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 More igneous →
Explore Mica (Biotite and Muscovite) in Matrix in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
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
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).
