THE CRYSTAL-CHEMISTRY OF MOGANITE AND AMETHYST

The crystal structure of moganite, SiO2, is described, analysed, and s hown to be (mimetic) Brazil-twinned alpha-quartz: it consists of {1 (1 ) over bar 01} layers of the latter, one tetrahedron thick, coherently joined at composition planes, with left- and right-handed quartz laye rs alternating. The structure of the region immediately adjacent to a composition plane is identical to that found in macroscopically twinne d 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 id ea that, in crystals, 'defects' are stable structural entities.