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The origin of a tetrahedral diamond

Published online by Cambridge University Press:  05 July 2018

A. F. Seager
A. F. Seager*
Affiliation:
Department of Geology, Birkbeck College, London W1P 1PA
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Summary

The symmetry of diamond is still disputed, and is usually regarded as 4/m3̄2/m or 4̄3m. The (rare) development of tetrahedral morphology has been cited in favour of the lower symmetry. A tetrahedral crystal of c. 4 mm edge-length was studied for evidence relevant to this controversy. Three similar quadrants have nearly plane tetrahedron faces, each surrounded by six curved surfaces belonging to forms {hkl}. The fourth (unique) quadrant, containing (IĪĪ), differs topographically. In the three similar quadrants the tetrahedron faces exhibit coplanar banding of trigons parallel to (IĪĪ), and the curved surfaces have striations parallel to it. The banding and striations are interpreted as stratigraphic etching of nitrogen-rich layers parallel to (IĪĪ) in a type I diamond. Slip, and apparently polygonization, occur in the unique quadrant. The three similar {III} faces were formed by cleavage, and the curved surfaces, banding, and striations by subsequent dissolution. It is probable that (IĪĪ) was part of an original octahedral surface, since tetrahedra of diamond are so rare. The tetrahedral morphology does not necessarily indicate that this crystal belongs to class 4̄3m. The doubt cast upon tetrahedral morphology (and the inferred twinning on {100} or about (100>) as evidence in favour of lower symmetry strengthens the case for assigning diamond to class 4/m3̄2/m.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 327 , September 1979 , pp. 377 - 387
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1979

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