Staurolite Twinning

Vernon J. Hurst1, J. D. H. Donnay and Gabrielle Donnay
The Johns Hopkins University, Baltimore 18, Maryland, U.S.A.
Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C., U.S.A.
1Present address: Department of Mines, Mining, and Geology, 425 State Capitol, Atlanta, Georgia, U.S.A.

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-orthorhombie, as shown optically by horizontal dispersion and morphologically by unequal developments of forms r(201) and r′(2¯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 orthorhombie 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 013/01¯3/300. 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.

Mineralogical Magazine; June 1956 v. 31; no. 233; p. 145-163; DOI: 10.1180/minmag.1956.031.233.04
© 1956, The Mineralogical Society
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