Abstract
Unit-cell dimensions of natural phlogopite from Franklin, New Jersey, have been determined at 1 atm and at 18, 35, and 47 kbar. The structure of phlogopite has been refined (wR = 14.7 percent) from three-dimensional X-ray diffraction data collected at 35 kbar, and is compared with previously published room-condition and high-temperature crystal-structure data. The change in unit-cell dimensions of monoclinic phlogopite between 1 atm and 47 kbar may be represented by a prolate spheroid, where compressibility perpendicular to the silicate layers is approximately five times the value within the layers. Mean metal-oxygen bond distances at 1 atm and 35 kbar, respectively, are 2.97 and 2.80A for inner K-O, 3.31 and 3.23A for outer K-O, 1.65 and 1.65A for (Si,Al)-O, 2.06 and 2.07A for M1-(O,F), and 2.06 and 2.03A for M2-(O,F). The weaker K-O bonds compress more than the average Mg-(O,F) bonds, which compress more than the (Si,Al)-O bonds.
Unit-cell dimensions of a natural IIb type chlorite were measured at 1 atm and at 22 and 40 kbar. The strain ellipsoid of compression is a prolate spheroid with axial ratio 3:1 and maximum compression direction perpendicular to the layers. The large anisotropies in mica and chlorite compression are due to the difference in bonding within the 2:1 layers versus the bonding between these layers.
