Aluminum substitution mechanisms in perovskite-type MgSiO<Subscript>3</Subscript>: an investigation by Rietveld analysis

Al-containing MgSiO₃ perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg_0.975Si_0.975Al_0.05O₃ and Mg_0.95Si_0.95Al_0.10O₃ considering only coupled substitution Mg²⁺ + Si⁴⁺ = 2Al³⁺, and nonstoichiometric compositions of Mg_0.99Si_0.96Al_0.05O_2.985 and Mg_0.97Si_0.93Al_0.10O_2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si⁴⁺ = 2Al³⁺ + V_O¨. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter–composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the X _Al range of 0–0.05, where X _Al is Al number on the basis of total cation of two (X _Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X _Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X _Al of 0.10, though they are the same as each other at X _Al of 0.05. The Si(Al)–O1 distance, Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O distance of the nonstoichiometric perovskites keep almost constant up to X _Al of 0.05, and then the Si(Al)–O1 increases and both of the Si(Al)–O1–Si(Al) angle and minimum Mg(Al)–O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X _Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)–O1 distance and one of two independent Si(Al)–O2 distances in Si(Al)O₆ octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.     

Vibrational spectroscopy of aluminate spinels at 1 atm and of MgAl2O4 to over 200 kbar

Single-crystal Raman and infrared reflectivity data including high pressure results to over 200 kbar on a natural, probably fully ordered MgAl₂O₄ spinel reveal that many of the reported frequencies from spectra of synthetic spinels are affected by disorder at the cation sites. The spectra are interpreted in terms of factor group analysis and show that the high energy modes are due to the octahedral internal modes, in contrast to the behavior of silicate spinels, but in agreement with previous data based on isotopic and chemical cation substitutions and with new Raman data on gahnite (～ ZnAl₂O₄) and new IR reflectivity data on both gahnite and hercynite (～Fe_0.58Mg_0.42Al₂O₄). Therefore, aluminate spinels are inappropriate as elastic or thermodynamic analogs for silicate spinels. Fluorescence sideband spectra yield complementary information on the vibrational modes and provide valuable information on the acoustic modes at high pressure. The transverse acoustic modes are nearly pressure independent, which is similar to the behavior of the shear modes previously measured by ultrasonic techniques. The pressure derivative of all acoustic modes become negative above 110 kbar, indicating a lattice instability, in agreement with previous predictions. This lattice instability lies at approximately the same pressure as the disproportionation of spinel to MgO and Al₂O₃ reported in high temperature, high pressure work.     

Investigation on mantle peridotites from Neyriz ophiolite, south of Iran: geodynamic signals

Neyriz ophiolite in Abadeh Tashk area appears as four major separated massifs in an area with 125 km², south of Iran. Peridotites including harzburgite, dunite, and lesser low-Cpx lherzolite are the major constituents of the ophiolite with very minor mafic rocks. Usual gabbros of ophiolite complexes are virtually absent from the study area. Mineral modality associated with bulk rock and mineral chemistry of the peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease in modal pyroxenes and in Al₂O₃, CaO, SiO₂, Sc, Ta, V, and Ga values of the studied rocks by approaching chromite deposits. The Neyriz peridotites vary from low-Cpx lherzolite (MgO, 41.97–43.1 wt.%; Al₂O₃, 0.8–1.3 wt.%) with low content of Cr# spinel (36.7–37.6) and Fo olivine (90.79–91.5) to harzburgite (MgO, 44.31–45.25 wt.%;Al₂O₃, 0.29–0.45 wt.%; Cr# spinel, 58.2–73.45; Fo olivine, 91.23–91.56), and then to dunite (MgO, 45.9–49.2 wt.%; Al₂O₃, 0.18–0.48 wt.%) with higher content of Cr# spinel (74.34–79.36) and Fo olivine (91.75–94.68). Compared to modern oceanic settings, mineral and rock composition of low-Cpx lherzolite plot within the field of mid-ocean-ridge environment, whereas those of harzburgite and dunite fall in the field of fore-arc peridotites. As a result of the studies on minerals and whole rock chemistry along with rock interrelationships, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt from which the high-Cr–Mg, low-Ti chromitites were formed within mantle wedge above the supra-subduction zone in a fore-arc setting.     

