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The orientation of the pericline twin lamellae in triclinic alkali felspars (With Plate II.)

Published online by Cambridge University Press:  14 March 2018

Wm. Scott MacKenzie
Wm. Scott MacKenzie*
Affiliation:
Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C., U.S.A.
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Summary

Cross-hatched twinning is characteristic of two alkali-felspar species, microclines and anorthoclases. Because of the difference in the crystallographic angles of these two species, the cross-hatched twinning is most clearly seen in (001) sections of microcline and in sections perpendicular to the a crystallographic axis in anorthoclases. If pericline twinning is present it is generally seen in (010) sections of microclines, but if the microcline is microperthitic the twin lamellae may not be seen. Strings or lamellae of perthitic albite can be distinguished from pericline twin lamellae by refractive index differences and also by the angle that their trace makes with the trace of (001) in (010) sections.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 31 , Issue 232 , March 1956 , pp. 41 - 46
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1956

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