Perthite is a textural variety of alkali feldspar formed by the exsolution of sodic plagioclase from a potassium feldspar host during cooling. It is easily identified by its characteristic lamellar, streaky, or wavy internal pattern, often visible in hand samples as subtle color variations. It is most frequently found as a major constituent of coarse-grained pegmatites and granitic rocks.
Is this perthite?
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 perthite with a known reference. Perthite sits at Mohs 6 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Perthite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Perthite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, gray, pink, brown, tan.
- 5Look at form & habitCrystal system: triclinic. Typical habit: massive.
Often confused with
Perthite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside perthite
Minerals reported to co-occur with perthite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (K,Na)AlSi₃O₈
- Mohs hardness
- 6
- Density
- 2.55-2.60 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Triclinic
- Crystal habit
- Massive
- Cleavage
- Perfect in Two Directions
- Rarity
- Common
- Uses
- Collector, Lapidary
- Host rock
- Granite Pegmatites
- Typical price
- $5-50 for small specimens
Where rockhounds find perthite
2 mapped spotsClassic worldwide localities
- Ontario, Canada
- Norway
- Madagascar
- USA
- Russia
Field-hunting tip
Look in granite pegmatites country — that is the host setting where perthite typically forms. If you start seeing quartz, muscovite, biotite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in New York, Virginia — start trip planning there.
Common questions
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Find perthite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
