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Biotite micas from igneous rocks differ from one another mainly in their contents of SiO 2 , Al 2 O 3 , MgO, total Fe as FeO and, to a less extent, in TiO 2 . The variation in amount of each of these oxides was investigated. It was found that some depend on the nature of minerals associated with the biotite, others on the general nature of the rock containing the biotite.
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S. R. Nockolds (1947) studied this question.
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