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
Mineralogical Society (www.minersoc.org)