Potassium Feldspar Identification Guide
A field guide to potassium feldspar (orthoclase/microcline), identifying its pink-to-cream color, blocky cleavage, hardness, and how to separate it from plagioclase and quartz.
Read the full Potassium Feldspar encyclopedia entry →
What Potassium Feldspar Looks Like
Potassium feldspar (K-feldspar) includes orthoclase, microcline, and sanidine. It is one of the most common rock-forming minerals, appearing as pink, salmon, cream, white, or pale tan blocky crystals and grains, and the green variety amazonite in microcline. Luster is vitreous to slightly pearly, and crystals are translucent to opaque, often stubby prismatic or tabular. It frequently shows perthitic streaks (wavy intergrowths of white plagioclase).
Step-by-Step Field ID Checklist
- Confirm a feldspar. Two cleavages near 90 degrees and a hardness of 6.
- Check color. Pink, salmon, or cream strongly favors K-feldspar over the typically gray-white plagioclase.
- Look for the absence of fine striations. K-feldspar lacks the polysynthetic albite striations seen on plagioclase cleavage faces.
- Hunt for perthite or grid twinning. Wavy perthitic streaks (or, in microcline, a tartan cross-hatch under magnification) confirm K-feldspar.
- Note the host rock. Common in granite, syenite, and pegmatite alongside quartz and mica.
Key Diagnostic Tests
- Mohs hardness: 6 to 6.5 — scratches glass, not quartz.
- Cleavage: Two good directions at about 90 degrees, blocky stepped surfaces.
- Streak: White.
- Specific gravity: About 2.55 to 2.63.
- Acid: No reaction to dilute HCl.
- Twinning: Carlsbad (simple) twinning common; microcline shows tartan twinning.
Common Look-Alikes and How to Tell Them Apart
- Plagioclase feldspar: Usually gray-white and shows fine parallel striations (albite twinning) on cleavage; K-feldspar lacks these and is more often pink.
- Quartz: No cleavage (conchoidal fracture), harder (7), glassy, forms hexagonal points; K-feldspar has blocky cleavage.
- Calcite: Softer (3), fizzes in acid, rhombohedral cleavage.
- Nepheline: Greasy luster, no good cleavage at 90 degrees, occurs in silica-undersaturated rocks.
- Pink dolomite/rhodochrosite: Softer and react/fizz with acid.
Where Potassium Feldspar Is Found
K-feldspar is ubiquitous in granite, syenite, rhyolite, and pegmatite, and as detrital grains in arkose sandstone. Large gem and specimen crystals (including amazonite microcline) come from pegmatites in Colorado, Virginia, Brazil, Madagascar, and Russia; sanidine occurs in young volcanic rocks.
Frequently asked questions
How can you tell if it's real potassium feldspar?
Look for pink-to-cream blocky crystals with two cleavages near 90 degrees, hardness about 6, a white streak, no acid reaction, and an absence of the fine striations seen on plagioclase. Perthitic streaks or tartan twinning confirm it.
What does potassium feldspar look like?
It is typically pink, salmon, cream, or white feldspar with a glassy luster and blocky cleavage, common in granite and pegmatite; the green microcline variety is amazonite.
Potassium feldspar vs plagioclase: what is the difference?
K-feldspar is often pink and lacks fine albite striations, sometimes showing perthite or tartan twinning, while plagioclase is usually gray-white with fine parallel striations on its cleavage faces.
Potassium feldspar vs quartz: how do you tell them apart?
K-feldspar has blocky cleavage at about 90 degrees and is slightly softer (6), while quartz has no cleavage, breaks conchoidally, is harder (7), and forms hexagonal crystals.
Potassium Feldspar identified by the community
Recent Potassium Feldspar specimens identified with Rock Identifier.
