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 6-6.5, 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, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: white, colorless, flesh-red, yellow, gray, greenish, grayish yellow, pink.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: blocky, massive, prismatic.
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
- 6-6.5
- Density
- 2.55-2.63 g/cm³
- Streak
- White
- Luster
- Vitreous, pearly on cleavages
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Blocky, massive, prismatic
- Cleavage
- Perfect on {001} and {010}; partings on {100}, {110}, {110}, and {201}
- Rarity
- Common
- Uses
- Collector, Industrial, Ceramics
- Host rock
- The common feldspar of granites, granite pegmatites, and syenites
Where rockhounds find orthoclase
9 mapped spotsClassic worldwide localities
- Paris and Buckfield, Oxford Co, Maine, USA
- Cornog, Chester Co., and Blue Hill and Lieperville, Delaware Co., Pennsylvania, USA
- Pala and Mesa Grande districts, San Diego Co., California, USA
- Mt. Antero, Chaffee Co., Colorado, USA
- St. Gotthard, Ticino, and at Val Giuv, Tavetsch, Graubünden, Switzerland
- Zillertal, Tirol, Austria
Field-hunting tip
Where it forms: The common feldspar of granites, granite pegmatites, and syenites. If you start seeing albite, muscovite, biotite in float, you are in the right ground. Typical habit: blocky, massive, prismatic. 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
How do you identify orthoclase?+
Where is orthoclase found?+
Can I find orthoclase in the United States?+
What rocks look like orthoclase?+
What minerals are found with orthoclase?+
What kind of rock does orthoclase form in?+
What is orthoclase used for?+
Find orthoclase on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