Raman spectra of MgSiO3·10% Al2O3-perovskite at various pressures and temperatures

Variations of Raman spectra of MgSiO₃·10% Al₂O₃-perovskite were investigated up to about 270 kbar at room temperature and in the range 108–425 °K at atmospheric pressure. Like MgSiO₃-perovskite, the Raman frequencies of MgSiO₃·10% Al₂O₃-perovskite increase nonlinearly with increasing pressure and decrease linearly with increasing temperature within the experimental uncertainties and the range investigated. A comparison of these data with those of MgSiO₃-perovskite suggests that MgSiO₃·10% Al₂O₃-perovskite is slightly more compressible than MgSiO₃-perovskite, and that the volume thermal expansion for MgSiO₃·10% Al₂O₃-perovskite is also slightly greater than that for MgSiO₃-perovskite.     

U–Cr-rich high Mg-Al granulites from Karimnagar Granulite Belt, India: implications for Neoarchean-Paleoproterozoic events in southern India

High Mg-Al granulite occurs as enclave within granite gneisses at Karimnagar, southern India, and it contains coarse granoblastic aggregates of orthopyroxene and sapphirine with minor amount of cordierite, spinel and phlogopite. An important chemical characteristic of these minerals is their extremely high MgO content and the high Cr₂O₃ in sapphirine and spinel. Textural analysis shows sapphirine + orthopyroxene + cordierite as the peak-metamorphic assemblage that possibly evolved though the breakdown of a spinel-bearing assemblage. Cation exchange geothermometers involving orthopyroxene, sapphirine and spinel yield temperatures of 600–800 °C with a maximum of 860 °C implying an event of high temperature (HT) metamorphism. Pseudosection analysis in the FeO–MgO–Al₂O₃–SiO₂ chemical system shows the stability of the peak- assemblage below 6.2 kbar. Subsequently, the rock underwent hydration and cooling with the appearance of phlogopite in the assemblage. Chromium enrichment is possibly inherited from the protolith and its presence presumably stabilized sapphirine and spinel below their high-temperature stability field. The recorded Rb–Sr age of ca. 2,500 Ma in host granite gneiss marks the upper age limit of HT metamorphism. Presence of patchy, lobate grains as well as veinlets of uraninite and brannerite is also a characteristic feature of the rock. Uranium mineralization took place during the post peak metamorphic stage, sulfide mineralization represented by tiny grains and veinlets of pyrite, millerite and pentlandite coincided with, and outlasted the uranium mineralization. The U–Th–Pb chemical ages of uraninite grains suggest ca. 2,200?±?12 Ma for the age of uranium mineralization in the granulite. Based on the field relations, it is surmised that the granulite metamorphism in the study area is older than ca. 2,500 Ma and is comparable with an event in the other parts of Eastern Dharwar Craton. It can be conceived as a widespread event in southern India.     

Petrology of spinel lherzolite xenoliths from Youkou volcano,Adamawa Massif,Cameroon Volcanic Line: mineralogical and geochemical fingerprints of sub-rift mantle processes

The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo_89.4?90.5), enstatite (En_89???91Fs_8.7?9.8Wo_0.82?1.13), clinopyroxene, spinel (Cr#_Sp?=?9.4–13.8), and in some cases amphibole (Mg_#?=?88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al₂O₃, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al₂O₃?=?6.07–6.56 wt% in clinopyroxene) and the low CaO/Al₂O₃ ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.     

