralogical specimens containing spinels. Received: 19 October 1998 / Revised, accepted: 14 June 1999     

Comparison of the raman microprobe spectra of (Mg,Fe)2SiO4 and Mg2GeO4 with olivine and spinel structures

Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg₂GeO₄ and (Mg, Fe)₂SiO₄. The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase.     

Synthesis of kosmochlor and phase equilibria in the join CaMgSi2O6-NaCrSi2O6

Kosmochlor (NaCrSi₂O₆) was synthesized by the flux method from melts along the join Na₂O·2 SiO₂-Na₂O·Cr₂O₃·4 SiO₂ at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi₂O₆-NaCrSi₂O₆ was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr₂O₃) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.     

High-temperature creep in a Ni2GeO4: a contribution to creep systematics in spinel

 High-temperature creep behavior in Ni₂GeO₄ spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4 microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to 1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following flow laws: where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion (Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less than 2 μm. Received: 18 June 2000 / Accepted: 3 April 2001     

Thermochemistry of phases in the system MgGa2O4-Mg2GeO4

The enthalpies of solution in molten 2PbO · B₂O₃ of phases synthesized at one atmosphere in the system MgGa₂O₄-Mg₂GeO₄ have been measured. A spinel solid solution, which is stable at 1400 °C from the MgGa₂O₄ end-member to 27 mole percent Mg₂GeO₄, shows endothermic heats of mixing of up to 10 kJ/mole at the solubility limit. The spinelloid phase, Mg₃Ga₂GeO₈, is energetically less stable than a mixture of terminal spinel solid solutions (0.73 MgGa₂O₄·0.27 Mg₂GeO₄(sp)+Mg₂GeO₄(sp)), by 3.63±3.64 kJ/mole. This indicates that the spinelloid is a high-entropy phase.The volume of the spinel solid solution, MgGa₂O₄-Mg₂GeO₄, shows a positive deviation from Vegard's Law. Modeling of the cation distribution in the solid solution indicates that this V is due to a change in the spinel type from inverse towards normal as the Mg₂GeO₄ content increases.     

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     

Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications

The equilibrium (Mg, Fe, Zn)₃Al₂Si₃O₁₂+2Al₂SiO₅=3(Mg, Fe, Zn)Al₂O₄+5SiO₂ garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100^*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al₂O₃–SiO₂–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths.     

The rheology of olivine and spinel magnesium germanate (Mg2GeO4): TEM study of the defect microstructures

Synthetic polycrystals of α-Mg₂GeO₄ (with the olivine structure) and γ-Mg₂GeO₄ (with the spinel structure) deformed at high temperature and pressure in their respective stability fields were investigated by analytical transmission electron microscopy. Specimens with a mean grain size of 20–30 µm deform by dislocation glide and/or climb. The predominance of glide versus climb depends on stress and grain orientation. The defect microstructures of both polymorphs are very similar to those observed in their respective silicate analogues, α- and γ-(Mg,Fe)₂SiO₄, and, in the case of the spinel phase, very similar to those observed in magnesium aluminate spinels. These observations suggest that Mg₂GeO₄ is a good rheological analogue for the Earth’s upper mantle. A spinel specimen deformed under the same conditions of temperature and strain rate as an olivine specimen was approximately three times stronger than olivine. In specimens of both phases deformed at or above 1400 K, a thin amorphous film composed of Mg, Ge, and O was detected along some grain boundaries. Grains ≤10 µm diameter surrounded by a film of amorphous phase (>10 nm thick) exhibited low dislocation densities, and deformation appeared to have occurred by grain boundary sliding.     

Temperature-dependent magnetic susceptibility behaviour of spinelloid and spinel solid solutions in the systems Fe2SiO4–Fe3O4 and (Fe,Mg)2SiO4–Fe3O4

