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Single crystal growth of the modified spinel (β) and spinel (γ) phases of (Mg,Fe)2SiO4 and some geophysical implications

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Abstract

Single crystals of ferromagnesian orthosilicates with modified spinel (β) and spinel (γ) structure as large as 500 μm have been grown by solid state crystallization at high temperature and high pressure using an MA8-type apparatus driven in a 2,000-ton uniaxial press. This system is capable of generating pressures of 24.0 (±0.3) GPa at 2,400 (±50)°C for one hour in a sample assembly volume of 0.14 cm3. Crystals larger than 100 μm were observed to grow only at pressures within 5 percent of the phase boundary between the stability fields of the β and γ phases.

Experimental determination of the phase boundaries between β or β+γ and γ phases for (Mg,Fe)2SiO4 has been extended to 22 GPa and 2,400°C. The effect of configurational entropy due to disordering is evaluated to be minimal on the basis of the cationic distribution in the synthesized samples; thus, we conclude that the phase boundary between β or β+γ and γ phases remains essentially linear to 2,400°C. In (Mg,Fe)2SiO4 solid solutions, the stability field of the γ phase shifts towards the lower pressures with increasing iron content at a rate of a 1 GPa for each 10 mole percent Fe.

Assignment of the β→β+γ→γ transition to the seismic 550 km discontinuity is rejected by the present phase diagram results for (Mg0.9Fe0.1)2SiO4 and measurement of acoustic velocities for β and γ Mg2SiO4, but the discontinuity may be caused by a phase transition of pyroxene to a garnet-like structure.

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  1. Department of Earth Sciences, Nagoya University, 464, Nagoya, Japan

    Hiroshi Sawamoto

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  1. Hiroshi Sawamoto
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Sawamoto, H. Single crystal growth of the modified spinel (β) and spinel (γ) phases of (Mg,Fe)2SiO4 and some geophysical implications. Phys Chem Minerals 13, 1–10 (1986). https://doi.org/10.1007/BF00307307

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  • DOI: https://doi.org/10.1007/BF00307307

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