Syun-iti Akimoto

Pyroxene-garnet solid-solution equilibria in the systems Mg4Si4O12Mg3Al2Si3O12 and Fe4Si4O12Fe3Al2Si3O12 at high pressures and temperatures

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg4Si4O12Mg3Al2Si3O12, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe4Si4O12Fe3Al2Si3O12. At 1,000°C, the solid solubility of enstatite (MgSiO3) in pyrope (Mg3Al2Si3O12) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg3(Al0.8Mg0.6Si0.6)Si3O12. In the MgSiO3-rich field, the three-phase assemblage of β- or γ-Mg2SiO4, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe4Si4O12Fe3Al2Si3O12 shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO3) in almandine (Fe3Al2Si3O12) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C. If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.

High-pressure phase equilibria in a garnet lherzolite, with special reference to Mg2+Fe2+ partitioning among constituent minerals

Abstract Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg 2+ Fe 2+ , among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg 2 SiO 4 Fe 2 SiO 4 . At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe 2+ /(Mg + Fe 2+ ) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe 2+ /(Mg + Fe 2+ ) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe 2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg 2+ Fe 2+ partition relation among coexisting minerals, an increase of about 1% in the Fe 2 SiO 4 component in (Mg,Fe) 2 SiO 4 modified spinel and spinel was observed compared with olivine. These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.

Direct determination of coesite- stishovite transition by in-situ X-ray measurements

The coesite—stishovite transition was determined over the temperature range 500–1100° C by means of in-situ X-ray measurements with NaCl as an internal pressure standard. A cubic-anvil type of high-pressure apparatus and a high-power X-ray diffraction system were used for this study. Based on Deckers new pressure scale (1971), the coesite—stishovite transformation curve was represented by the linear equation P(kbar) = (80 ± 2) + (0.011 ± 0.003)(° C). Both the transition pressure at 1000° C and the slope dP/dT obtained in the present investigation are a little smaller than the previous determinations by the quenching method. Comparison with the thermodynamic data suggests that some uncertainty is still involved in the determination of the entropy of stishovite.

Compositions of coexisting liquid and solid phases formed upon melting of natural garnet and spinel lherzolites at high pressures: a preliminary report

Abstract The coexisting liquid and solid phases formed upon partial melting of a garnet lherzolite inclusion from kimberlite and a spinel lherzolite inclusion from tuff have been analyzed with the electron probe microanalyser. The experiments show that partial melting of 10–15% of garnet lherzolite with 2% phlogopite produces alkali picritic liquid at 30 kb (∼ 100 km) under anhydrous conditions. The compositions of olivine, orthopyroxene, clinopyroxene and garnet change regularly with temperature; for instance, Cr, Al and Ca contents in olivine and orthopyroxene coexisting with clinopyroxene and garnet or spinel increase with increasing temperature. Under hydrous conditions, partial melting of about 20% of the spinel lherzolite produces andesitic or dacitic liquid at pressures at least near 25 kb. The residuum after partial melting of these lherzolites still has lherzolite mineral assemblage.

Stability of phlogopite at high pressures and possible presence of phlogopite in the earth's upper mantle

Abstract High-pressure experiments have been made on the stability of natural and synthetic phlogopites by the tetrahedral-anvil type high-pressure apparatus in the pressure range from 41 to 96 kb (kilobars) and in the temperature range from 1030° to 1680°C. Water pressure is probably equal to or slightly lower than total pressure during the runs. It is found that phlogopite breaks down to garnet and unidentified mineral at pressures higher than at least 40 kb at about 1200°C. The garnet formed by the breakdown of phlogopite most likely contains about 5 wt % K 2 O, as determined by the electron probe microanalyser. It is suggested that phlogopite could be a possible potassium-bearing mineral in the upper mantle down to 150–200 km in the continental areas, forming phlogopite-bearing peridotites. In the deeper parts of the upper mantle garnet may be a possible potassium-bearing mineral. At shallower levels of the upper mantle (e.g., less than 100 km) amphibole may be a potassium-bearing mineral.instead of or in addition to phlogopite. Kimberlites and some potassium-rich basalts may be formed by the partial melting of the phlogopite-bearing peridotites of the upper mantle.

