Anorthosite Identification Guide

What Anorthosite Looks Like

Anorthosite is a coarse-grained intrusive (plutonic) igneous rock made up of at least 90 percent plagioclase feldspar, usually calcic (labradorite to bytownite). It is typically pale gray to bluish gray, off-white, or greenish, and often looks almost monomineralic. Many anorthosites flash blue, green, or gold (labradorescence/schiller) on cleavage faces because the feldspar is labradorite, the same mineral sold as the gemstone.

• Luster: dull to vitreous on fresh feldspar faces
• Texture: coarse-grained, interlocking blocky feldspar crystals, sometimes very large
• Color: gray, blue-gray, white, greenish, occasionally dark when mafic-rich
• Standout feature: broad spectral color flashes from labradorite grains

Step-by-Step Field ID Checklist

1. Assess grain size: confirm it is coarse-grained and crystalline (plutonic), not fine glassy or layered.
2. Estimate mineralogy: the rock should be dominated by one mineral, blocky feldspar, with only minor dark minerals (pyroxene, olivine, oxides).
3. Look for cleavage flashes: rotate a fresh face; striations and stepped reflections on feldspar plus color play support labradorite-rich anorthosite.
4. Check the dark mineral content: if dark minerals exceed roughly 10 percent, it grades toward gabbro/norite rather than anorthosite.
5. Test hardness: the feldspar should scratch glass (about 6 to 6.5).

Key Diagnostic Tests

• Mohs hardness: dominated by plagioclase, about 6 to 6.5
• Streak: white (from the feldspar)
• Cleavage: plagioclase shows two good cleavages near 90 degrees plus fine parallel twinning striations on cleavage faces (a key plagioclase signature)
• Fracture: rock breaks irregularly across interlocking grains
• Specific gravity: about 2.6 to 2.8
• Acid: no reaction (silicate, not carbonate)
• Magnetism: generally none unless minor magnetite is present

Common Look-Alikes and How to Tell Them Apart

• Gabbro/norite: these are also coarse and dark-bearing, but contain abundant pyroxene (and olivine), so they are much darker overall. Anorthosite is pale and overwhelmingly feldspar.
• Granite: granite is also coarse but contains visible quartz (glassy, no cleavage) and potassium feldspar (often pink), plus mica. Anorthosite lacks quartz and is dominated by gray plagioclase with twinning striations.
• Marble: pale marble can look similar in color but is much softer (Mohs 3), fizzes in dilute acid, and shows no feldspar cleavage.
• Larvikite (monzonite): sold for countertops with blue schiller, but larvikite contains alkali feldspar and more dark minerals; anorthosite is more nearly pure plagioclase.

Where Anorthosite Is Typically Found

Massif-type anorthosites form huge Proterozoic intrusions: the Adirondacks (New York), the Grenville Province of Canada, Norway (the labradorite-rich Larvik area is monzonitic but nearby), the Rogaland complex, and Labrador (source of "Labradorite"). Layered intrusions (Bushveld, Stillwater) contain anorthosite layers. Notably, the lunar highlands are largely anorthosite, making it a famous Moon rock.