Shuangmeng Zhai

Equation of state of γ-tricalcium phosphate, γ-Ca3(PO4)2, to lower mantle pressures

Abstract The γ-tricalcium phosphate phase (γ-TCP), γ-Ca3(PO4)2, is a high-pressure polymorph of tricalcium phosphate with a potential important implication as the reservoir of rare-earth elements and very large lithophile elements in the deep mantle. In situ synchrotron X-ray diffraction measurements of the γ-TCP phase have been carried out using a diamond-anvil cell to 40.29 GPa at room temperature, with a methanol-ethanol mixture as the pressure medium. The pressures in the measurements have been determined by using gold metal as the internal pressure calibrant. The third-order Birch-Murnaghan equation of state fitted to the experimentally defined unit-cell parameters suggests for the γ-TCP phase a density of ρ0 = 3.461(1) g/cm3, an isothermal bulk modulus of KT = 100.2(13) GPa, and first pressure derivative of K′T = 5.48(16). When K′T is fixed at 4, the derived KT is 113.1(12) GPa.

Determination of high-pressure phase equilibria of Fe2O3 using the Kawai-type apparatus equipped with sintered diamond anvils

Abstract Phase equilibria of Fe2O3 have been studied up to 58 GPa and 1400 K using the Kawai-type multi anvil apparatus equipped with sintered diamond anvils. Identification of phases and pressure determination has been carried out by means of in situ X-ray observation using synchrotron radiation at SPring-8. Hematite (phase I) successively transforms to the Rh2O3(II)-type structure (phase II) and then to an orthorhombic structure (phase III) with increasing pressure. The transformations of hematite into high-pressure phases have been observed only at temperatures higher than 500 K, which is not concordant with previous results obtained by using the diamond anvil cell. Volume changes accompanied by the I-II and II-III transformations are calculated to be -2.8 and -5.0%, respectively. The phase boundary between I and II phases and that between II and III have been proposed to be P (GPa) = -0.015 T (K) + 44.2 and P (GPa) = -0.005 T (K) + 48.7, respectively. Possible correlation between a Mott transition and the phase stabilities may be concealed at room temperature due to slow reaction kinetics of the structural transformations.

Si-Al distribution in high-pressure CaAl4Si2O11 phase: A 29Si and 27Al NMR study

Abstract High-resolution 29Si and 27Al NMR techniques have been applied to resolve the Si-Al distribution and coordination in the high-pressure CaAl4Si2O11 (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The 29Si MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400~1600°C show two broad, asymmetric peaks near -92.7 and -182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and Al, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The 27Al MAS and 3Q MAS NMR spectra contain only one unresolved peak typical of octahedral Al with a range of quadrupolar coupling constants

Phase boundary between perovskite and post-perovskite structures in MnGeO3 determined by in situ X-ray diffraction measurements using sintered diamond anvils

Abstract To determine the phase boundary between the perovskite and post-perovskite structures in MnGeO3, in situ X-ray observations were carried out at pressures of 57-68 GPa and temperatures of 1000-1900 K using the Kawai-type high-pressure apparatus equipped with sintered diamond anvils interfaced with synchrotron radiation. The phase boundary was determined to be P (GPa) = 39.2 + 0.013T (K) based on Tsuchiya’s (2003) gold pressure scale. The Clapeyron slope, dP/dT, of 13(+12/-5) MPa/K, determined in the present study is larger that of MgGeO3 and MgSiO3.

P-V-T relations of γ-Ca3(PO4)2 tuite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus

Abstract Tuite, γ-Ca3(PO4)2, is regarded as an important phosphate mineral in the deep mantle playing a crucial role as a host for rare earth elements, large ion lithophile elements, and phosphorus. In this study we report the thermoelastic properties of synthetic γ-Ca3(PO4)2 at simultaneously high pressures and temperatures of up to 35.4 GPa and 1300 K, respectively, as determined by means of in situ energydispersive X‑ray diffraction in a large-volume multi-anvil apparatus. The pressure-volume-temperature data obtained for γ-Ca3(PO4)2 were fitted by the high-temperature Birch-Murnaghan equation of state to yield V2 = 447.4(4) Å3, KT0 = 100.8(18) GPa, K′T0 = 5.74(13), (∂KT/∂T)P = -0.020(1) GPa/K, and αT = 3.26(18) × 10-5 + 1.76(24) × 10-8 T. In addition, fitting the present data to the Mie-Grüneisen-Debye equation of state gives γ0 = 1.35(6), Θ0 = 944(136) K, and q = 0.37(29). Based on the thermoelastic properties obtained in our study, the density profiles of γ-Ca3(PO4)2 tuite along typical cold and hot slab geotherms were calculated and are compared with those of the coexisting silicate minerals in subducting mid-ocean ridge basalt.

