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Corundum twins from Transvaal

Published online by Cambridge University Press:  14 March 2018

L. J. Spencer
L. J. Spencer*
Affiliation:
British Museum (Natural History)
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Extract

Lamellar twinning is of frequent occurrence in corundum, as evidenced by rulings on the surfaces and by the well-marked pseudo-cleavages (or partings) parallel to the faces of the primary rhombohedron and also parallel to the basal plane. Distinctly developed juxtaposition and interpenetration twins would, however, appear to be rare, for only a few isolated examples have hitherto been described.

There are two twin-laws for corundum crystals, the twin-plane being either r (101̄l) = (100) or c (0001) = (111). The former of these is the one considered in the present note.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 21 , Issue 118 , September 1927 , pp. 329 - 336
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1927

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References

page 329 note 1 On lamellar twinning see, for example, Judd, J. W., On the structure-planes of corundum. Min. Mag., 1895, vol. 11, pp. 49-55 Google Scholar.

page 330 note 1 V. Goldschmidt, Atlas der Krystallformen, 1918, vol. 5, pls. 18-26. r-twins are shown in figs. 47, 56-59, 89,119-121 ; c-twins in figs. 71, 88, 94, 96, 104-106.

page 331 note 1 Dr. A. L. Hall in his memoir (loc. cit., p. 67) mentions such twins measuring 6 inches across with a thickness of one-eighth of an inch, and he has informed me that twins of this size are by no means uncommon at the locality.

page 332 note 1 Plumasite was first described from Plumas Co., California, by A. C. Lawson in 1903 as an oligoelase-corundum rock. It has been formed by the desilicification of granitic magma intrusive into ultra-basic rocks. [Min. Abstr., vol. 1, pp. 16, 210, 282; vol. 2, p. 80 ; vol. 3, p. 84.]

page 333 note 1 Lewis, W. J., Min. Mag., 1908, vol. 15, p. 62 Google Scholar.

page 336 note 1 Tertsch, H., Trachten der Kristalle. Berlin, 1926 Google Scholar [Min. Abstr., vol. 3, p. 223]. See also Beeke, F., Über die Ausbildung der Zwillingskristalle. Fortschr. Min. Krist. Petr., 1911, vol. 1, pp. 68-85 Google Scholar ; and Hilton, H., The energy of twin-crystals. Min. Mag., 1909, vol. 15, pp. 245-246 Google Scholar.