igneous
Anorthosite
Anorthosite (dominated by Plagioclase Feldspar)
Hardness: 6-6.5 (Mohs scale); Color: light grey to medium grey, often with bluish or greenish tints; Luster: Pearly to vitreous; Crystal Structure: Triclinic; Cleavage: Perfect in two directions; Specific Gravity: 2.62-2.76.
- Hardness
- 6-6
- Luster
- Pearly to vitreous
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Physical properties
Hardness: 6-6.5 (Mohs scale); Color: light grey to medium grey, often with bluish or greenish tints; Luster: Pearly to vitreous; Crystal Structure: Triclinic; Cleavage: Perfect in two directions; Specific Gravity: 2.62-2.76.
Formation & geological history
Formed from the slow cooling of magma deep beneath the Earth's crust (plutonic). It is characterized by the accumulation of plagioclase crystals in a magma chamber. These rocks are often ancient, dating back to the Proterozoic eon (over 1 billion years ago).
Uses & applications
Used primarily as decorative stone (countertops, memorials), dimension stone in construction, or crushed for railroad ballast. High-quality iridescent varieties (Labradorite) are used in jewelry.
Geological facts
Anorthosite makes up the majority of the light-colored 'highlands' on the Earth's moon. It is far more common on the Moon than on the Earth's surface.
Field identification & locations
Identify in the field by its light-to-medium grey color and the prevalence of large, shiny cleavage surfaces of feldspar crystals. Often found in large massifs like the Adirondack Mountains or the Canadian Shield.
