Orthoclase is a primary rock-forming tectosilicate mineral characteristic of granitic rocks. It is easily identified in the field by its distinct two-directional cleavage at nearly 90 degrees and its tendency to form characteristic Carlsbad twin crystals.
Is this orthoclase?
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 orthoclase with a known reference. Orthoclase 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. Orthoclase leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Orthoclase typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, flesh-red, yellow, gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, often twinned as Carlsbad twins.
Often confused with
Orthoclase vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside orthoclase
Minerals reported to co-occur with orthoclase. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KAlSi₃O₈
- Mohs hardness
- 6
- Density
- 2.56 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic Crystals, Often Twinned as Carlsbad Twins
- Cleavage
- Perfect Basal, Good Prismatic
- Rarity
- Common
- Uses
- Collector, Industrial, Ceramics
- Host rock
- Granite, Granitic Pegmatites, Syenite
- Typical price
- $5-50 for typical specimens, $100+ for large well-formed twins
Where rockhounds find orthoclase
9 mapped spotsClassic worldwide localities
- St. Gotthard, Switzerland
- Baveno, Italy
- Madagascar
- Karur, India
- Colorado, USA
Field-hunting tip
Look in granite, granitic pegmatites, syenite country — that is the host setting where orthoclase typically forms. If you start seeing quartz, muscovite, biotite in float, you are in the right ground. Field specimens usually show a prismatic crystals, often twinned as carlsbad twins habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah, California, Missouri — start trip planning there.
Common questions
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Find orthoclase on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
