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
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.