Biotite K(Mg,Fe)3(AlSi3O10(OH)2
Biotite mica, a complex silicate, is a common rock-forming mineral, particularly in granitic rocks. It crystallizes with eight-sided crystals, though these are rare.
Biotite often takes the place of muscovite in pegmatites, particularly when rare earths are present. It is found in pegmatite dikes in large sheets, of which these specimens are an example, and occurs in a larger variety of rocks than muscovite does.
Like muscovite, thin cleavage plates are flexible and elastic, though less so than muscovite. Students should be able to identify the two common rock-forming micas, muscovite and biotite.
This biotite was collected from a pegmatite at Cardiff, near Bancroft, Ontario, Canada.
The specimens in the pair shown with just a black and silver pen are thinner. You get both.
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