igneous

Mica (Biotite and Muscovite) in Matrix

Potassium aluminum silicate with iron/magnesium (KAl2(AlSi3O10)(F,OH)2 / K(Mg,Fe)3AlSi3O10(F,OH)2)

Hardness: 2.5-3.0 (Mohs scale); Color: Black, brown, and silvery-tan; Luster: Vitreous to pearly; Crystal Structure: Monoclinic; Cleavage: Perfect basal (easily peels into thin sheets); Specific Gravity: 2.7–3.3

Hardness: 2
Color: Black, brown, and silvery-tan
Luster: Vitreous to pearly

Identified April 8, 2026More igneous →

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Physical properties

Formation & geological history

Formed in felsic igneous rocks, specifically pegmatites, through the cooling of molten rock. These large-crystal specimens typically form in the late stages of magma crystallization where high water content allows for rapid mineral growth.

Uses & applications

Historically used as an electrical insulator due to its heat resistance and dielectric properties. Today, ground mica is used in makeup, paints, and drywall compound. Sheet mica is used in electronics and stove windows.

Geological facts

Mica was often called 'Isinglass' and used for windows in ovens and wood stoves before tempered glass was common. It is famous for its 'books' (thick stacks of sheets) that can be peeled apart with a fingernail.

Field identification & locations

Identify in the field by its distinct shiny, platy layers that can be flaked off with a knife or fingernail. It is a primary constituent of granite and pegmatites worldwide, particularly in Brazil, India, and the United States (Appalachian Mountains).

Other igneous specimens

See all

Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral