THE NEW THERMOSTABILITY CRITERION OF INDUSTRIAL PHLOGOPITE CRYSTALS. G.I. Kosmacheva. V.M Kalikhman, G.A. Kuznetsova, M.S. Metsik. Irkutsk State University, Irkutsk, Russia.

Crystal structure thermostability of industrial phlogopite has been investigated.The structural thermostability was estimated as a temperature of nonreversible decrease of basal interplanar distances d₀₀₁₂ at the heating during 30 min.14 various genetic type specimens of different chemical composition from Aldan and Carel-Colsky mica provinces have been studied.

The interplanar distances varied from 1,018 nm to 1,028 nm, the thermostability ranged between 1170-870 K with admixture changes of F 0,4-5,5%, Fe -2-9%,Ti -0,3- 0,8%.The most thermostable crystals (T=1170 K) contained iron less than 5%. The thermostability decreased to 870 K with the increase of the iron content, especially Fe³⁺ even if the F-content was 4-5%.

Structure phlogopite thermostability has been experimentally determined to depend on iron concentration in the octahedral layer but not depend on fluorine content, which substituted the hydroxyl in mica lattice.

The octahedral substitution of Mg to Fe³⁺ resulted the formation of localized quasimuscovite defective domains with lower thermostability.The formation of these domains promoted the crystal dehydroxylating process and decreased the thermostability at the heating with the iron conctntration over 5%.