Abstract

An alkaline gabbro from the Brome complex, in Quebec, contains abundant biotite that is highly enriched in Ba and Ti. A bulk-mica sample has been analyzed for structural H 2 O, FeO and Fe 2 O 3 , and by Fe and Ti K-edge and Ti L-edge X-ray absorption spectroscopy to provide information on the valency and coordination states of Fe and Ti. Fe (super 3+) makes up a maximum of 10% of total iron, whereas no Ti (super 3+) can be detected. There is no spectroscopic evidence suggesting the presence of significant Fe (super 3+) or Ti (super 4+) in tetrahedral coordination. By combining all this information, it is shown that the bulk biotite contains about 50% of an oxy-mica component (O (super 2-) <-->OH (super -) ) with few or no vacancies in cation sites. Electron-microprobe analyses define core to rim zoning trends of outwardly decreasing Ba, Ti, Al, mgPSnd increasing Si, (Fe+Mn+Mg), (Na+K). The substitution mechanism in the Brome biotite is deduced to be: XII 0.5Ba (super 2+) + VI 0.33Ti (super 4+) +0.375 IV Al (super 3+) +0.045O (super 2-) <--> 0.465 XII (K+Na) (super +) + 0.375 VI (Fe+Mg) (super 2+) + 0.015 VI Fe (super 3+) + 0.535 IV Si (super 4+) + 0.045(OH) (super -) . Cation-site deficiencies commonly reported 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 O (super 2-) for OH (super -) (oxy-mica substitution).

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