Synthesis of cordierite using industrial waste fly ash

Fly ash is a product arising from coal combustion in thermal power plants. It represents a major source of environmental pollution. It is well known by its chemical composition rich of SiO₂ and Al₂O₃. With the aim of preserving the environment against this contamination, fly ash was used along with the starting materials for producing glass cordierite (2MgO, 2Al₂O₃, 5SiO₂). Four formulations were developed by mixing the silica gel, magnesium chloride (MgCl₂.6H₂O) and fly ash in the percentages enclosing the stoichiometry of cordierite (2MgO, 2Al₂O₃, 5SiO₂). Different experimental techniques (DTA/TGA, X-ray diffraction, FTIR and SEM) were used to characterise the prepared formulations. The results shown that for all formulations, a cordierite phase was obtained at 1200 °C along with several secondary phases such as mullite, cristobalite, silicon oxide, enstatite and spinel. At 1300 °C, pure indialite (α-cordierite) was obtained along with a small amount of spinel. The four formulations sintered at 1200 °C exhibit a homogenous morphology and high porosity. The acicular-shaped indialite grains were observed in both formulations with excess of alumina and excess of magnesia.     

Spinel-anthophyllite rocks of the Kokchetav massif (northern Kazakhstan)

Spinel-anthophyllite rocks that may be classified as ultrabasic low-Ca spinel amphibolites have been first discovered in the Kokchetav collision zone (northern Kazakhstan). They outcrop 2 km west of Enbek-Berlyk Village among schists and quartzites and are closely associated with spinel harzburgites and garnet pyroxenites. The main hosted minerals are spinel (hercynite) and anthophyllite. The rocks bear magnetite-hornblende-spinel-anthophyllite pseudomorphs with rounded and polygonal sections, which might have been resulted from the replacement of garnet grains. The prismatic anthophyllite crystals and scarce olivine relics contain elongate parallel spinel inclusions resembling spinel-olivine syntactic intergrowths in the Enbek-Berlyk spinel harzburgites. The spinel-anthophyllite rocks are similar to the associated spinel harzburgites in CaO, MnO, SiO₂, and Al₂O₃ contents but are richer in FeO and poorer in MgO (F = FeO/(FeO + MgO) = 57% against 35% in the harzburgites). Geological, mineralogical, and geochemical data suggest that the spinel-anthophyllite rocks formed during the isochemical contact metamorphism of garnet-bearing spinel harzburgites, which contained more FeO and less MgO than garnet-free harzburgites of the same area. Variations in FeO and MgO contents in both types of harzburgites seem to be due to different chemical compositions of the chlorite protoliths of these rocks.     

辽宁瓦房店金伯利岩中尖晶石族矿物种类划分及指示意义

晶体矿物学理论认为不同成岩环境金伯利岩中尖晶石族矿物由于形成物理化学条件不同,其晶体结构和化学成分会发生明显的变化,通过对无矿、贫矿、富矿金伯利岩岩管中的尖晶石族矿物晶胞参数和化学成分的测定,研究尖晶石族矿物化学成分和晶胞参数变化与无矿、贫矿、富矿金伯利岩的内在关系,可以提高金伯利岩型金刚石矿床找矿效率。为了确定辽宁瓦房店金伯利岩中的尖晶石族矿物种属,探讨辽宁瓦房店金伯利岩中尖晶石族矿物化学成分和晶胞参数与金伯利岩含矿性关系,本文运用电子探针波谱仪对50件尖晶石族矿物中的MgO、FeO、TiO2、Al2O3、MnO及Cr2O3进行微区化学成分分析,运用单晶X射线衍射仪对136个尖晶石族矿物晶胞参数进行测定。数据统计显示:瓦房店金伯利岩中尖晶石族矿物为铬铁矿和镁铬铁矿,以化学分子式中A、B组主要阳离子占位特征为基础,可把矿区的尖晶石族矿物划分为10个亚种;如果用尖晶石族矿物化学成分中Cr2O3与(Cr2O3+Al2O3)含量的比值Cr'来表示尖晶石族矿物与金伯利岩含矿性的关系,金伯利岩岩体含矿性由富矿→中等含矿→贫矿,相应岩体中尖晶石族矿物Cr'值分别为89.5%、83.4%~87.1%和70.2%,逐渐变低;从无矿金伯利岩岩体→贫矿和中等含矿金伯利岩岩体→富矿金伯利岩岩体,金伯利岩体中第一世代尖晶石族矿物晶胞参数分别为0.831~0.832 nm、0.834~0.836 nm、0.837 nm,有逐渐变大的趋势。本文认为,辽宁瓦房店金伯利岩中第一世代尖晶石族矿物晶胞参数大小和Cr'参数可以作为判断辽宁瓦房店金伯利岩含矿性的指示标型。     