A comparison of the Ca3(PO4)2 and CaSiO3 systems, with a new structure refinement of tuite synthesized at 15 GPa and 1300 °C

Abstract Tuite, the high-pressure γ-form of the Ca3(PO4)2 system, has been synthesized from chlorapatite at 15 GPa and 1300 °C using a multi-anvil apparatus. Its crystal structure was determined with single-crystal X‑ray diffraction. It is isostructural with palmierite, with space group R3̅m and unit-cell parameters a = 5.2522(9) and c = 18.690(3) Å. The structure of tuite is characterized by three distinct polyhedra, PO4, Ca1O12, and Ca2O10, that are translationally interconnected in the sequence of PO4- Ca2O10-Ca1O12-Ca2O10-PO4 along the c axis. Comparison of the CaSiO3 and Ca3(PO4)2 polymorphic systems shows a striking resemblance in the evolution of atomic packing arrangements as the polymorphic density increases. In both cases, the Ca atoms are progressively incorporated into the (Ca+O) close-packed monolayers, consistent with the hypothesis that close packing is a consequence of volume decrease as density increases. Based on this observation, we predict a possible high-pressure post-tuite phase.

Compressibility of pyrochlore-type MgZrSi2O7 determined by in situ X-ray diffraction in a large-volume high pressure apparatus

The pyrochlore-type MgZrSi2O7 has been investigated by in situ synchrotron energy-dispersive X-ray diffraction measurements up 24.3 GPa at room temperature in a multianvil high pressure apparatus. The pressure has been calculated using an internal gold metal pressure calibrant. No phase transformation was observed in this study. A third-order Birch–Murnaghan equation of state was used to fit the pressure–volume data. The fitting gives a volume of V 0=802.8±0.2 Å3, an isothermal bulk modulus of K T =249±12 GPa, and first pressure derivative of K′ T =3.2±1.3. If K′ T is fixed as 4, K T is obtained as 241.8±2.6 GPa. Compared with zirconate or titanate pyrochlores, MgZrSi2O7 shows a smaller compressibility due to a larger cation radii ratio (r Mg,  Zr /r Si).

Effects of pre-heated pyrophyllite gaskets on high-pressure generation in the Kawai-type multi-anvil experiments

Pyrophyllite gaskets heated up to 900 °C for 1 h and raw gaskets (without heating treatment) were characterized by X-ray diffraction and Raman spectroscopy. These gaskets were used in multi-anvil high-pressure experiments to investigate the effect of pre-heating on pressure generation in Kawai-type apparatus. The results show that the gaskets heated at low temperatures (200–500 °C) have no change in structure and those heated at higher temperatures (600–900 °C) dehydroxylate gradually. No structural OH is observed in 900 °C pre-heated gasket from Raman spectroscopy indicating a complete dehydroxylation. The low temperature (200–500 °C) pre-heated gasket causes a limited effect on pressure generation, and the high-temperature (600–900 °C) pre-heated gasket has a relatively large negative effect on pressure generation.

High-pressure Raman spectra of Sr-substituted γ-Ca3−x Sr x (PO4)2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of Sr-substituted γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2) were carried out up to 30.7, 32.4 and 31.4 GPa, respectively. No pressure-induced phase transformation was observed in the present studies. The behaviors of the phosphate modes in γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2) below 14.4 GPa were quantitatively analyzed since a methanol:ethanol:water (16:3:1) mixture was used as the pressure medium in DAC. The Raman wavenumbers of all PO4 vibrations increased linearly and continuously with pressure for γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2). With the increasing content of Sr, the corresponding pressure coefficients α changed, and the v i0 shifts to higher wavenumbers except that for ν2 bending vibration.