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

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineraluncommonPlagioclase Feldspar Group family

Anorthite

Hardness: 6
Luster: Vitreous
Streak: White
Transparency: 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, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Anorthite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Anorthite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, colorless, gray, yellowish.
5
Look at form & habit
Crystal system: triclinic. Typical habit: tabular crystals, massive, granular.

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
Density: 2.74-2.76 g/cm³
Colors: White Colorless Gray Yellowish
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Triclinic
Crystal habit: Tabular Crystals, Massive, Granular
Cleavage: Perfect Basal, Good Prismatic
Rarity: Uncommon
Uses: Collector, Scientific Research
Host rock: Mafic Igneous Rocks, Metamorphic Contact Zones
Typical price: $10-100 per specimen

Where rockhounds find anorthite

Classic worldwide localities

Mount Somma, Italy
Miyake-jima, Japan
Franklin, New Jersey, USA
Bushveld Complex, South Africa

Field-hunting tip

Look in mafic igneous rocks, metamorphic contact zones country, the host setting where anorthite typically forms. If you start seeing pyroxene, olivine, magnetite in float, you are in the right ground. Field specimens usually show a tabular crystals, massive, granular habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify anorthite?+

Mohs hardness is 6. It typically shows a vitreous luster. The streak is white. Common colors include white, colorless, gray, yellowish.

Where is anorthite found?+

Notable localities include Mount Somma, Italy; Miyake-jima, Japan; Franklin, New Jersey, USA; Bushveld Complex, South Africa.

How much is anorthite worth?+

Typical asking prices fall in the range of $10-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

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 Pyroxene, Olivine, Magnetite, Hornblende. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does anorthite form in?+

Anorthite typically forms in mafic igneous rocks, metamorphic contact zones. 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.