Modeling the thermodynamic parameters of six endmember garnets at ambient and high pressures from vibrational data

Raman and infrared spectroscopic data at ambient and high pressures were used to compute the lattice contribution to the heat capacities and entropies of six endmember garnets: pyrope, almandine, spessartine, grossular, andradite and uvarovite. Electronic, configurational and magnetic contributions are obtained from comparing available calorimetric data to the computed lattice contributions. For garnets with entropy in excess of the computed lattice contribution, the overwhelming majority is found in the subambient temperature regime. At room temperature, the non-lattice entropy is approximately 11.5 J/mol-K for pyrope, 49 J/mol-K for almandine, and 19 J/mol-K for andradite. The non-lattice entropy for pyrope and some for almandine cannot be accounted for by magnetic or electronic contributions and is likely to be configurational in nature. Estimates of low temperature non-lattice entropies for both spessartine and uvarovite are made in absence of calorimetric measurements and are based on low temperature calorimetry of other minerals containing the Mn²⁺ and Cr³⁺ cations as well as on solid solution garnets containing these cations. The estimate for uvarovite non-lattice entropy is approximately 18 J/mol-K, while for spessartine, approximately 45 J/mol-K. Neither of these cations is expected to provide electronic contributions to the entropy. For both iron-bearing garnets, a small electronic or magnetic entropy contribution continues above ambient temperatures. High pressure data on pyrope, grossular and andradite permit calculation of the thermodynamic parameters at high pressures, which are important for computation of processes in the Earth’s mantle. Thermal expansion coefficients of these materials were found to be 1.6, 1.5, 1.6×10⁻⁵ K⁻¹ at 298 K, respectively, using a Maxwell relation. These closely match the literature values at ambient conditions.     

Composition and pressure dependence of lattice thermal conductivity of (Mg,Fe)O solid solutions

We measured the lattice thermal conductivities of Fe_0.98O wüstite and iron-rich (Mg,Fe)O magnesiowüstite using the pulsed light heating thermoreflectance technique with a diamond anvil cell up to 61 GPa at 300 K. We found that the thermal conductivity of wüstite does not show a monotonic increase as a function of pressure, contrary to that of MgO periclase. Rocksalt (B1) to rhombohedral B1 transition is likely to induce an abnormal pressure response in the conductivity of wüstite. Our results also show that magnesiowüstite has a lower conductivity than that of MgO and FeO endmembers due to a strong iron impurity effect, which is well reproduced by a model considering phonon-impurity scattering in a binary solid solution.     

Progress and new directions in high temperature calorimetry

Calorimetry at 600–900° C, using Calvet-type microcalorimeters, has provided data in several areas of interest to geology. The best-developed application is solution calorimetry in molten oxide solvents (lead borate, sodium molybdate) to determine the enthalpies of formation of anhydrous silicates and related minerals, the enthalpies of phase transformations and order-disorder reactions, and the enthalpies of mixing in molten salts, glasses, and solid solutions. An attractive feature of the technique is the ability to use rather small samples; ～ 300 mg total often being sufficient for the study of a phase synthesized at high pressure. Current developments include the improvement of precision by careful control of solvent composition and water content, the development of alkali borate or borosilicate solvents for use under atmospheres of controlled oxygen fugacity, and the study of compounds containing fluorine as well as oxygen. In addition, direct-reaction calorimetry at high temperature has been used to study rapid phase transformations and gas-solid reactions. The latter include the oxidation of metals and the thermodynamics of oxides having wide homogeneity ranges, for example wüstite. A recent development is calorimetry at pressures up to 2 kb, using a cold-seal pressure vessel inserted in the calorimeter. Applications include the study of geothermal fluids and, eventually, the study of equilibria involving hydrous phases and water-containing melts and glasses.     

Entropies and enthalpies of aluminosilicate garnets

Activity-composition data have been used to obtain excess entropies and enthalpies of mixing for almandine-grossular and Mg-rich pyrope-grossular solid solutions. Excess free energies of Fe-rich almandine-grossular garnets are negative and imply the formation of a stable garnet compound with a different structure from the end members in this series.X-ray investigations of natural and synthetic calcium-poor aluminosilicate garnets indicate a lower space group symmetry than Ia3d, that of the end members. The most probable space group is I2₁3 in which there are two crystallographically distinct 8-coordinate sites present in equal numbers in the garnet structure.Excess entropies of mixing for almandine-grossular garnets are asymmetric and can be explained by Fe-Ca ordering in calcium-poor garnets together with Fe positional disorder on sub-sites within the 8-coordinate polyhedra. In the middle of the series and towards the Ca-rich end, a high degree of sub-site disorder with little or no Fe-Ca ordering may be responsible for the high positive entropies. Excess entropies of calcium-poor pyrope-grossular garnets show a similar trend, but are slightly more positive. Excess enthalpies of mixing for Mg-rich pyropegrossular garnets are in good agreement with the high-temperature calorimetric measurements of other workers.     

