Twinning of Monoclinic Crystals as a Function of Habit Type and Centrosymmetry

Abstract

Twinning in 118 species of monoclinic crystals has been surveyed. Eighty percent of such twinning occurs on planes with a k index of zero. Some of this twinning is mechanical, but most is due to a mechanism, called the SA mechanism, described in this work. It operates only in centrosymmetric crystals and in noncentrosymmetric ones that are pseudo‐orthorhombic. In most cases, SA twinning and mechanical twinning can be distinguished on the basis of the crystal's growth habit. Growth habits of monoclinic crystals can be classified into eight general types, two of which account for 75% of all monoclinic crystals. Precise criteria for pseudosymmetry are suggested.