A HIGH-PRESSURE STRUCTURAL STUDY OF MICROCLINE (KALSI3O8) TO 7 GPA

Unit-cell parameters of a microcline of average composition Na0.01K0.9 7Si3.02 Al0.98O4 have been determined at twelve pressures from atmosph eric pressure to a maximum of 7.1 GPa by single-crystal X-ray diffract ion in a diamond-anvil pressure cell. The unit-cell angles alpha and b eta show small changes in their variation with pressure at approximate ly 3.8 GPa, in agreement with a previous study. There is no measurable discontinuity in cell volume accompanying these changes. The volume-p ressure data are described by a third order Birch-Murnaghan equation o f state with V-0 = 719.8(1.1) Angstrom, K-0 = 58.3(2.0) GPa with K' fi xed equal to 4. The crystal structure was also refined to X-ray intens ity data collected at six pressures to a maximum pressure of 7.1 GPa. The results indicate that the symmetry of microcline remains C (l) ove r bar over this entire pressure range. The volume compression is accom modated by flexing of the T-O-T bond angles of the framework, and shor tening of the M-O distances. The largest T-O-T angle changes are T1o-O bo-T2o and T1m-Obm-T2m which decrease by 15 degrees and 18 degrees res pectively between room pressure and 7.1 GPa. The flexing of the framew ork reduces the volume of the cage occupied by the M cation by 14 +/- 2% in this pressure range. The response of the microcline structure to pressure changes between 2.9 and 4.4 GPa. At low pressures the M posi tion at approximate fractional coordinates 0.29, 0.00, 0.14 moves in t he [(1) over bar 00] direction as pressure is increased, but at higher pressures there is an additional component of displacement along [001 ]. This change in the pattern of displacement of the M site is linked to changes in the pressure dependence of both of the T-Ob-T angles in the tetrahedral framework and the M-Oa1', M-0a2, M-Obo and M-Odm bond lengths.