Mössbauer spectroscopic studies on the iron forms of deep-sea sediments

Mössbauer spectroscopy was applied to characterize the valence states Fe(II) and Fe(III) in sedimentary minerals from a core of the Peru Basin. The procedure in unraveling this information includes temperature-dependent measurements from 275?K to very low temperature (300?mK) in zero–field and also at 4.2?K in an applied field (up to 6.2?T) and by mathematical procedures (least-squares fits and spectral simulations) in order to resolve individual spectral components. The depth distribution of the amount of Fe(II) is about 11% of the total Fe to a depth of 19?cm with a subsequent steep increase (within 3?cm) to about 37%, after which it remains constant to the lower end of the sediment core (at about 40?cm). The steep increase of the amount of Fe(II) defines a redox boundary which coincides with the position where the tan/green color transition of the sediment occurs. The isomer shifts and quadrupole splittings of Fe(II) and Fe(III) in the sediment are consistent with hexacoordination by oxygen or hydroxide ligands as in oxide and silicate minerals. Goethite and traces of hematite are observed only above the redox boundary, with a linear gradient extending from about 20% of the total Fe close to the sediment surface to about zero at the redox boundary. The superparamagnetic relaxation behavior allows to estimate the order of magnitude for the size of the largest goethite and hematite particles within the particle-site distribution, e.g. ～170?Å and ～50?Å, respectively. The composition of the sediment spectra recorded at 300?mK in zero-field, apart from the contributions due to goethite and hematite, resembles that of the sheet silicates smectite, illite and chlorite, which have been identified as major constituents of the sediment in the <2?μm fraction by X-ray diffraction. The specific “ferromagnetic” type of magnetic ordering in the sediment, as detected at 4.2?K in an applied field, also resembles that observed in sheet silicates and indicates that both Fe(II) and Fe(III) are involved in magnetic ordering. This “ferromagnetic” behavior is probably due to the double-exchange mechanism known from other mixed-valence Fe(II)–Fe(III) systems. A significant part of the clay-mineral iron is redox sensitive. It is proposed that the color change of the sediment at the redox boundary from tan to green is related to the increase of Fe(II)–Fe(III) pairs in the layer silicates, because of the intervalence electron transfer bands which are caused by such pairs.     

About short and long range ordering in wüstites, Fe1-xO

This short contribution consists of several comments on a paper recently published by T.R. Welberry and A.G. Christy in Physics and Chemistry of Minerals (volume 24, Number 1, January 1997, pages 24–38). Due to the fact that some of the previous works were not taken into account by these authors, it seems necessary to present briefly a short review of the main results concerning the short and long range ordering of point defects in wüstites Fe_1-xO. Using direct imaging found in the literature, it was clearly shown in papers of our group, from 1981 to 1991, that one of the most probable and numerous clusters in wüstite under equilibrium is the (10/4) one, which is of blende ZnS type. The P' and P'' quenched phases were also described from this cluster.     

主编絮语

自2011年第2期起,我刊推出"主编絮语"这个栏目,目的在于及时表达我刊的办刊思想和办刊动态,推介重点文章,加强与读者、作者和各位编委的信息沟通和交流。正如在本卷卷首语中指出的,文章质量是刊物的生命线。因此,如何提高发表在我刊文章的质量,是本届编委会的第一要务和工作重点。编委会将通过多种途径,从     

Thermodynamics of open networks: Ordering and entropy in NaAlSiO4 glass,liquid, and polymorphs

The thermodynamic properties of carnegieite and NaAlSiO₄ glass and liquid have been investigated through C _p determinations from 10 to 1800 K and solution-calorimetry measurements. The relative entropies S ₂₉₈-S₀ of carnegieite and NaAlSiO₄ glass are 118.7 and 124.8 J/mol K, respectively. The low-high carnegieite transition has been observed at 966 K with an enthalpy of transition of 8.1±0.3 kJ/mol, and the enthalpy of fusion of carnegieite at the congruent melting point of 1799 K is 21.7±3 kJ/mol. These results are consistent with the reported temperature of the nepheline-carnegieite transition and available thermodynamic data for nepheline. The entropy of quenched NaAlSiO₄ glass at 0 K is 9.7±2 J/mol K and indicates considerable ordering among AlO₄ and SiO₄ tetrahedra. In the liquid state, progressive, temperature-induced Si, Al disordering could account for the high configurational heat capacity. Finally, the differences between the entropies and heat capacities of nepheline and carnegieite do not seem to conform to current polyhedral modeling of these properties     

