Rock and Mineral Identification Reference

How to Identify Orthoclase (Potassium Feldspar): Hardness, Streak, Luster and Tests

Quick answer

Orthoclase (Potassium Feldspar) (mineral): 6 identifying properties, and how to tell it from Plagioclase Feldspar, quoted from geology department and survey sources.

Updated 2026-09-10 · Sources: University of Minnesota Common Minerals

Scratch, streak and acid tests mark the specimen, and some produce sharp fragments or dust. Wear eye protection, work in a ventilated space, keep acid off skin, and never taste a mineral you have not already identified.

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What are the key facts?

Type: Mineral
Mohs hardness: 6 (harder than glass)
Streak: White
Luster: Vitreous to porcelaneous (porcelain-like) luster.
Color: Usually off-white, or shades of red, orange, and brown; occasionally green.
Cleavage or fracture: Two cleavage directions meeting at nearly 90 degrees.
Specific gravity: 2.6

What Orthoclase (Potassium Feldspar) looks like

The properties that identify Orthoclase (Potassium Feldspar) — mohs hardness: 6 (harder than glass); streak: White; luster: Vitreous to porcelaneous (porcelain-like) luster; color: Usually off-white, or shades of red, orange, and brown; occasionally green; cleavage or fracture: Two cleavage directions meeting at nearly 90 degrees; specific gravity: 2.6. Hardness, streak and cleavage are the reliable ones; color alone rarely settles it.

Often confused with

[Plagioclase Feldspar](/lookup/rocks-and-minerals/plagioclase-feldspar): Potassium feldspar cleavage surfaces lack the fine parallel striations (polysynthetic twinning lines) commonly seen on plagioclase cleavage surfaces.

Where it is found in North America

An important constituent of felsic igneous rocks across the Upper Midwest.

How to photograph it for identification

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

FAQ

How hard is Orthoclase (Potassium Feldspar) on the Mohs scale?

6 (harder than glass). A fingernail is about 2.5, a copper coin about 3, a steel knife 5.5 and glass about 5.5, so test against those.

What color streak does Orthoclase (Potassium Feldspar) leave?

White. Rub the specimen on an unglazed porcelain tile; the powder color is the streak, and it is more consistent than the specimen color.

How do I tell Orthoclase (Potassium Feldspar) from Plagioclase Feldspar?

Potassium feldspar cleavage surfaces lack the fine parallel striations (polysynthetic twinning lines) commonly seen on plagioclase cleavage surfaces.

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Sources

University of Minnesota Common Mineralscommonminerals.esci.umn.edu/minerals-o-s/potassium-feldspar

Compiled by the RockLens editorial team

Every fact on this page is taken from the sources listed above; numbers and prices are copied from the source, and prices name the guide or auction and its date. Research and drafting were assisted by AI, and each claim was then checked against the cited sources. Checked 2026-09-10. We do not buy, sell or grade items; the app identifies what you photograph and points you to these references.