Anorthosite
Anorthosite is a coarse-grained igneous rock made mostly of plagioclase feldspar. It often looks light gray to white, sometimes with dark mafic minerals or bluish feldspar flashes.
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Anorthosite properties at a glance
| Type | Igneous Rocks |
|---|---|
| Chemical formula | Plagioclase feldspar-rich intrusive rock |
| Mohs hardness | 6 |
| Luster | pearly |
| Streak | White to pale gray |
| Cleavage | Variable by feldspar grains |
| Fracture | Uneven to granular |
| Specific gravity | 2.75 |
| Transparency | opaque |
| Common colors | White, Gray, Blue gray, Black, Pale pink |
| Rarity | common |
Geology & background
Forms as large bodies of magma crystallize and concentrate plagioclase feldspar, sometimes with minor pyroxene, olivine, or oxide minerals.
Anorthosite helps geologists study large igneous intrusions, crustal differentiation, and feldspar-rich planetary crust.
How to identify Anorthosite
- Look for coarse feldspar-rich texture with mostly white, gray, or bluish plagioclase.
- Anorthosite usually has little visible quartz compared with granite.
- Dark patches can be pyroxene, ilmenite, magnetite, or related minerals.
- Fresh broken surfaces show feldspar grains better than weathered surfaces.
Where Anorthosite is found
Reported from multiple igneous terrains worldwide; verify locality against specimen provenance when available.
How much is Anorthosite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Anorthosite is usually valued for teaching, field context, locality, texture, color, mineral content, or unusually attractive specimens rather than high gem value.
Common uses
- Field identification
- Education
- Decorative stone when attractive
- Igneous intrusion study
Interesting facts
- Anorthosite is abundant in the Moon's highlands and also occurs in large bodies on Earth.
- Some anorthosite contains dark oxide-rich bands or patches.
- The feldspar-rich look can separate it from darker gabbro or more quartz-rich granite.
Care & safety
Anorthosite can vary in toughness, porosity, vesicles, glass content, and weathering depending on cooling history and mineral mix.
Avoid inhaling dust. Do not cut, grind, or polish unknown igneous rocks without eye protection and a properly rated respirator.
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