Orthoclase Identification Guide
Identify orthoclase, the Mohs-6 reference potassium feldspar, by its two cleavages, pink-to-white color, and Carlsbad twinning.
Read the full Orthoclase encyclopedia entry →
What Orthoclase Looks Like
Orthoclase is a potassium feldspar (KAlSi3O8) and one of the most common rock-forming minerals. It is the standard reference for Mohs hardness 6. Color is usually white, cream, pale pink, or salmon, sometimes pale yellow (the gem variety) or with a moonstone-like sheen (adularia). It forms blocky prismatic crystals and is a key constituent of granite.
- Color: white, cream, pink, salmon, pale yellow
- Luster: vitreous to slightly pearly on cleavages
- Transparency: transparent to translucent
- Form: blocky prismatic crystals; common Carlsbad twins; massive in granite
Step-by-Step Field ID Checklist
- Test hardness. Orthoclase scratches glass and is the very benchmark of Mohs 6; quartz (7) will scratch it.
- Find two cleavages. Two good cleavage directions meeting at about 90 degrees produce stepped, blocky flat faces.
- Look at color. Pink-to-salmon blocky grains in granite are usually orthoclase.
- Check for twinning. Carlsbad twins (two intergrown halves) are common; lack of fine parallel striations distinguishes it from plagioclase.
- Note the pearly sheen on some cleavage faces.
Key Diagnostic Tests
- Hardness: Mohs 6 (the reference standard).
- Streak: white.
- Cleavage: two directions at ~90 degrees (the name means "straight fracture").
- Density: about 2.55-2.63 g/cm3.
- Acid: no reaction.
- Striations: none on cleavage faces (unlike plagioclase's fine albite twinning lines).
Common Look-Alikes
- Plagioclase feldspar (albite/oligoclase): shows fine parallel striations on the best cleavage from polysynthetic twinning; orthoclase does not.
- Microcline (incl. amazonite): chemically the same K-feldspar but triclinic; shows tartan/cross-hatch twinning under a microscope, and amazonite is green.
- Quartz: harder (7), no cleavage (conchoidal fracture), often glassy gray and clear.
- Calcite: much softer (3), rhombohedral cleavage, fizzes in acid.
Where It Is Found
Orthoclase is ubiquitous in granite, syenite, pegmatites, and gneiss worldwide. Notable gem orthoclase (yellow) comes from Madagascar; adularia/moonstone from Sri Lanka, India, and the Alps. It is a primary igneous and metamorphic feldspar found on every continent.
Frequently asked questions
How can you tell if a mineral is orthoclase?
Orthoclase has hardness exactly 6 (the Mohs reference), two cleavage directions meeting near 90 degrees, white streak, no acid reaction, and often pink to cream color with Carlsbad twinning but no fine striations.
What is the difference between orthoclase and plagioclase?
Plagioclase shows fine parallel striations on its cleavage faces from polysynthetic twinning, while orthoclase lacks these striations. Orthoclase is potassium-rich and often pink; plagioclase is sodium-calcium and often white-gray.
Orthoclase vs quartz, how do you tell them apart?
Quartz is harder (7 vs 6), has no cleavage and breaks conchoidally, and is usually glassy and clear, while orthoclase has two good cleavages at about 90 degrees and is often pink or cream.
What does orthoclase look like?
It typically appears as blocky white, cream, pink, or salmon crystals or grains with a glassy-to-pearly luster and stepped flat cleavage faces, common in granite.
Orthoclase identified by the community
Recent Orthoclase specimens identified with Rock Identifier.
