Cause of Twinning of Crystals

Abstract

IN the course of an investigation¹ of optically active uniaxial crystals, I expressed the view that the possibility of twinning of a crystal was connected with a definite pseudo-symmetrical character of its structure as a whole. This statement was seriously attacked by Prof. Friedel, as may be clear from the following quotation of his paper:² “La formation de la macle est totalement indifférente à la symétrie ou à la pseudo-symétrie de l'édifice cristallin ou des groupes d'atomes qui le constituent. Elle est due uniquement à la rencontre de symétries ou de pseudo-symétries dans les périodes (mailles, simples ou multiples) de cet édifice.”