Twinning of tridymite from the Steinbach meteorite

Abstract

Tridymite from the Steinbach meteorite is monoclinic with cell dimensions: a 18.54, b 5.00, c 23.83 Å, β 105.70° and space group C2⁄c or Cc. Its diffraction pattern shows an extremely large number of structural absences, leading to pseudo-orthorhombic superlattice with cell dimensions A 51.86, B 30.00, C 49.20 Å. The method of twin analysis (Ito and Sadanaga, 1970) was applied uniquely to derive a crystal lattice from the twin pattern. Pseudohexagonal twinning about the [-201] row of the true monoclinic lattice satisfactorily accounts for these unusual structural extinctions. The symmetry of the twin is represented by the six-color symmetry symbolism. A possible mechanism of twin formation is proposed, based on the recently determined low tridymite structure.