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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: white, gray, colorless, pale yellow, pale green, bluish white, reddish white, greenish white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: massive.
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
- 2.61-2.76 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Massive
- Cleavage
- Good on {001}; good on {010}
- Rarity
- Common
- Uses
- Collector, Industrial
- Host rock
- Igneous and metamorphic rocks
Where rockhounds find plagioclase
5 mapped spotsClassic worldwide localities
- Norway
- Canada
- USA
- Italy
- Madagascar
Field-hunting tip
Where it forms: Igneous and metamorphic rocks. If you start seeing quartz, biotite, hornblende in float, you are in the right ground. Typical habit: massive. 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
How do you identify plagioclase?+
Where is plagioclase found?+
Can I find plagioclase in the United States?+
What rocks look like plagioclase?+
What minerals are found with plagioclase?+
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Find plagioclase on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
