THE STRUCTURAL PRESSURE DEPENDENCE OF MICROCLINE (KAlSi₃O₈) TO 7 Gpa. D. R. Allan, R. J. Angel, Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

Members of the feldspar group of minerals are the most abundant constituents of igneous rocks and form a major component of the Earth's crust. A knowledge of their high-pressure and high-temperature behaviour is, therefore, crucially important for the understanding of crustal chemical and thermodynamic processes. Previous high-pressure single-crystal x-ray diffraction studies of well-ordered K-rich microcline have identified discontinuities in the pressure dependence of the [[alpha]] and [[beta]] unit-cell angles at approximately 3.8 GPa while, in contrast, the rate of change of [[gamma]] was found to be continuous. The volume reduction accompanying this change in behaviour was found to be too small to be measurable and studies conducted above ~3.8 GPa did not reveal a change in symmetry. We have now repeated these earlier unit-cell measurements, but over a pressure range extended from ~4.8 GPa to ~7.1 GPa and we have also determined the underlying pressure-induced changes in the crystal structure for the first time.

Our preliminary results confirm the findings of the previous study and indicate that the structural changes with pressure are remarkably strong. The potassium atom, for example, initially undergoes a large continuous shift which is principally directed along the a-axis and at pressures above 3.8 GPa there is an abrupt change with a strong additional c-axis component. Accompanying the potassium atom displacement some of the key Si(Al)-O-Si(Al) linking bond angles exhibit very large changes of up to 18deg. at 7.1 GPa. Indeed, the sudden change in the pressure dependence of some of the linking bond angles at 3.8 GPa may partly account for the discontinuity in the rate of change of the unit-cell angle [[beta]] at the same pressure.