The crystal structure of proto-enstatite, MgSiO3

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 sluggishness of the transformations between rhombic enstatite and the other two forms, and because of the ease of the inversion between the proto-and clinoforms.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.Protoenstatite 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 protoenstatite.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 clinoenstatite.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.The author is indebted to Dr F. 1~.Boyd of the Geophysical Laboratory, Washington, D.C. for attempting to grow large crystals of proto-enstatite.In 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. AC 12MgSiOa glass in a hydrothermal bomb at 1080 °C.for four days at 500 bars H20.