The single-crystal elastic moduli of alpha-quartz were measured to above 20
GPa in a diamond-anvil cell by Brillouin spectroscopy. The behavior of the
elastic moduli indicates that the high-pressure phase transition in quartz
is ferroelastic in nature and is driven by softening of C-44 through one o
f the Born stability criteria. The trends in elastic moduli confirm theoret
ical predictions, but there are important differences, particularly with re
spect to the magnitudes of the B-i. The quartz I-II transition occurs prior
to complete softening of the mode and amorphization.