Biotite Composition, Crystallization & Structure
Composition. (H,K)2(Mg,Fe)2Al2 (SiO4)3.
Crystallization. Monoclinic. In tabular or short prismatic crystals with prominent basal planes. Crystals rare, frequently pseudorhombohedral.
Structure. Usually in irregular foliated masses¡ often in disseminated scales or in Sealy aggregates.
Physical Properties. Perfect basal cleavage. Folia flexible and elastic. H. = 2.5-3. G. = 2.95-3. Spending luster. Color usually dark green and brown to black. More rarely lighter yellow. Thin sheets usually have a smoky color (differing from the almost colorless muscovite).
Test. Difficultly fusible at 5. unattached by hydrochloric acid. Decomposed by boiling concentrated sulfuric acid, giving a milky solution. Characterized by its micaceous structure, cleavage and dark color.
Occurrence. An important and widely distributed rocks-making mineral, but not as common as muscovite. Occurs in igneous rocks, especially those in which feldspar is prominent, such as granite and syenite. At times in pegmatite veins in large sheets. Found also in many felsites lavas and porphyries. Less common in the Ferromagnesium rocks. Is also present in some metamorphosed rocks, as gneiss and shish. Occurs in fine crystals in the lava of Versuvius.