The first find of spinel peridotite in the Southern Kazakhstan: Structure,composition, and parameters of high-pressure metamorphism

Spinel peridotite, metamorphosed in high-pressure conditions, was first described within the Western part of the Central Asian Orogenic Belt. The spinel peridotite has the characteristics of Mg–Cr ultramafites indicating the mantle origin of its protolith. The preliminary estimation of the metamorphism peak for the model system MgO–Al₂O₃—SiO₂–Cr₂O₃ (MASCr) is 10–19 kbar at 680–800°C.     

Garnet and spinel lherzolite assemblages in MgO–Al2O3–SiO2 and CaO–MgO–Al2O3–SiO2: thermodynamic models and an experimental conflict

The recent publication of an updated thermodynamic dataset for petrological calculations provides an opportunity to illustrate the relationship between experimental data and the dataset, in the context of a new set of activity–composition models for several key minerals. These models represent orthopyroxene, clinopyroxene and garnet in the system CaO–MgO–Al₂O₃–SiO₂ (CMAS), and are valid up to 50 kbar and at least 1800 °C; they are the first high‐temperature models for these phases to be developed for the Holland & Powell dataset. The models are calibrated with reference to phase‐relation data in the subsystems CaO–MgO–SiO₂ (CMS) and MgO–Al₂O₃–SiO₂ (MAS), and will themselves form the basis of models in larger systems, suitable for calculating phase equilibria in the crust and mantle. In the course of calibrating the models, it was necessary to consider the reaction orthopyroxene + clinopyroxene + spinel = garnet + forsterite in CMAS, representing a univariant transition between simple spinel and garnet lherzolite assemblages. The high‐temperature segment of this reaction has been much disputed. We offer a powerful thermodynamic argument relating this reaction to the equivalent reaction in MAS, that forces us to choose between good model fits to the data in MAS or to the more recent data in CMAS. We favour the fit to the MAS data, preserving conformity with a large body of experimental and thermodynamic data that are incorporated as constraints on the activity–composition modelling via the internally consistent thermodynamic dataset.     

The stability of sapphirine + quartz: calculated phase equilibria in FeO–MgO–Al2O3–SiO2–TiO2–O

The presence in rocks of coexisting sapphirine + quartz has been widely used to diagnose conditions of ultra‐high‐temperature (UHT) metamorphism (>900 °C), an inference based on the restriction of this assemblage to temperatures >980 °C in the conventionally considered FeO–MgO–Al₂O₃–SiO₂ (FMAS) chemical system. With a new thermodynamic model for sapphirine that includes Fe₂O₃, phase equilibra modelling using thermocalc software has been undertaken in the FeO–MgO–Al₂O₃–SiO₂–O (FMASO) and FeO–MgO–Al₂O₃–SiO₂– TiO₂–O (FMASTO) chemical systems. Using a variety of calculated phase diagrams for quartz‐saturated systems, the effects of Fe₂O₃ and TiO₂ on FMAS phase relations are shown to be considerable. Importantly, the stability field of sapphirine + quartz assemblages extends down temperature to 850 °C in oxidized systems and thus out of the UHT range.     

