Departments of Geology and Chemistry

A number of polytypes, existing over short intervals of structure, have been observed in en-statites. These include the27,36, and 54 A periodicit ies, in addition to the well known 9 A monoclinic enstatite (CLEN) and l8 A orthorhombic enstatite (OREN) polymorphs. The polytypes, as well as all other features described in this paper, can be explained on the basis of a twinning model. This includes "antiphase boundaries, " which we interpret in terms of the normal (100) twin planes stepping across adjacent (100) planes. Parting planes can be seen in their earliest stages of development as simple extensions of the (100) twin planes. They presumably occur in regions where lattice strains have been most highly localized. These strains can be observed as dark contrast in the electron photomicrographs. Parting is produced by shear plus rotation. It is possible to distinguish between CLEN that formed by solid state transformation (a) directly from OREN in the absence of shearing, (b) from OREN by shearing, and (c) by temperature quench from the protoenstatite stabil ity f ield. This is done by measuring the width of CLEN fields within OREN and noting the degree of CLEN twinning. Most meteoritic CLEN formed by shearing