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High pressure single-crystal synthesis, structure and compressibility of the garnet Mn2+ 3Mn3+ 2[SiO4]3 |
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| Authors: | T Arlt T Armbruster R Miletich P Ulmer T Peters |
| Institution: | Mineralogisch-petrographisches Institut, Universit?t Bern, Baltzerstr. 1, CH-3012 Bern, Switzerland, CH Laboratorium für Chemische und Mineralogische Kristallographie, Universit?t Bern, Freiestr. 3, CH-3012 Bern, Switzerland, CH Bayerisches Geoinstitut, Universit?t Bayreuth, D-95440 Bayreuth, Germany, DE Mineralogisch-petrographisches Institut, ETH-Zentrum, Sonneggstr. 5, CH-8092 Zürich, Switzerland, CH |
| Abstract: | Single crystals of the garnet Mn2+ 3Mn3+ 2SiO4]3 and coesite were synthesised from MnO2-SiO2 oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca3Mn3+ 2GeO4]3 (space group I41/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn3+-O=1.995 (4) Å (octahedron), Mn2+-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K 0=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn2+ 3Mn3+ 2SiO4]3 were assigned based on a calderite Mn2+ 3Fe3+ 2SiO4]3 model extrapolated from andradite and grossular literature data. |
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