High-pressure stability, structure and compressibility of Cmcm -MgAl2O4: an ab initio study

 Quantum-mechanical solid-state calculations have been performed on the highest-pressure polymorph of magnesium aluminate (CaTi₂O₄-type structure, Cmcm space group), as well as on the low-pressure (Fd3ˉm) spinel phase and on MgO and Al₂O₃. An ab initio all-electron periodic scheme with localized basis functions (Gaussian-type atomic orbitals) has been used, employing density-functional-theory Hamiltonians based on LDA and B3LYP functionals. Least-enthalpy structure optimizations in the pressure range 0 to 60 GPa have allowed us to predict: (1) the full crystal structure, the pV equation of state and the compressibility of Cmcm-MgAl₂O₄ as a function of pressure; (2) the phase diagram of the MgO–Al₂O₃–MgAl₂O₄ system (with exclusion of CaFe₂O₄-type Pmcn-MgAl₂O₄), and the equilibrium pressures for the reactions of formation/decomposition of the Fd3ˉm and Cmcm polymorphs of MgAl₂O₄ from the MgO + Al₂O₃ assemblage. Cmcm-MgAl₂O₄ is predicted to form at 39 and 57 GPa by LDA and B3LYP calculations, with K ₀=248 (K′=3.3) and 222 GPa (K′=3.8), respectively. Results are compared to experimental data, where available, and the performance of different DFT functionals is discussed. Received: 31 January 2001 / Accepted: 16 May 2001     

Enthalpies of formation at 970 K of compounds in the system MgO-Al2O3-SiO2 from high temperature solution calorimetry

The enthalpies of solution of petrologically important phases in the system MgO-Al₂O₃-SiO ₂ were measured in a melt of composition 2PbO · B₂O₃ at 970 ± 2K. The substances investigated included synthetic and natural (meteoritic) enstatite (MgSiO₃), synthetic aluminous enstatite (MgSiO_30.9Al₂O_30.1), synthetic and natural cordierite (Mg₂Al₄Si₅O₁₈), synthetic and natural sapphirine (approx. 7MgO·9Al₂O₃ · 3SiO₂), synthetic spinel (MgAl₂O₄), natural sillimanite (Al₂SiO₅), synthetic forsterite (Mg₂SiO₄), synthetic pyrope (Mg₃Al₂Si₃O₁₂), natural quartz (SiO₂), synthetic periclase (MgO) and corundum (Al₂O₃). Improvement in standardization of the calorimeter solvent made possible greater precision in this study than obtainable in former work in this laboratory on some of the same substances.The enthalpies of formation of enstatite, synthetic cordierite, forsterite and spinel are in reasonable agreement with values previously determined by solution calorimetry. The enthalpy of formation of enstatite is about 0.7 kcal less negative than the value for clinoenstatite resulting from the HF calorimetry of Torgesen and Sahama (J. Amer. Chem. Soc.70. 2156–2160, 1948), and is in accord with predictions based on analysis of published pyroxene equilibrium work. Aluminous enstatite with 10 wt.% Al₂O₃ shows an enthalpy of solution markedly lower than pure MgSiO₃: the measurements lead to an estimate of the enthalpy of formation at 970 K for MgAl₂SiO₆ (Mg-Tschermak) orthopyroxene of + 9.4 ± 1.5 kcal/mole from MgSiO₃ and Al₂O₃.Comparison of the enthalpies of formation of synthetic cordierite and anhydrous natural low-iron cordierite shows that they are energetically quite similar and that the synthetic cordierite is not likely to have large amounts of (Al, Si) tetrahedral disorder. Comparison of the enthalpies of formation of synthetic sapphirine and natural low-iron sapphirine shows, on the other hand, that the former is not a good stability model for the latter. The lower enthalpy of formation of the high-temperature synthetic sample is undoubtedly a consequence of cation disordering.The enthalpy of formation of natural sillimanite is considerably less negative than given by the tables of Robie andWaldbaum (U.S. Geol. Surv. Bull.1259 1968).The measured enthalpy of formation of synthetic pyrope is consistent with that deduced from published equilibrium diagrams in conjunction with the present measured enthalpy of formation of aluminous enstatite. Calculation of the entropy of synthetic pyrope from the present data yields surprisingly high values and suggests that synthetic pyrope is not a good stability model for natural pyrope-rich garnets. Hence, considerable doubt exists about the direct quantitative application of experimental diagrams involving pyropic garnet to discussions of the garnet stability field in the Earth's outer regions.     

