A high-pressure structural study of microcline (KAlSi3O8) to 7 GPa
Allan, David R.; Angel, Ross J.
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 263 - 276
published: Jun 26, 1997
manuscript accepted: Oct 31, 1996
manuscript received: May 17, 1996
ArtNo. ESP147050902004, Price: 29.00 €
Abstract Unit-cell parameters of a microcline of average composition Na0.01K0.97Si3.02 AI0.98O4 have been determined at twelve pressures from atmospheric pressure to a maximum of 7.1 GPa by single-crystal X-ray diffraction in a diamond-anvil pressure cell. The unit-cell angles α and ß show small changes in their variation with pressure at approximately 3.8 GPa, in agreement with a previous study. There is no measurable discontinuity in cell volume accompanying these changes. The volume-pressure data are described by a third order Birch-Murnaghan equation of state with V0 = 719.8(1.1) Å3, KQ = 58.3(2.0) GPa with K' fixed equal to 4. The crystal structure was also refined to X-ray intensity data collected at j>ix pressures to a maximum pressure of 7.1 GPa. The results indicate that the symmetry of microcline remains Cl over this entire pressure range. The volume compression is accommodated by flexing of the T-O-T bond angles of the framework, and shortening of the M-0 distances. The largest T-O-T angle changes are T1o-Obo-T2o and Tlm-Obm-T2m which decrease by 15° and 18° respectively between room pressure and 7.1 GPa . The flexing of the framework 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 position at approximate fractional coordinates 0.29, 0.00, 0.14 moves in the  direction as pressure is increased, but at higher pressures there is an additional component of displacement along . 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-Oa2, M-Obo and M-Odm bond lengths.