PHASE TRANSITIONS WlTHOUT SYMMETRY CHANGE, AND RELATED HIGH PRESSURE PHENOMENA. R.J. Angel, Bayerisches Geoinstitut, Universitat Bayreuth, D-95440 Bayreuth, Germany.

Recent in-situ diffraction studies of a number of compounds at high pressures and temperatures have revealed the existence of phase transitions that occur without symmetry change. A variety of phase transition behaviour is displayed by such systems:

1: In materials such as KTiOPO₄ and its chemical derivatives the phase transitions are strongly first order in character. The transition is purely displacive in nature involving deformation of the structural framework of the material.

2: In (Mg,Fe)SiO₃ orthopyroxenes the transition at pressure is either very-weakly first order or continuous, and results from a change in compression mechanism without major distortion of the structure.

3: In both anorthite-rich feldspars and (Mg,Fe)-clinopyroxenes there are two structurally distinct phases with the same symmetry, one stable at high temperatures and the other at high pressures. In-situ diffraction studies have revealed that in both systems the high-temperature and the high-pressure phases are structurally quite distinct. The stability fields in P-T space of the two phases are therefore expected to be separated by a transition line or crossover without symmetry change. In the case of anorthite the crossover has been observed directly by in-situ high-P,T single-crystal diffraction.

A review of available high-pressure data suggests that the appearance of such transitions is restricted to structures that have signifcant degrees of internal structural freedom that allow them to take up different conformations between which transformations can occur that do not involve primary bond breaking.