Gaps in cubic closest packing: from MgO via spinel to the pharmacosiderite crystal structure type

Numerous ordered defect structures are known that are related to the sodium chloride (or MgO) structure type, thus they are basically cubic closest packed (ccp) arrangements with vacancies. For example the NbO type is an MgO type in which one quarter each of the anions and the cations are missing compared to the ccp in such a way that both anions and cations are in square-planar coordination. In spinel, Al₂MgO₄, one half of the octahedrally coordinated cations are missing compared with the MgO type and only one eighth of the tetrahedrally coordinated sites within the ccp are occupied. What these cases have in common is that all these derivatives are rather dense. This is different in pharmacosiderite, K[Fe₄(OH)₄As₃O₁₂]. 6 to 7H₂O, where one half of the anion positions, three quarters of the octahedral sites and five eighth of the tetrahedral sites remain vacant, compared to the spinel type. Pharmacosiderite is a wide open porous structure with zeolitic properties. We are illustrating these relationships using a Bärnighausen symmetry tree and by tables relating the various structure types to each other.     

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 interpretation of reaction textures in Fe‐rich metapelitic granulites of the Musgrave Block,central Australia: constraints from mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Fe‐rich metapelitic granulites of the Musgrave Block, central Australia, contain several symplectic and coronal reaction textures that post‐date a peak S2 metamorphic assemblage involving garnet, sillimanite, spinel, ilmenite, K‐feldspar and quartz. The earliest reaction textures involve spinel‐ and quartz‐bearing symplectites that enclose garnet and to a lesser extent sillimanite. The symplectic spinel and quartz are in places separated by later garnet and/or sillimanite coronas. The metamorphic effects of a later, D3, event are restricted to zones of moderate to high strain where a metamorphic assemblage of garnet, sillimanite, K‐feldspar, magnetite, ilmenite, quartz and biotite is preserved. Quantitative mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using Thermocalc 3.0 and the accompanying internally consistent dataset provide important constraints on the influence of TiO₂ and Fe₂O₃ on biotite‐bearing and spinel‐bearing equilibria, respectively. Biotite‐bearing equilibria are shifted to higher temperatures and spinel‐bearing equilibria to higher pressures and lower temperatures in comparison to the equivalent equilibria in K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH). The sequence of reaction textures involving spinel is consistent with a D2 P–T path that involved a small amount of decompression followed predominantly by cooling within a single mineral assemblage stability field. Thus, the reaction textures reflect changes in modal proportions within an equilibrium assemblage rather than the crossing of a univariant reaction. The D3 metamorphic assemblage is consistent with lower temperatures than those inferred for D2.     

Stability of phengite and biotite in eclogites and characteristics of biotite- or orthopyroxene-bearing eclogites

Stability of phengite and biotite in eclogite is discussed using petrological data of natural eclogites, and the observational data are examined by thermodynamic calculations. Generally, phengite is a major K phase in natural eclogite and is stable in wide range of bulk composition. However, in eclogites from several localities of the Caledonides, biotite occurs as a stable eclogite-facies mineral, and is often associated with orthopyroxene. Bulk compositions of such biotite- or orthopyroxene-bearing eclogites are compared with those of eclogites from the Dabie–Sulu region, China, where phengite is a major K phase in eclogite. The biotite- or orthopyroxene-bearing eclogites from the Western Gneiss Region of the Caledonides are rich in MgO (10–15 wt%) and relatively poor in CaO (7–8 wt%) and Al₂O₃ (12–16 wt%). The CaO/MgO ratios of the biotite- or orthopyroxene-bearing eclogites are clearly lower than those of eclogites from the Dabie–Sulu region, indicating that MgO-rich and CaO-poor environments should be important for stabilizing of biotite and orthopyroxene in eclogite. Biotite-bearing eclogite from the North-East Greenland Eclogite Province is rich in MgO (≈16 wt%) and CaO (≈15.5 wt%) and extremely poor in Al₂O₃ (≈8 wt%). To stabilize biotite in eclogite, Al₂O₃-poor environments are also important. Bulk compositions of these biotite- or orthopyroxene-bearing eclogites are similar to picrite basaltic compositions. To examine these observational data, thermodynamic calculations were carried out in a seven-component system KH₂O_1.5–Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂, which includes garnet, kyanite, phengite, biotite, quartz, omphacite, orthopyroxene and olivine in conjunction with mass-balance calculations. Firstly, calculations were performed on the average bulk composition of eclogites from the Dabie–Sulu region to lherzolite (KLB-1). The calculation results confirmed that phengite should be stable in eclogite with 'ordinary' basaltic composition, whereas biotite and orthopyroxene should be stable in picrite basaltic compositions (e.g. MgO >11.0 wt%, CaO <9.8 wt%, Al₂O₃ <15.2 wt% at 700 °C, 2.5 GPa). Further calculations in basaltic system confirmed that increase of MgO content and decrease of CaO and Al₂O₃ contents were important to stabilize biotite and orthopyroxene in eclogite. Thus, mineral assemblage in picrite basalt system should be completely different from that in normal basaltic system.     