Isotopic systematics of the rocks of the Khalyuta carbonatite complex of western Transbaikalia

This paper reports the results of isotopic investigations of the rocks of the Khalyuta carbonatite complex (carbonatites, comagmatic silicate rocks, fenites, and hydrothermal rocks) and host limestones and granites. Pyroxene, amphibole, magnetite, potassium feldspar, apatite, phlogopite, calcite, dolomite, strontianite, celestite, and barite were investigated. The isotopic compositions of C, O, Sr, and S were analyzed. The character of the distribution of oxygen isotopic composition in minerals (carbonates, silicates, phosphates, and oxides) suggests their equilibrium formation. It was supposed that the evolutionary trend of C and O isotopic compositions is mainly related to the processes of differentiation in the melt-fluid system and indicates the absence of significant contamination by carbon and oxygen from a crustal source during rock formation from the magmatic stage to the hydrothermal stage. The isotopic compositions of S and Sr did not change.     

Experimental determination of the effects of pressure and temperature on the stoichiometry and phase relations of wüstite

When quenched metastable wüstite (Fe_.924O and Fe_.947O) is held at 300°C at pressures up to 200 kbar in a diamond anvil cell, a mixture of magnetite, metallic iron and wüstite is found. We interpret this to indicate that magnetite plus metallic iron constitute the stable phase assemblage at pressures and temperatures below this boundary is stoichiometric FeO ($\text{a}{}_0\text{= 4.332 ± 0.001}\text{A}\text{?}$) at pressures below 110 kbar at 300°C. However, just below the boundary in the pressure range 110 kbar to 200 kbar at 300°C, the residuál wüstite is non-stoichiometric ($\text{a}{}_0\text{< 4.332}\text{A}\text{?}$). Data collected at pressures and temperatures above the boundary indicate that non-stoichiometric wüstite (Fe_xO) plus metallic iron constitute the stable phase assemblage and that the value of x in Fe_xO increases as pressure is increased isothermally to 100 kbar and then decreases as pressure is increased above 100 kbar.     

Calculated silica activities in carbonatite liquids

Carbonatite magmas precipitate silicates, in addition to the abundant carbonates, oxides, and phosphates. Calculated silica activities for equilibria involving silicates and a silica component in magmatic liquids predict specific assemblages for silicate and oxide phases in carbonatites. These assemblages provide tests of alternative sources (carbonatite magma, coeval silicate magma, or older rock) for silicate minerals in carbonatites. Quartz, feldspars, and orthopyroxene are unlikely to be primary magmatic phases in carbonatites, because the silica activity in carbonatite magmas is too low to stabilize these minerals. Zircon and titanite should be unstable relative to baddeleyite and perovskite, respectively, but they do occur in carbonatites. Liquids dominated by carbonate are strongly nonideal with respect to dissolved silica. Consequently, activity coefficients for a silica component in carbonatite liquids are >>1, so that small mole fractions of SiO₂ translate into silica activities sufficient to stabilize phlogopite, clinopyroxene, amphibole, monticellite, and forsterite, among other silicates. Examination of silicate mineral assemblages in carbonatites in the light of silica activity indicates that many carbonatites are contaminated by solid silicate phases from external sources but these xenocrysts can be discriminated from magmatic minerals.     

新疆萨尔托海高铝铬铁矿中异常矿物群的发现及意义

在萨尔托海高铝型铬铁矿中发现20余种矿物,包括金刚石、单质铬、自然铁和单质硅等自然元素类;碳化物碳硅石;铁镍、铁镍铬合金等金属合金;方铁矿、金红石、赤铁矿、磁铁矿、钛铁矿、石英和铬尖晶石等氧化物类;方铅矿、闪锌矿、针镍矿、赫硫镍矿和毒砂等硫化物类;镁橄榄石、顽火辉石、透辉石、蛇纹石、锆石和长石等硅酸岩类。这些超高压、强还原性和壳源矿物与俄罗斯极地乌拉尔以及西藏罗布莎铬铁矿可以对比,暗示萨尔托海高铝铬铁矿和高铬铬铁矿一样,可能存在深部地幔成矿阶段。深部地幔矿物以及浅部壳源矿物的发现,暗示萨尔托海铬铁矿的形成可能经历了深部地幔预富集和浅部再造富集成矿两个阶段。     

