Paper
CRYSTAL STRUCTURES OF POTASSIUM TETRA-SILICIC FLUORO-MICAS
2017 Volume 21 Issue 3 Pages 41-54
Details
2017 Volume 21 Issue 3 Pages 41-54
Potassium tetra-silicic fluoro-micas (K-TSFMs) having variable amounts of K and Li were synthesized and crystallized from melts of the designated compositions; the ideal, K-deficient and Li-bearing yangzhumingite [K1.0Mg2.5Si4O10F2, K0.8Mg2.6Si4O10F2 and K0.7Li0.3Mg2.5Si4O10], as well as tainiolite [K0.9Li0.5Mg2.3Si4O10F2]. Their crystal structures were refined to examine the crystal chemistry of Li in the K-TSFMs. Single crystal XRD data were obtained using automated 4-circle diffractometers with synchrotron radiation in addition to the monochromatized Mo Kα radiation. The lattice parameters, a=5.2471(7), b=9.0883(16), c=10.1483(16) Å, β=99.909(12)°, Z=476.72(13) Å3 (ideal yamgthumingite); a=5.2470(17), b=9.090(4), c=10.172(6) Å, β=99.9092(7)°, Z=477.9(4) Å3 (K-deficient yangzhumingite); a=5.2503(13), b=9.092(3), c=10.148(3) Å, β=99.94(2)°, Z=477.2(2) Å3 (Li-bearing yangzhumingite); a=5.2511(6), b=9.0914(14), c=10.1544(13) Å, β=99.883(10)°, Z=477.58(11) Å3 (tainiolite), showed no remarkable fluctuation among them, and are intermediate between those of trioctahedral and dioctahedral micas. The octahedral volumes of K-TSFMs are comparable to each other, and are larger than di-octahedral micas, but smaller than tri-octahedral micas. It is suggested that Li+ can replace Mg2+ without volume change in the octahedral sites in K-TSFMs by the equivalent octahedral volumes between tainiolite and Li-free yangzhumingite. The refinement on crystal structure of Li-bearing yangzhumingite suggests that Li+ can occupy the octahedral sites together with Mg2+, as well as Li+ in tainiolite.
