Epitaxic Relations between Coexisting Pyroxenes

J. Tarney
Department of Geology, The University, Birmingham 15

Summary: Many examples of epitaxic relations between coexisting pyroxenes are found in the picritic dykes that cut the Lewisian gneisses of the Lochinver area, Sutherland. In some dykes augite forms oriented overgrowths on the (100) face of orthopyroxene; in others orthopyroxene forms similar overgrowths on augite. A number of unusual variants of the first case are described together with a complex ‘induced’ twinning found in the augite overgrowths. Analyses of the coexisting pyroxenes suggest that the overgrowth relation has been facilitated by increased solid solution between the pyroxenes, so reducing the crystallographic misfit along the c-axis. However, crystallization of the pyroxenes in an ordered sequence rather than simultaneously also seems to have been an important factor.

Mineralogical Magazine; March 1969 v. 37; no. 285; p. 115-122; DOI: 10.1180/minmag.1969.037.285.11
© 1969, The Mineralogical Society
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