The magnetic behaviour and Curie temperatures (T _C ) of spinelloids and spinels in the Fe₃O₄–Fe₂SiO₄ and Fe₃O₄–(Mg,Fe)₂SiO₄ systems have been determined from magnetic susceptibility (k) measurements in the temperature range –192 to 700 °C. Spinelloid II is ferrimagnetic at room temperature and the k measurements display a characteristic asymmetric hump before reaching a T _C at 190 °C. Spinelloid V from the Mg-free system is paramagnetic at room temperature and hysteresis loops at various low temperatures indicate a ferri- to superparamagnetic transition before reaching the T _{C .} The T _C shows a non-linear variation with composition between –50 and –183 °C with decreasing magnetite component (X _Fe3O₄). The substitution of Mg in spinelloid V further decreases T _C . Spinelloid III is paramagnetic over nearly the total temperature range. Ferrimagnetic models for spinelloid II and spinelloid V are proposed. The T _C of Fe₃O₄–Fe₂SiO₄ spinel solid solutions gradually decrease with increasing Si content. Spinel is ferrimagnetic at least to a composition of X _Fe3O₄=0.20, constraining a ferrimagnetic to antiferromagnetic transition to occur at a composition of X _Fe3O₄<0.20. A contribution of the studied ferrimagnetic phases for crustal anomalies on the Earth can be excluded because they lose their magnetization at relatively low temperatures. However, their relevance for magnetic anomalies on other planets (Mars?), where these high-pressure Fe-rich minerals could survive their exhumation or were formed by impacts, has to be considered.     

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.     

Single crystal growth of the modified spinel (β) and spinel (γ) phases of (Mg,Fe)2SiO4 and some geophysical implications

Single crystals of ferromagnesian orthosilicates with modified spinel (β) and spinel (γ) structure as large as 500 μm have been grown by solid state crystallization at high temperature and high pressure using an MA8-type apparatus driven in a 2,000-ton uniaxial press. This system is capable of generating pressures of 24.0 (±0.3) GPa at 2,400 (±50)°C for one hour in a sample assembly volume of 0.14 cm³. Crystals larger than 100 μm were observed to grow only at pressures within 5 percent of the phase boundary between the stability fields of the β and γ phases. Experimental determination of the phase boundaries between β or β+γ and γ phases for (Mg,Fe)₂SiO₄ has been extended to 22 GPa and 2,400°C. The effect of configurational entropy due to disordering is evaluated to be minimal on the basis of the cationic distribution in the synthesized samples; thus, we conclude that the phase boundary between β or β+γ and γ phases remains essentially linear to 2,400°C. In (Mg,Fe)₂SiO₄ solid solutions, the stability field of the γ phase shifts towards the lower pressures with increasing iron content at a rate of a 1 GPa for each 10 mole percent Fe. Assignment of the β→β+γ→γ transition to the seismic 550 km discontinuity is rejected by the present phase diagram results for (Mg_0.9Fe_0.1)₂SiO₄ and measurement of acoustic velocities for β and γ Mg₂SiO₄, but the discontinuity may be caused by a phase transition of pyroxene to a garnet-like structure.     

Meixnerit,Mg6Al2(OH)18·4H2O,ein neues Magnesium-Aluminium-Hydroxid-Mineral

Zusammenfassung Meixnerit, Mg₆Al₂(OH)₁₈·4H₂O, kommt als sekundäres Mineral in Klüften eines Serpentins in der Nähe von Ybbs-Persenbeug (Niederösterreich) vor. Die Kristalle sind blättchenförmig, farblos durchsichtig, optisch ein-achsig-negativ mitn ₀=1,517. Die Spaltbarkeit parallel (00.1) ist perfekt. Meixnerit ist trigonal-rhomboedrisch, RaumgruppeR m,a=3,0463 (15) Å,c=22,93 (2) Å,Z=3/8. Strukturell ist er eng mit dem Hydrotalkit verwandt. Das neue Mineral wurde zu Ehren von Herrn Prof. Dr.H. Meixner, Salzburg, benannt.

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Meixnerite, Mg₆Al₂(OH)₁₈·4H₂O, a new magnesium-aluminum-hydroxide mineral

Summary Meixnerite, Mg₆Al₂(OH)₁₈·4H₂O, occurs as a secondary mineral in cracks of a serpentine rock near Ybbs-Persenbeug (Lower Austria). The crystals are tabular, colorless transparent, optically uni-axial negative withn ₀=1.517, cleavage parallel (00.1) perfect. Meixnerite is trigonal rhombohedral, space groupR m,a=3.0463 (15) Å,c=22.93 (2) Å,Z=3/8. It is structurally related to hydrotalcite. The new mineral is named in honour of Prof. Dr.H. Meixner, Salzburg.

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Potassic primary melts of vulsini (Roman Province): evidence from mineralogy and melt inclusions

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo_92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo_92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H₂O-rich ( 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeO^total=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO₂=46.5-50, K₂O=5.3-2.8, P₂O₅=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al₂O₃= 0.8-1.15), with negative correlations between SiO₂ and K₂O, Al₂O₃ and CaO, as well as positive correlations between K₂O, and P₂O₅, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe²⁺)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).