Plagioclase is a series of tectosilicate minerals that form the most abundant mineral group in the Earth's crust. Collectors typically identify them by their characteristic polysynthetic twinning, which appears as fine, parallel striations on cleavage surfaces.
Is this plagioclase?
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 plagioclase with a known reference. Plagioclase sits at Mohs 6-6.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Plagioclase leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Plagioclase typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, gray, colorless, pale yellow, pale green.
- 5Look at form & habitCrystal system: triclinic. Typical habit: tabular crystals, massive, granular.
Often confused with
Plagioclase vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside plagioclase
Minerals reported to co-occur with plagioclase. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,Ca)(Al,Si)₄O₈
- Mohs hardness
- 6-6.5
- Density
- 2.6-2.8 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Triclinic
- Crystal habit
- Tabular Crystals, Massive, Granular
- Cleavage
- Perfect in Two Directions At Nearly 90 Degrees
- Rarity
- Common
- Uses
- Collector, Industrial
- Host rock
- Igneous Rocks Including Granite, Basalt, And Gabbro
- Typical price
- $5-30 for typical mineral specimens
Where rockhounds find plagioclase
5 mapped spotsClassic worldwide localities
- Norway
- Canada
- USA
- Italy
- Madagascar
Field-hunting tip
Look in igneous rocks including granite, basalt, and gabbro country — that is the host setting where plagioclase typically forms. If you start seeing quartz, biotite, hornblende 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. In the U.S., the densest reported localities are in Wisconsin, Missouri, Washington — start trip planning there.
Common questions
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Find plagioclase on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
