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Staurolite twinning (With Plates V-VI.)

Published online by Cambridge University Press:  14 March 2018

Vernon J. Hurst ,
J. D. H. Donnay  and
Gabrielle Donnay
Vernon J. Hurst
Affiliation:
The Johns Hopkins University, Baltimore 18, Maryland, U.S.A.
J. D. H. Donnay
Affiliation:
The Johns Hopkins University, Baltimore 18, Maryland, U.S.A.
Gabrielle Donnay
Affiliation:
Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C., U.S.A.
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Summary

The space group Ccmm of the accepted staurolite structure (7·82:16·52:5·63 kX 0·473:1:0·341, Náray-Szabó 1929) is a pseudo-space group; the true one has no c glide plane. Staurolite is monoclinie pseudo-orthorhombic, as shown optically by horizontal dispersion and morphologically by unequal developments of forms r(201) and r′(01). It has variable cell dimensions, 7·83–7·95:16·50–16·82:5·62 5·71 Å., constant β 90° + 3′, aspect C*, and probable space group C2/m (negative pyroelectric test). This leads to the prediction of a new type of twin, ‘by high order merohedry’, a penetration twin that simulates an orthorhombic single crystal. It is known (Friedel, 1922) that the 90°-cross can be accounted for by two different twin laws and the 60°-cross by five. The twin operations are respectively the pseudo-symmetry operations of a pseudo-tetragonal cell, obtained by transformation 010/003/100, and those of Mallard's pseudo-cube, resulting from transformation 013/03/300. By precession methods more than one twin law is established for each type of cross: both laws [100]90° and [013]180° are found for 90°-crosses: [313]180 and [102]120°, for 60°-crosses.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 31 , Issue 233 , June 1956 , pp. 145 - 163
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1956

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