Hydrostatic compression of four corundum‐type compounds: α‐Al2O3, V2O3, Cr2O3, and α‐Fe2O3

Accurate x‐ray diffraction experiments for hydrostatic compression of corundum α‐Al2O3 and transition‐metal sesquioxides with the corundum structure, V2O3, Cr2O3, and α‐Fe2O3, were performed using a cubic‐anvil type of high‐pressure apparatus at room temperature up to about 120 kbar. An isotropic compression of the unit cell was found in α‐Al2O3 and Cr2O3. In V2O3 and α‐Fe2O3, anisotropic behaviors of compression were observed: The ratio c/a in the hexagonal cell increases in V2O3 and decreases in α‐Fe2O3 with increasing pressure. The isothermal bulk modulus KT=1.75±0.03 Mbar of V2O3 is unusually small among the studied compounds with the corundum structure. Values of 2.39±0.04 Mbar and 2.31±0.05 Mbar were calculated for KT of α‐Al2O3 and Cr2O3, and a value of 2.31±0.10 Mbar for KT of α‐Fe2O3 in the pressure range below 30 kbar.

Structure Relations of Hexagonal Perovskite-Like Compounds ABX3 at High Pressure

Phase stability relations among four hexagonal perovskite-like structures as well as the cubic perovskite structure have been studied for several oxides (BaMnO 3 and SrMnO 3 ) and fluorides (CsMnF 3 , RbNiF 3 and TlNiF 3 ) at high pressure. A series of high pressure transformations are found to occur in the order of the packing sequence along the hexagonal c axis (or cubic 111 axis) of ( a b ), ( a b a b c b c a c ), ( a b a c ), ( a b c a c b ) and ( a b c ) with increasing pressure. This order is corresponding with the increasing order of the proportion of the cubic close-packed layers in the hexagonal close-packed structure. It is suggested that the tolerance factor of the perovskite structure and the Coulomb repulsive force play an important role in determining the crystal structure and its order in the series of phase transformations at high pressure.

High pressure transformations in zinc silicates

Phase transformations in Zn2SiO4 and ZnSiO3 have been investigated at high pressure up to 170 kbar and temperature to 1500°C. Crystal structures of high pressure polymorphs have been studied by means of single crystal and powder X-ray diffraction analyses. Chemical compositions have been determined by electron microprobe and wet chemical analyses. Five polymorphs have been identified in Zn2SiO4, designated as I–V in the order of increasing pressure. Coordination numbers of metal ions in the crystal structures of Zn2SiO4 II–IV are four, the same as those in Zn2SiO4 I with the phenacite structure. The crystal structure of Zn2SiO4 II is composed of an approximately body-centered tetragonal arrangement of oxygen ions. Zn2SiO4 III and IV are suggested to be nonstoichiometric. Zn2SiO4 V, appearing above 130 kbar, is identified to be of the modified spinel structure. Zn2+ ions enter the octahedrally coordinated sites in it, accompanied by a large density increase. No olivine-like structures could be found among five polymorphs in Zn2SiO4. The solubility limit of Zn2SiO4 in Mg2SiO4 with the olivine structure is determined to be close to 75% at 90 kbar. Only a clinopyroxene form of ZnSiO3 is found to be stable over a relatively wide region in the pressure-temperature diagram. However, it has anomalous unit cell parameters when compared with more conventional pyroxenes. The extreme instability of the olivine structure in Zn2SiO4, and unusual cell parameters in ZnSiO3 pyroxene are discussed in terms of crystal structures. Stability of the modified spinel structure is also inferred in some detail. It is suggested from the study of phase transformations in Zn2GeO4 and ZnGeO3 that simple analogy in the mode of the high-pressure transformation between silicates and the corresponding germanates should be reexamined carefully.

Crystal chemistry of MSeO3 and MTeO3 (M = Mg, Mn, Co, Ni, Cu, and Zn)

Abstract New selenites and tellurites MgSeO3, MnSeO3, CoSeO3, NiSeO3, CuSeO3, MnTeO3, CoTeO3, and NiTeO3 were synthesized under high pressures and temperatures. All the compounds are isomorphous and their crystal system is orthorhombic. Structure analyses were carried out for all the selenites and CoTeO3. The structure is described as a salt of M2+ and SeO32− or TeO32− ions or as a distorted perovskite. In these compounds an Se or Te atom is closely linked to three oxygen atoms to form a flattened trigonal pyramid. The features of this coordination are discussed. At low temperature, magnetic order appears in all the compounds containing iron group ions, among which CuSeO3 is a ferromagnet with the Curie temperature of 26°K.

Thermal expansion of stishovite

Abstract The thermal expansion of stishovite has been determined by an X-ray camera technique in a temperature range of 18 – 600°C at an atmospheric pressure. The thermal-expansion coefficients along the crystallographic a - and c -axes at 300 K are α a = (6.0 ± 0.6) · 10 −6 K −1 and α c = (1.4 ± 0.5) · 10 −6 K −1 , respectively. The volume coefficient at 300 K is α ν = (13.5 ± 0.6) · 10 −6 K −1 .