A high-pressure structural study of microcline (KAlSi (sub 3) O (sub 8) ) to 7 GPa
A high-pressure structural study of microcline (KAlSi (sub 3) O (sub 8) ) to 7 GPa (in EMPG-VI; Experimental mineralogy, petrology and geochemistry, E. A. J. Burke (editor), C. Chopin (editor), W. V. Maresch (editor), L. Ungaretti (editor) and C. Willaime (editor))
European Journal of Mineralogy (April 1997) 9 (2): 263-275
Unit-cell parameters of a microcline of average composition Na (sub 0.01) K (sub 0.97) Si (sub 3.02) O (sub 4) were determined at 12 points from 1 atm to 7.1 GPa by single-crystal XRD in a diamond-anvil press. The angles alpha and beta show small changes in their variation with P at approximately 3.8 GPa, but with no measurable discontinuity in cell volume; the data are described by V (sub 0) = 719.8 Aa (super 3) , K (sub 0) = 58.3 GPa with K' fixed at 4. The crystal structure was refined to data collected at six pressures to a maximum P of 7.1 GPa; the symmetry remains Cl over this entire P range, the V compression being accommodated by flexing one of the T-O-T bond angles (by 15-18 degrees ) of the framework and shortening of the M-O distances. This flexing reduces the volume of the cage occupied by the M cation by 14 + or - 2% in this P range; at low P the M position moves in the [<$E 1 back 40 up 40 ->00] direction as P increases, but at higher P there is an additional component of displacement along .