Enstatite, Mg2Si2O6: A neutron diffraction refinement of the crystal structure and a rigid-body analysis of the thermal vibration

Synthetic enstatite, Mg2Si2O6, is orthorhombic, space group Pbca, with eight formula units per cell and lattice parameters a = 18.235(3), b = 8.818(1), c = 5.179(1) A at 230C. A least-squares structure refinement based on 1790 neutron intensity data converged with an agreement factor R(F2) = 0.032, yielding Mg-O and Si-O bond lengths with standard deviations of 0.0007 and 0.0008 A, respectively. The variations observed in the Si-O bond lengths within the silicate tetrahedra A and B are caused by the differences in primary coordination of the oxygen atoms and the proximity of the magnesium ions to the silicon atoms. The latter effect is most pronounced for the bridging bonds of tetrahedron. A. The smallest O-Si-O angle is the result of edge-sharing by the Mg(2) octahedron and the A tetrahedron. An analysis of rigid-body thermal vibrations of the two crystallographically independent [SiO4] tetrahedra indicates considerable librational motion, leading to a thermal correction of apparent Si-O bond lengths as large as +0.002 A at room temperature. (orig.)