Anorthosite is a phaneritic, intrusive igneous rock composed almost entirely of plagioclase feldspar, typically containing 90% or more of this mineral. It is often gray to white in color and is notable for forming large plutonic bodies on Earth as well as making up the bulk of the lunar highlands. Collectors typically look for variations containing accessory minerals like ilmenite or labradorite with iridescent optical properties.
Is this anorthosite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch anorthosite with a known reference. Anorthosite sits at Mohs 6, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Phaneritic.
- 3Pick out the mineralsComposition: 90 to 100 percent plagioclase feldspar, often labradorite, with pyroxene, ilmenite, magnetite or olivine. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 5Match the color rangeCompare against the expected color range: white, gray, bluish-gray, black.
Often confused with
Anorthosite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside anorthosite
Minerals reported to co-occur with anorthosite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 6
- Density
- 2.6-2.9 g/cm³
- Luster
- Vitreous
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Building stone, Decorative stone, Collector
- Host rock
- plutonic igneous rocks
- Texture
- Phaneritic
- Composition
- 90 to 100 percent plagioclase feldspar, often labradorite, with pyroxene, ilmenite, magnetite or olivine
Where rockhounds find anorthosite
Classic worldwide localities
- Adirondack Mountains, USA
- Labrador, Canada
- Rogaland, Norway
- Madagascar
- Moon (Lunar Highlands)
Field-hunting tip
Where it forms: plutonic igneous rocks. If you start seeing plagioclase, ilmenite, magnetite in float, you are in the right ground.
Common questions
How do you identify anorthosite?+
Where is anorthosite found?+
What rocks look like anorthosite?+
What minerals are found with anorthosite?+
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What is anorthosite used for?+
Find anorthosite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