Ultra-calcic Magmas Generated from Ca-depleted Mantle: an Experimental Study on the Origin of Ankaramites

Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitousin arc, ocean-island and mid-ocean ridge settings. They areprimitive in character (X_Mg > 0·65) and have highCaO contents (>14 wt %) and CaO/Al₂O₃ (>1·1). Experimentson an ankaramite from Epi, Vanuatu arc, demonstrate that itsliquidus surface has only clinopyroxene at pressures of 15 and20 kbar, with X_CO2 in the volatile component from 0 to 0·86.The parental Epi ankaramite is thus not an unfractionated magma.However, forcing the ankaramite experimentally into saturationwith olivine, orthopyroxene and spinel results in more magnesian,ultra-calcic melts with CaO/Al₂O₃ of 1·21–1·58.The experimental melts are not extremely Ca-rich but high inCaO/Al₂O₃ and in MgO (up to 18.5 wt %), and would evolve tohigh-CaO melts through olivine fractionation. Fractionationmodels show that the Epi parent magma can be derived from suchultra-calcic experimental melts through mainly olivine fractionation.We show that the experimental ultra-calcic melts could formthrough low-degree melting of somewhat refractory mantle. Thelatter would have been depleted by previous melt extraction,which increases the CaO/Al₂O₃ in the residue as long as someclinopyroxene remains residual. This finding corrects the commonassumption that ultra-calcic magmas must come from a Ca-richpyroxenite-type source. The temperatures necessary for the generationof ultra-calcic magmas are     

Crystal and molecular structure and spectroscopic behavior of isotypic synthetic analogs of the oxalate minerals stepanovite and zhemchuzhnikovite

The crystal structure of synthetic stepanovite, Na[Mg(H₂O)₆][Fe(C₂O₄)₃]·3H₂O, and zhemchuzhnikovite, Na[Mg(H₂O)₆][Al_0.55Fe_0.45(C₂O₄)₃]·3H₂O, has been determined by single-crystal X-ray diffraction methods. The compounds are isotypic to each other and to the previously reported Na[Mg(H₂O)₆][M(C₂O₄)₃]·3H₂O (M: Cr, Al). They crystallize in the trigonal P3c1 space group with Z = 6 molecules per unit cell and (hexagonal axes) a = 17.0483(4), c = 12.4218(4) Å for the iron compound, and a = 16.8852(5), c = 12.5368(5) Å for the Al/Fe solid solution. Comparison of our crystallographic results with previous X-ray diffraction and chemical data of type stepanovite and zhemchuzhnikovite minerals provides compelling evidence that these natural materials possess the same crystal and molecular structure as their synthetic counterparts. It is shown that the originally reported unit cell for stepanovite represents a pronounced sub-cell and that the correct unit cell and space group are based on weak superstructure reflections. The infrared and Raman spectra of both synthetic analogs were also recorded and are briefly discussed.