The substitution mechanism of Ba and Ti into phyllosilicate phases: The example of barium-titanium biotite

Extremely high abundances of BaO (up to 21.46 wt. %) and TiO2 (up 13.85 wt. %) are reported from brown biotite in calcite carbonatite inclusions from Jbel Saghro, Anti-Atlas Morocco. The substitution of Ba into the biotite st ructure can be explained by the scheme Ba + Al double left right arrow K Si. At least 50 % of the K on the interlayer sites of biotites from Saghro is replaced by Ba. Al compensates for tetrahedral Si deficiencies, and Ti i s probably limited to octahedral substitution for Mg and Fe. A substitution scheme of the type Ba + 2Ti + 3Al double left right arrow K + 3(Mg, Fe) 3Si is proposed.