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Mineralogical Magazine; October 1998; v. 62; no. 5; p. 687-695
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Barian-titanian micas from Ilha da Trindade, South Atlantic

J. C. Greenwood

University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom

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.

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