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Transmission electron microscopy of coexisting low-tridymite polymorphs

Published online by Cambridge University Press:  05 July 2018

J. R. Ashworth
J. R. Ashworth*
Affiliation:
School of Earth Sciences, University of Birmingham, P.O. Box 363, Birmingham B15 2TT
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Abstract

Three polymorphs of tridymite (MC, PO-10 and MX) have been examined by transmission electron microscopy (TEM), in crystals which also contain lamellae of cristobalite. MX is the least common, and is highly unstable in the electron beam. It occurs as small regions (< 1 µm2) in PO-10 tridymite and at MC/PO-10 boundaries; these regions are interpreted as being strained. A consistent association between PO-10 and the cristobalite lamellae is attributed to the fact that PO-10 can more closely match the low-cristobalite structure than MC can, at boundaries parallel to the plane in which the crystal structures contain layers of SiO4 tetrahedra. An analogous interpretation explains the observation that twin boundaries (associated with 60° or 120° rotation of the layer) are commonly parallel to the layer plane in PO-10 but at high angles to it in MC. Abundant lamellar features, parallel to the same plane in PO-10, are tentatively interpreted as representing twinning by reflection and 180° rotation, which may also account for c* streaking in diffraction patterns.

Keywords

tridymitepolymorphismtwinningtransmission electron microscopy
Type
Mineralogy
Information
Mineralogical Magazine , Volume 53 , Issue 369 , March 1989 , pp. 89 - 97
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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