Polytypes of the phlogopite-biotite series and their mutual relations

Abstract

Chemical compositions and polytypes of twenty specimens of trioctahedral micas from various occurrences of the phlogopite-biotite series have been determined. The results (Tables 1 and 2) are as follows: 1) Almost all of the phlogopites and biotites in basalts and andesites are 1M type. 1M type biotite transforms to 2M type biotite (anormite) by the oxidation reaction, Fe2+(OH)-→Fe3+O2-+H↑. 2) Most biotites in granites and granodiorites are 3T type and a few 3 Td or 1M type. 3) In gneiss as compared with gneissose granodiorite, 3T type decreases and 3Td or 1M type increases. 4) The biotites in pegmatites are 1M or 3 Td or 3T type. 5) The biotites and phlogopites in quartz-biotite schists and in the skim zone of contact deposits are almost 1M type.In the synthesis of phlogopite and muscovite, at low temperatures the nucleation of 1Md type mica takes place easily, and so does the nucleation of 1M type mica at temperatures above 500°C. In the hydrothermal treatments of natural phlogopite and biotite, lMd type micas transfrom to 1M type, and 3Td type and 1M type micas transform to 3T type. In the basis of these experimental results, the mutual phase relations of polytypes of trioctahedral micas of the phlogopite-biotite series have been discussed and figured as in figures 3 and 4.