igneous

Granitic Pegmatite

Granite Pegmatite (principally KAlSi3O8, SiO2, and KAl2(AlSi3O10)(OH)2)

Hardness: 6-7 (Mohs scale); Color: White, cream, tan, and smoky grey; Luster: Vitreous to pearly; Crystal Structure: Very large interlocking crystals (>1cm); Cleavage: Perfect in mica and feldspar components; Specific Gravity: 2.6-2.7

Hardness: 6-7 (Mohs scale)
Color: White, cream, tan, and smoky grey
Luster: Vitreous to pearly

Identified March 16, 2026More igneous →

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Physical properties

Formation & geological history

Formed during the final stages of a magma's crystallization, usually in hydrothermal veins or at the margins of large plutons. The high water content allows for rapid crystal growth resulting in massive mineral sizes. Most often associated with Precambrian or Paleozoic geological eras.

Uses & applications

Primary source of industrial minerals like sheet mica, feldspar for ceramics/glass, and rare elements (lithium, tantalum). High-quality specimens are mined for gemstones like tourmaline, beryl, and spodumene.

Geological facts

Pegmatites can contain the world's largest crystals; beryl crystals over 30 feet long have been found in pegmatite deposits. They are sometimes called 'nature's treasure chest' because they concentrate rare earth elements.

Field identification & locations

Identify in the field by the exceptionally large crystal size compared to surrounding rock. Look for the 'graphic' texture (intergrown quartz and feldspar) and the presence of sheet-like books of mica. Found in mountainous or shield regions like the Black Hills (SD), Brazil, and Ontario.

Other igneous specimens

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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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral