NEW PHENOMENA IN MINERALS AT HIGH-PRESSURES

Increasing precision in high-pressure single-crystal diffraction exper iments has resulted in the detection of a number of apparent phase tra nsitions at high pressures that are not accompanied by any detectable symmetry change. Such ''Isosymmetric Transitions'' have been detected mostly in complex framework structures with large degrees of internal structural freedom. The experimental evidence for such transitions in two minerals, orthopyroxene and anorthite, is reviewed and discussed.