igneous
Granite with Biotite and Quartz
Phaneritic Felsic Igneous Rock (composed of SiO2, KAlSi3O8, and K(Mg,Fe)3AlSi3O10(OH)2)
Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper appearance with white/grey quartz and black biotite; Luster: Vitreous to pearly; Crystal structure: Phaneritic (coarse-grained, visible crystals); Cleavage: Perfect in biotite flakes, none in quartz.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Vitreous to pearly
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Salt-and-pepper appearance with white/grey quartz and black biotite; Luster: Vitreous to pearly; Crystal structure: Phaneritic (coarse-grained, visible crystals); Cleavage: Perfect in biotite flakes, none in quartz.
Formation & geological history
Formed through the slow cooling of silica-rich magma deep within the Earth's crust (intrusive cooling). These rocks can date back to several billion years (Precambrian) or be as young as a few million years depending on the tectonic setting.
Uses & applications
Used extensively in construction for countertops, floor tiles, and paving stones. High-quality specimens with large crystals are popular for geological teaching collections.
Geological facts
Granite makes up the majority of the Earth's continental crust. Its durability and resistance to weathering are why it is frequently used for public monuments and Egyptian obelisks.
Field identification & locations
Identify in the field by looking for a coarse-grained texture where individual mineral grains are visible to the naked eye. It is commonly found in mountain ranges and continental shields (e.g., the Rockies or the Appalachians). Collectors should look for distinct crystalline boundaries.
