Anorthite is the calcium-rich endmember of the plagioclase feldspar solid solution series. It is typically found in calcium-rich mafic igneous rocks or thermally metamorphosed limestone, often appearing as glassy, colorless to white tabular crystals.

Hardness

6-6.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent to translucent

mineraluncommonPlagioclase Feldspar Group family

Anorthite

Hardness: 6-6.5
Luster: Vitreous
Streak: White
Transparency: Transparent to translucent

Is this anorthite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch anorthite with a known reference. Anorthite sits at Mohs 6-6.5, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: white, colorless, gray, yellowish, gray, red, reddish gray.
5
Look at form & habit
Crystal system: triclinic. Typical habit: euhedral crystals, granular, striated.

Often confused with

Anorthite vs. its common look-alikes, and how to tell them apart in the field.

Often found alongside anorthite

Minerals reported to co-occur with anorthite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: CaAl₂Si₂O₈
Mohs hardness: 6-6.5
Density: 2.74-2.76 g/cm³
Colors: White Colorless Gray Yellowish Gray Red Reddish gray
Streak: White
Luster: Vitreous
Transparency: Transparent to translucent
Crystal system: Triclinic
Crystal habit: Euhedral crystals, granular, striated
Cleavage: Perfect on {001}, less so on {010}, imperfect on {110}
Rarity: Uncommon
Uses: Collector, Scientific research
Host rock: A rare constituent of mafic plutonic and volcanic rocks

Where rockhounds find anorthite

Classic worldwide localities

Great Sitkin Island, Aleutian Islands, Alaska, USA
Grass Valley, Nevada Co., California, USA
Monte Somma and Vesuvius, Campania, Italy
Mt. Monzoni, Val di Fassa, Trentino-Alto Adige, Italy
Cyclopean Islands, Italy
Tunaberg, Södermanland, Sweden

Field-hunting tip

Where it forms: A rare constituent of mafic plutonic and volcanic rocks. If you start seeing olivine, pyroxene, corundum in float, you are in the right ground. Typical habit: euhedral crystals, granular, striated. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify anorthite?+

Mohs hardness is 6-6.5. Luster: vitreous. The streak is white. Common colors include white, colorless, gray, yellowish.

Where is anorthite found?+

Notable localities include Great Sitkin Island, Aleutian Islands, Alaska, USA; Grass Valley, Nevada Co., California, USA; Monte Somma and Vesuvius, Campania, Italy; Mt. Monzoni, Val di Fassa, Trentino-Alto Adige, Italy; Cyclopean Islands, Italy.

What rocks look like anorthite?+

Anorthite is most often confused with Albite, Labradorite, Bytownite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with anorthite?+

Anorthite commonly co-occurs with Olivine, Pyroxene, Corundum. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does anorthite form in?+

Where it forms: A rare constituent of mafic plutonic and volcanic rocks. Working float back to the host body is the standard way to chase a fresh occurrence.

What is anorthite used for?+

Anorthite is used in collector, scientific research.

Find anorthite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.