华南下寒武统Ni-Mo-Se多金属层S-Se同位素体系

为了进一步理解华南下寒武统Ni-Mo-Se多金属层的物质来源及形成环境,文中分析了遵义中南村和张家界后坪两个Ni-Mo-Se矿层及其围岩的黄铁矿硫同位素和全岩的硒同位素组成。硫同位素组成显示两个Ni-Mo-Se矿层形成时的环境存在区域性差别,中南村矿层形成于间歇开放的海洋环境,而后坪矿层形成于封闭的缺氧(静海)环境。较大的硫同位素范围暗示硫酸盐还原菌控制硫同位素的分馏,而热液流体可能提供了大量金属元素,从而导致矿层富集大量的硫化物和稀有金属。硒同位素组成指示牛蹄塘组底部热液流体的Se可能重新经历了氧化还原循环,而Se的富集过程可能受有机质和粘土矿物吸附或类质同象过程控制。因此,控制多金属富集的因素主要为富集金属的热液流体的参与和缺氧环境下的自生沉积。     

Melting relations and major element partitioning in an oxidized bulk Earth model composition at 15–26 GPa

Melting experiments were performed on an FeO-rich bulk Earth model composition in the CMFAS system in order to investigate the partitioning of major elements between coexisting minerals and melts. The starting material (34.2% SiO₂, 3.86% Al₂O₃, 35.2% FeO, 25.0% MgO and 1.88% CaO), contained in Re-capsules, was a mixture of crystalline forsterite and fayalite, and a glass containing SiO₂, Al₂O₃, and CaO. Olivine is the first liquidus phase at 10 GPa but is replaced by majoritic garnet (ga) in the 15–26 GPa range. Magnesiowüstite (mw) crystallizes close to the liquidus and is joined by perovskite (pv) at 26 GPa.

The quenched melt compositions are homogeneous throughout the melt region of the charges and are only slightly enriched in Si, Ca and Fe, and depleted in Mg, relative to the starting composition. The Fe/Mg and Ca/Al ratios in all of the minerals increase rapidly below the liquidus to become compatible with the bulk composition at the solidus. At 26 GPa, a relative density sequence of mw>pv>melt>ga is observed. This indicates that majorite floating, combined with the sinking of magnesiowüstite and perovskite can be expected during the solidification of a Hadean magma ocean and in hot mantle plumes early in the Earth's history. The mineral–melt partitioning relations indicate that fractional crystallization or partial melting in the transition zone and the upper part of the lower mantle would increase the Fe/Mg and Ca/Al ratios of the melt, even if magnesiowüstite was predominant in the solid fraction. A significant contribution of accumulated mw to the segregation of the protocore is therefore unlikely. The suggested process of perovskite fractionation to the lower mantle is not capable of increasing the Mg/Si ratio in the residual melt, and the combined fractionation of perovskite and magnesiowüstite produces a melt with elevated ratios of Si/Mg, Ca/Al and Fe/Mg.     

Fundamentals of the mantle carbonatite concept of diamond genesis

In the mantle carbonatite concept of diamond genesis, the data of a physicochemical experiment and analytical mineralogy of inclusions in diamond conform well and solutions to the following genetic problems are generalized: (1) we substantiate that upper mantle diamond-forming melts have peridotite/eclogite–carbonatite–carbon compositions, melts of the transition zone have (wadsleyite ? ringwoodite)–majorite–stishovite–carbonatite–carbon compositions, and lower mantle melts have periclase/wüstite–bridgmanite–Ca-perovskite–stishovite–carbonatite–carbon compositions; (2) we plot generalized diagrams of diamondforming media illustrating the variable compositions of growth melts of diamonds and paragenetic phases, their genetic relationships with mantle matter, and classification relationships between primary inclusions; (3) we study experimentally equilibrium diagrams of syngenesis of diamonds and primary inclusions characterizing the diamond nucleation and growth conditions and capture of paragenetic and xenogenic minerals; (4) we determine the fractional phase diagrams of syngenesis of diamonds and inclusions illustrating regularities in the ultrabasic–basic evolution and paragenetic transitions in diamond-forming systems of the upper and lower mantle. We obtain evidence for physicochemically similar melt–solution ways of diamond genesis at mantle depths with different mineral compositions.     

