STRUCTURAL TRANSFORMATIONS OF QUARTZ AND BERLINITE ALPO4 AT HIGH-PRESSURE AND ROOM-TEMPERATURE - A TRANSMISSION ELECTRON-MICROSCOPY STUDY

Single crystals of alpha-quartz and alpha-berlinite AlPO4 have been co mpressed at high pressure and room temperature in a diamond anvil cell (DAC). The pressure-induced microstructures have been studied on reco vered specimens using transmission electron microscopy. As previously reported, quartz is shown to exhibit an amorphous transition at high p ressure (almost-equal-to 30 GPa). Under the markedly non-hydrostatic c onditions of the present study, a wide mixed-phase regime in which amo rphous lamellae form within the crystalline matrix is encountered at l ower values of the mean stress. The amorphous lamellae are interpreted as shear lamellae. The formation of these shear lamellae as well as t heir habit planes are described by the evolution with pressure of shea r moduli mu as computed in anisotropic elasticity. Our calculations al so show instabilities at higher pressure of the elastic moduli (i.e. o f the alpha-quartz structure) which are related to the amorphous trans ition. Berlinite exhibits a more ductile behavior with simultaneous di slocation activity and shear on amorphous lamellae which become pervas ive at high pressure (almost-equal-to 10 GPa). These amorphous lamella e of berlinite do not revert to crystal when pressure is released.