CRYSTAL STRUCTURE OF ORTHOENSTATITE

The crystal structure of a natural orthorhombic enstatite (orthoenstatite) from the Bishopville meteorite, has been refined by the least-squares method using 680 three-dimensional reflections. The space group and the cell dimensions of this orthoenstatite are Pbca and a = 18.210, b = 8.812 and c = 5.178 A. The crystal structure of orthoenstatite is compared with that of clinoenstatite and the geometric relationship between the two polymorphs is described. The orthoenstatite structure is shown to be an exact twin of clinoenstatite on a unit-cell scale. The orthorhombic cell is composed of two monoclinic cells of clinoenstatite joined on (100) through a 6-glide plane. The orthoenstatite structure is also compared with the structures of hypersthene and orthoferrosilite. The corresponding interatomic distances show the following characteristics. With increasing Fe replacement of Mg the interatomic distances M-O in the metal octahedra increase. The mean interatomic distances, M-O, of the octahedra can thus be used to determine the Mg:Fe ratio in the octahedral sites. More surprisingly, an increase of Fe in Mg positions is accompanied by a decrease of the mean Si-O distances in the silicate tetrahedra. © 1969, Walter de Gruyter. All rights reserved.