igneous

Granitic Pegmatite with Biotite

Pegmatitic Granite (contains KAlSi3O8, SiO2, K(Mg,Fe)3AlSi3O10(OH)2)

Hardness: 6-7 (feldspar/quartz) and 2.5-3 (mica); Color: Tan/white base with black flaky inclusions; Luster: Vitreous to pearly; Crystal structure: Monoclinic and Hexagonal; Large grain size.

Hardness: 6-7 (feldspar/quartz) and 2
Color: Tan/white base with black flaky inclusions
Luster: Vitreous to pearly

Identified April 9, 2026More igneous →

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Physical properties

Hardness: 6-7 (feldspar/quartz) and 2.5-3 (mica); Color: Tan/white base with black flaky inclusions; Luster: Vitreous to pearly; Crystal structure: Monoclinic and Hexagonal; Large grain size.

Formation & geological history

Formed during the final stages of a magma's crystallization. Highly fluid, water-rich melts allow for the growth of exceptionally large crystals. These are typically found in batholiths and dikes from the Proterozoic to Cenozoic eras.

Uses & applications

Used as a source of industrial minerals (feldspar and quartz), and historically as a source of mica for electrical insulation. Large specimens are popular with mineral collectors.

Geological facts

Pegmatites are famous for containing the largest crystals ever recorded on Earth, sometimes exceeding 10 meters in length. The black mica (biotite) in this specimen can often be peeled into thin, flexible sheets.

Field identification & locations

Identify in the field by the exceptionally large size of individual mineral grains (over 1cm). Commonly found in mountainous regions near granite intrusions. Look for the 'books' of dark, flaky mica embedded in light quartz or feldspar.

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