STRUCTURAL CONTROL OF POLYHEDRAL COMPRESSION IN SYNTHETIC BRAUNITE, MN2+MN3+6O8SIO4

Citation
R. Miletich et al., STRUCTURAL CONTROL OF POLYHEDRAL COMPRESSION IN SYNTHETIC BRAUNITE, MN2+MN3+6O8SIO4, Physics and chemistry of minerals, 25(3), 1998, pp. 183-192
Citations number
51
Categorie Soggetti
Mineralogy,"Material Science
ISSN journal
03421791
Volume
25
Issue
3
Year of publication
1998
Pages
183 - 192
Database
ISI
SICI code
0342-1791(1998)25:3<183:SCOPCI>2.0.ZU;2-9
Abstract
The compression of synthetic braunite, Mn2+Mn63+O8SiO4, was studied by high-pressure single crystal X-ray diffraction carried out in a diamo nd-anvil cell. The equation of state at room temperature (third-or-der Birch-Murnaghan equation of state: V-0=1661.15(8) Angstrom(3), K-0.29 8=180.7+/-0.9 GPa, K'=6.5+/-0.3) was determined from unit-cell volume data to 9.18 GPa. Crystal structures were determined at 6 different pr essures to 7.69 GPa. Compression of the structure (space group I4(1)/a cd) was found to be slightly anisotropic (a(0)=9.4262(4) Angstrom, K-a =499+/-4 GPa, K-a'=19.7+/-0.9; c(0)=18.6964(6) Angstrom, K-c=657+/-6 G Pa, K-c'=15.7+/-1.4) which can be attributed to the fact that the Mn3-O bonds, which are the most compressible bonds, are aligned closer to the (001) plane than to the c axis. The large bulk modulus is the res ult of the structural topology in which 2/3 and 1/2 of the edges of th e Mn2+O8 and Mn3+O6 polyhedra share edges with other polyhedra. The Mn 2+O8 polyhedra were found to compress isotropically, whereas anisotrop ic compressional behaviour was observed for all three Mn3+O6 octahedra . Although the polyhedral geometry of all three crystallographically i ndependent Mn3+ sites shows the same type of uniaxially elongated dist ortion, the compression of the individual octahedral configurations wa s found to be strongly dependent upon both the geometry of the polyhed ron itself and the types of, and the connectivity to, the neighbouring polyhedra. The differences in the configuration of the different oxyg en atoms, and therefore the structural topology, is one of the major f actors determining the type and degree of the pressure-induced distort ion, while the Jahn-Teller effect plays a subordinate role.