Orientation of the Boundaries of Out-of-Step Domains in Anorthite

Paul H. Ribbe and Alan A. Colville
Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia
Department of Geology, California State College at Los Angeles, Los Angeles, California

Summary: The orientation of the boundaries of out-of-step domains in ‘transitional’ anorthite has been deduced from the direction of elongation of the diffuse X-ray reflections h + h + l = 2n + 1 in selected precession photographs. The reflections are elongated normal to (23¯1¯) which is in the zone [111¯ ] and contains the body-centring vector (a+b−c)/2. The structure is presumed to be perfect parallel to (223¯1¯). Atoms in essentially identical, primitive anorthite-like domains are related across the out-of-step domain boundaries by body-centring vectors. Increasing diffuseness of the h + k + l = 2n + 1 reflections indicates progressively smaller domains (more frequent domain boundaries), and when thesereflections disappear, the structure has become body-centred on the average.

Mineralogical Magazine; June 1968 v. 36; no. 282; p. 814-819; DOI: 10.1180/minmag.1968.036.282.08
© 1968, The Mineralogical Society
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