Growth and post growth histories of high quartz as revealed by natural and laboratorical etching.

Through a comparison of etch figure morphologies observed on natural { 10_??_1} surfaces of high quartz phenocrysts occurring in a dacitic tuff and those produced by laboratorical etching, growth and post growth histories of the crystals were analysed. It was demonstrated that the crystals grew by the spiral growth mechanism, and then experienced a very short dissolution period by disequilibrium above the phase transition temperature, 573°C, and possibly a weak plastic deformation. This was followed by a rapid quenching, due to adiabatic expansion of the magma, which resulted in crack formation and phase transition. Through phase transition from high to low quartz, Dauphine twins were formed, associated by the multiplication of dislocations due to the attendant volume change.