The Crystal Chemistry of Moganite and Amethyst

Abstract

The crystal structure of moganite , SiO₂, is described, analysed, and shown to be (mimetic) Brazil-twinned α-quartz: it consists of {1101} layers of the latter, one tetrahedron thick, coherently joined at composition planes, with left- and right-handed quartz layers alternating. The structure of the region immediately adjacent to a composition plane is identical to that found in macroscopically twinned quartz, e.g. in amethyst. Thus the 'defect' structure in the latter is the total structure of the former-a classic example of Wadsley's idea that, in crystals, 'defects' are stable structural entities.