Barian-titanian micas from Ilha da Trindade, South Atlantic

Extremely high abundances of TiO ₂ (up to 12.57 wt.%) and BaO (up to 7.11 wt.%) are reported from dark reddish-brown phlogopite-biotite rims around paler phlogopite cores in a lamprophyric dyke from Ilha da Trindade, South Atlantic. Both TiO ₂ and BaO contents increase towards the rims of the micas and in groundmass crystals. This is accompanied by corresponding increases in FeO (super *) , MnO and Mg number; whereas SiO ₂ , Al ₂ O ₃ , Cr ₂ O ₃ , MgO and K ₂ O abundances all decrease. The substitution of Ba into the biotite structure can be explained by the scheme Ba+Al = K+Si. Ti-substitution schemes however are much more complex, as no general equation can account for all the Ti incorporated into the structure and therefore several schemes are proposed.