HRTEM INVESTIGATION OF MICROSTRUCTURES IN LENGTH-SLOW CHALCEDONY

High-resolution transmission electron microscopy reveals dense Brazil- twin boundaries on the unit-cell scale in length-slow chalcedony. The twins can be considered as the result of stacking of left-and right-ha nded quartz with a (101) twin composition plane. Although most twin se quences on the unit-cell scale are nonperiodic, moganite-type domains result where they an periodic. It is proposed that the twins formed du ring rapid crystallization rather than as transformation products of a precursor phase such as moganite. The twin boundaries are energetical ly less stable than twin-foe areas and may indicate non-equilibrium cr ystallization at a high supersaturation of aqueous SiO2 species in the parent fluid.