BA-RICH AND TI-RICH PRIMARY BIOTITE FROM THE BROME ALKALINE IGNEOUS COMPLEX, MONTEREGIAN HILLS, QUEBEC - MECHANISMS OF SUBSTITUTION

An alkaline gabbro, from the Brome complex, in Quebec, contains abunda nt biotite that is highly enriched in Ba and Ti. A bulk-mica sample ha s been analyzed for structural H2O, FeO and Fe2O3, and by Fe and Ti K- edge and Ti L-edge X-ray absorption spectroscopy to provide informatio n on the valency and coordination states of Fe and Ti. Fe3+ makes up a maximum of 10% of total iron, whereas no Ti3+ can be detected. There is no spectroscopic evidence suggesting the presence of significant Fe 3+ or Ti4+ in tetrahedral coordination By combining all this informati on, it is shown that the bulk biotite contains about 50% of an oxy-mic a component (O2- <----> OH-) with few or no vacancies in cation sites. Electron-microprobe analyses define core to rim zoning trends of outw ardly decreasing Ba, Ti, Al, mg# and increasing Si, (Fe + Mn + Mg), (N a + K). The substitution mechanism in the Brome biotite is deduced to be: (XII)0.5Ba(2+) + (VI)0.33Ti(4+) 0.375(IV)Al(3+) + 0.045O(2-) <---- > 0.465(XII)(K + Na)(+) + 0.375(VI)(Fe + Mg)(2+) + 0.015(VI)Fe(3+) + 0 .535(IV)Si(4+) + 0.045(OH)(-) Cation-site deficiencies commonly report ed for very Ba-Ti-rich biotite in alkaline, basic igneous rocks may be artefacts of using 22-oxygen cell formulae rather than taking account of major replacement of excess O2- for OH- (oxy-mica substitution).