The crystal structure of proto-enstatite, MgSiO3

Abstract

Proto-enstatite, at room temperature, is orthorhombie with 8 units of MgSiO3 in a cell of dimensions a-9.25, b-8.74, c = 5.32 A, pseudo space group Pbcn. The atomic coordinates of the pseudo space group were determined from X-ray powder data. The SiO3 chain is fully extended. One of the Mg atoms has an irregular coordination, which may be the reason for the instability of proto-enstatite at low temperature. The structure is very similar to that predicted by Atlas (1952). Proto-enstatite is one of the five forms of MgSi03 that have so far been described (see Atlas, 1952 for literature survey). Only three of the five have been thought to have a field of stability-rhombic enstatite, clino-enstatite and proto-enstatite. Determination of the phase relations is difficult because of the slug-gishness of the transformations between rhombic enstatite and the other two forms, and because of the ease of the inversion between the proto-and clino-forms. Whether clino-or proto-enstatite is the stable form at high temperature or whether both have a field of stability is still open to doubt, (Atlas, 1952; Foster, 1951; Boyd & Schairer, 1957), but it is certain that rhombic enstatite is the form stable at low temperatures and that proto-enstatite is unstable at the temperature of this structure determination. Proto-enstatite changes into clino-enstatite upon heating at 1400 °C., upon long standing at room temperature, and upon grinding. Thus the structure of the two forms must be related and, indeed, Atlas has used this as a basis for predicting a crystal structure for proto-enstatite. This investigation ha~ three objectives: establishing reliable crystallographic data for proto-enstatite, providing information on the atomic movements that occur during the phase transformations of MgSi03 and looking for a structural reason why stress can markedly effect the transformation for proto-to clino-enstatite. In this paper the first objective will be described: the second and third will be discussed later in collaboration with Dr N. Morimoto, who is currently engaged on a refinement of the structure of clino-enstatite. spite of considerable labour, all attempts were unsuccessful and the structure determination has had, perforce, to be carried out on a powder sample. The conditions of crystallization were: treatment of an * Contribution No. 58-15, College of Mineral Industries. The X-ray powder diffraction pattern (Table 1) was indexed on an orthorhombie cell with a = 9-25, b-8.74, c = 5.32/~ (±0-005). Atlas obtained values of 9.25, 8.92, 5.25/~ for a specimen inverted from an iron-and aluminium-bearing anthophyllite. All reflections but one, (210), would obey the rules for the space group Pbcn, although little reliance could be placed on this pseudo space group because of the low number of unequivocally indexed reflections in the principal zones. The axes x and y should be interchanged to conform to common usage: however, the above order of axes is given to show the relation to clino-enstatite whose 8.8 J~ axis is the symmetry axis. The volume of the unit cell is 430 /~a and the calculated specific gravity for 8 units of MgSiOa is 3.10. Intensity data were obtained from powdered crystals smeared on a glass slide that was mounted on a diffractometer. Cu Ka radiation and a proportional counter were used. The counter was moved at a rate of 0"1 ° 20 per minute and the total number of counts printed out ten times per minute. A profile of each peak was plotted and the integrated intensity obtained. At low angles no difficulty was experienced in estimating the background level, but at angles greater than 20 30 ° the background was irregular, possibly because of the presence of peaks too small for recognition but sufficiently large for an upset of the background level. Above 20 70 ° the reflections were so irregular and difficult to index that no measurements were made. Divergence slits of 4 ° were used so that all the specimen was irradiated at all angles, permitting a simple correction for the Lorentz-polarization factor. Measurements of lattice spacings and peak intensities (Table 1) for the X-ray Powder Data File were 35