Description: Biotite is a group name for micas including phlogopite and others with varying composition. Micas have a single perfect cleavage displaying a platy texture with flexible “plates”. Biotite can come in enormous crystal sheets that can weigh several hundred pounds. Thin sheets can be peeled off as layers, and the thinner a layer is peeled the greater its transparency. Biotite is a common rock-forming mineral, and is especially noted in metamorphic rocks such as schist and gneiss.
Chemical Formula: K(Mg,Fe)3(AlSi3O10)(F,OH)2
Crystal Form: Crystals are in thick flakes, micaceous masses and groupings, and in tabular, foliated, flaky, and scaly forms.
Crystal System: Monoclinic
Color: Blackish brown, dark brown, greenish brown,, yellow, even white
Streak: grey
Luster: Vitreous, pearly
Fracture: Uneven
Cleavage: Perfect, in one direction
Hardness: 2.5 to 3.0 on Moh’s scale (can be scratched with fingernail)
Density/Specific Gravity: 2.8 - 3.4 g/cm3
Magnetism: N/A
Taste: N/A
Hydrochloric acid: N/A
Radioactivity: barely detectable
Fluorescence: N/A
Distinguishing Physical Properties:
Color: dark, black
Cleavage: one perfect direction of cleavage
Appearance: perfect cleavage results in mineral peeling into thin, flexible sheets
Similar properties to other micas, Muscovite
Photograph Attribution: Biotite specimen photograph by Sean C. Murphy, 2020.