HR: 0800h
AN: MR31A-0129    [Abstracts]
TI: High-pressure Phase Relation In The MgAl2O4-Mg2SiO4 System
AU: * Kojitani, H
EM: hiroshi.kojitani@gakushuin.ac.jp
AF: Gakushuin University, Dept. of Chemistry, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan
AU: Hisatomi, R
EM: 03142013@gakushuin.ac.jp
AF: Gakushuin University, Dept. of Chemistry, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan
AU: Akaogi, M
EM: masaki.akaogi@gakushuin.ac.jp
AF: Gakushuin University, Dept. of Chemistry, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan
AB: High-pressure and high-temperature experiments indicate that high-pressure phases of oceanic basalts contain Al-rich phases. MgAl₂O₄ with calcium ferrite-type crystal structure is considered as a main component of such the Al-rich phases. Since the calcium ferrite-type MgAl₂O₄ can be synthesized at only the maximum pressure of a Kawai-type high-pressure apparatus with tungsten carbide (WC) anvils, the amount of a synthesized sample is very limited. Therefore, the crystal structure of the calcium ferrite-type MgAl₂O₄ has been hardly known in detail due to these difficulties in sample synthesis. In our high-pressure experiments in the MgO-Al₂O₃-SiO₂ system, it was shown that Mg₂SiO₄ component could be dissolved in the MgAl₂O₄ calcium ferrite. In this study, we tried to synthesize a single phase MgAl₂O₄ calcium ferrite sample and to make the Rietveld refinement of the XRD pattern of the sample. The high-pressure phase relations in the MgAl₂O₄-Mg₂SiO₄ system were studied to know the stability field of the MgAl₂O₄-Mg₂SiO₄ calcium ferrite solid solutions. Lattice parameters-composition relation of the MgAl₂O₄-Mg₂SiO₄ calcium ferrite solid solutions was also determined. High-pressure and high-temperature experiments were performed by using a Kawai-type high-pressure apparatus at Gakushuin University. WC anvils with truncated edge length of 1.5 mm were used. Heating was made by a Re heater. Temperature was measured by a Pt/Pt-13%Rh thermocouple. Starting materials for the phase relation experiments were the mixture of MgO, Al₂O₃ and SiO₂ with bulk compositions of MgAl₂O₄:Mg₂SiO₄ = 90:10, 78:22, 70:30 and 50:50. The starting materials were held at 21-27 GPa and 1600 °C for 3 hours and then were recovered by the quenching method. The MgAl₂O₄ calcium ferrite sample for the Rietveld analysis was prepared by heating MgAl₂O₄ spinel at 27 GPa and about 2200 °C for one hour. Powder X-ray diffraction (XRD) profiles of obtained samples were measured by using a X-ray diffractometer at Gakushuin University (RINT 2500V, Cr Kα, 45 kV, 250 mA). Composition analysis of the recovered samples was made using SEM-DES. The RIETAN-2000 program was used to perform the Rietveld refinement. The results of the high-pressure phase relation experiments show that stability field of single phase of MgAl₂O₄-Mg₂SiO₄ solid solutions spreads at lower pressure than that of pure MgAl₂O₄ calcium ferrite. The lowest pressure at which the calcium ferrite solid solution can be synthesized is about 23 GPa. The maximum solubility of Mg₂SiO₄ component is about 35%. Lattice parameters of pure MgAl₂O₄ calcium ferrite were determined as a = 9.9495(6) Å, b = 8.6466(5) Å, c = 2.7901(2) Å ( Pbnm space group) by the Rietveld refinement. Obtained atomic positions for calcium ferrite-type MgAl₂O₄ are very similar to those of CaFe₂O₄ calcium ferrite. Lattice parameters of MgAl₂O₄-Mg₂SiO₄ calcium ferrite solid solutions with various compositions indicate that c-axis does not change with the composition and that a- and b-axes have a linear increase and decrease trend with increasing Mg₂SiO₄ component, respectively.
DE: 3924 High-pressure behavior
DE: 3954 X-ray, neutron, and electron spectroscopy and diffraction
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005