SYNTHETIC BIOTITE OXIDATION UNDER HYDROTHERMAL CONDITIONS

The oxidation of synthetic ferruginous biotite in hydrothermal systems was studied for two ideal compositions, annite, KFe3AlSi3O10(OH)(2), and siderophyllite, KFe2.5Al2Si2.5O10(OH)(2). These biotite samples we re annealed in Shaw-type vessels under controlled f(H2) at a total pre ssure of 1 kbar. The ratio of Fe2+ to total Fe, determined by wet-chem ical analysis, is directly proportional to the f(H2) regardless of tem perature variations. For f(H2) values of < 1 bar to 102 bars, this rat io varies from 0.72 to 0.90 for annite and from 0.76 to 0.96 for sider ophyllite. The H contents of the biotite do not vary with Fe3+ content for either composition and result in 94 and 100% of the OH sites bein g occupied in annite and siderophyllite, respectively. A consistent de crease in total octahedral cation content with increasing Fe3+ content for both annite and siderophyllite suggests that the Fe-vacancy subst itution 3Fe(2+) reversible arrow 2Fe(3+) + ([6])Vac plays a dominant r ole in the oxidation of these synthetic biotite samples.