中国降水同位素信息熵时空分布及其水汽输送示踪

研究水汽输送过程有助于更好地理解极端降水发生过程。但是,降水系统的复杂性和降水同位素的随机性使得利用降水同位素示踪水汽输送过程具有较大的不确定性。利用信息熵研究了中国降水同位素组成概率分布特征,发现降水氢氧同位素信息熵之间呈现很强的线性关系,且斜率近似为1;对比分析了降水同位素信息熵和其平均值的时空分布特征,发现降水同位素信息熵空间分布可以很好地揭示水汽由海洋向大陆的运移过程,而这一特征并没有反映在降水同位素平均值空间分布上;利用降水同位素信息熵对影响中国的由季风形成的3条水汽通道进行了示踪分析,发现降水同位素信息熵空间分布很好地指示出3条水汽通道的水汽来源和水汽运移路径及其季节变化。     

Refinements in a site-mixing model for illites: local electrostatic balance and the quasi-chemical approximation

A quasi-chemical model for illites has been derived, and local electrostatic balance has been added to a random regular solution site-mixing model for illites (Stoessell, 1979). Each model assumes similar order-disorder conditions for both the end-members micas and the solid solution. Thermodynamic properties of illites predicted by the random, electrostatic, and quasi-chemical models are compared as a function of composition. For natural illite compositions, molar entropies of mixing in the electrostatic model are about 1 entropy unit less than those in the random model. Intermediate values are given by the quasi-chemical model. Each model predicts an increased entropy of mixing in dominantly trioctahedral illites as compared to dioctahedral illites. Each model also predicts destabilization of trioctahedral illites using absolute molar exchange energies greater than 2 RT/Z_x, where Z_x is the number of adjacent cation interactions per site in the Xth site class. The most negative free energies of mixing are predicted by the quasi-chemical model. Intermediate values predicted by the random model are apparently the result of error cancellation due to overestimation of both the entropy and enthalpy of mixing.     

Heat capacity of liquid silicates: new measurements on NaAlSi3O8 and K2Si4O9

Drop calorimetric measurements of H_T-H₂₇₃ are reported for glassy and liquid albite and potassium tetrasilicate for the temperature interval 600–1500 K. Analysis of these observations as well as data for 13 other stable and supercooled silicate liquids suggests strongly that the isobaric heat capacities of stable and supercooled liquids are equal and thus temperature independent. Available evidence indicates that the isochoric heat capacities of liquid alkali silicates are also temperature independent within present experimental uncertainties.     

Melting of wüstite and iron up to pressures of 600 kbar

The effect of pressure on melting temperature of wüstite and iron has been measured with laser-heated diamond anvil cell. The temperature was determined by measuring the thermal radiation emitted by the sample as a function of wavelength in the range from 600 nm to 900 nm to which Planck's radiation function was fitted; the pressure was measured by ruby-fluorescence technique. The melting curve of wüstite in this study when extrapolated to low pressures agrees with Lindsley's (1966) data. Our data are similar to the recent data of Boehler (1992) and close to that of Ringwood and Hibberson (1990) at pressure of 160 kbar, but the melting temperature does not rise as rapidly with increasing pressure as reported by Knittle and Jeanloz (1991). If tungsten emissivity is used in the temperature calculation, the melting curve of iron matches those of Boehler et al. (1990). Use of emissivity of iron in the temperature calculation results in somewhat higher temperatures than those reported by Boehler et al. (1990).     

Clinopyroxene-perovskite phase transition of FeGeO3 under high pressure and room temperature

In order to confirm the possible existence of FeGeO₃ perovskite, we have performed in situ X-ray diffraction measurements of FeGeO₃ clinopyroxene at pressures up to 40 GPa at room temperature. The transition of FeGeO₃ clinopyroxene into orthorhombic perovskite is observed at about 33GPa. The cell parameters of FeGeO₃ perovskite are a=4.93(2) Å, b=5.06(6) Å, c=6.66(3) Å and V=166(3) ų at 40 GPa. On release of pressure, the perovskite phase transformed into lithium niobate structure. The previously reported decomposition process of clino-pyroxene into Fe₂GeO₄ (spinel)+GeO₂ (rutile) or FeO (wüstite) +GeO₂ (rutile) was not observed. This shows that the transition of pyroxene to perovskite is kinetically accessible compared to the decomposition processes under low-temperature pressurization.