Crystal structure and compression of an iron-bearing Phase A to 33 GPa |
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| Authors: | C M Holl J R Smyth M H Manghnani G M Amulele M Sekar D J Frost V B Prakapenka G Shen |
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| Institution: | (1) Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA;(2) Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA;(3) Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany;(4) Advanced Photon Source, Argonne National Laboratory, University of Chicago, Chicago, IL 60637, USA |
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| Abstract: | Fe-bearing dense hydrous magnesium silicate Phase A, Mg6.85Fe0.14Si2.00O8(OH)6 has been studied by single-crystal X-ray diffraction at ambient conditions and by high-pressure powder diffraction using synchrotron radiation to 33 GPa. Unit cell parameters at room temperature and pressure from single crystal diffraction are a=7.8678 (4) Å, c=9.5771 (5) Å, and V=513.43 (4) Å3. Fitting of the P–V data to a third-order Birch-Murnaghan isothermal equation of state yields V
0=512.3 (3) Å3, K
T,0=102.9 (28) GPa and K′=6.4 (3). Compression is strongly anisotropic with the a-axes, which lie in the plane of the distorted close-packed layers, approximately 26% more compressible than the c-axis, which is normal to the plane. Structure refinement from single-crystal X-ray intensity data reveals expansion of the structure with Fe substitution, mainly by expansion of M-site octahedra. The short Si2–O6 distance becomes nearly 1% shorter with ~2% Fe substitution for Mg, possibly providing additional rigidity in the c-direction over the Mg end member. K
T obtained for the Fe-bearing sample is ~5.5% greater than reported previously for Fe-free Phase A, despite the larger unit cell volume. This study represents a direct comparison of structure and K
T–ρ relations between two compositions of a F-free dense hydrous magnesium silicate (DHMS) phase, and may help to characterize the effect of Fe substitution on the properties of other DHMS phases from studies of the Fe-free end-members. |
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| Keywords: | Phase A High pressure Hydrous silicates DHMS Crystal structure Bulk modulus |
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