Rock and Mineral Identification Reference

How to Identify Plagioclase Feldspar: Hardness, Streak, Luster and Tests

Quick answer

Plagioclase Feldspar (mineral): 5 identifying properties, 1 home test, and how to tell it from Orthoclase (Potassium 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: 6–6.5 (harder than glass)
Streak: White
Color: White to gray; can also range from colorless to pale yellow, pink, reddish brown, or nearly black.
Cleavage or fracture: Two cleavage directions meeting at nearly 90 degrees.
Specific gravity: 2.6–2.8, increasing with calcium content

What Plagioclase Feldspar looks like

The properties that identify Plagioclase Feldspar — mohs hardness: 6–6.5 (harder than glass); streak: White; color: White to gray; can also range from colorless to pale yellow, pink, reddish brown, or nearly black; cleavage or fracture: Two cleavage directions meeting at nearly 90 degrees; specific gravity: 2.6–2.8, increasing with calcium content. Hardness, streak and cleavage are the reliable ones; color alone rarely settles it.

Tests you can do at home

Fine parallel striations (polysynthetic twinning lines) on cleavage surfaces are the key feature that distinguishes plagioclase from potassium feldspar.

Often confused with

[Orthoclase (Potassium Feldspar)](/lookup/rocks-and-minerals/orthoclase-feldspar): Plagioclase cleavage surfaces show fine parallel striations; potassium feldspar does not.

Where it is found in North America

Widespread in igneous and metamorphic rocks across the Upper Midwest, concentrated in north-central and northeastern Minnesota.

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 Plagioclase Feldspar on the Mohs scale?

6–6.5 (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 Plagioclase 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 Plagioclase Feldspar from Orthoclase (Potassium Feldspar)?

Plagioclase cleavage surfaces show fine parallel striations; potassium feldspar does not.

What is the simplest test for Plagioclase Feldspar?

Fine parallel striations (polysynthetic twinning lines) on cleavage surfaces are the key feature that distinguishes plagioclase from potassium feldspar.

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Sources

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