How to identify Potassium Feldspar

To correctly identify Potassium Feldspar (Orthoclase), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically white, pink, salmon-orange, or reddish; less commonly gray, yellow, or colorless. The color is often uniform.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically massive in igneous and metamorphic rocks, but can form well-developed euhedral to subhedral crystals, especially in pegmatites.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic system, often forming prismatic or tabular crystals. Carlsbad twinning is common, appearing as a single crystal divided into two halves by a planar boundary.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavage planes intersecting at approximately 90 degrees (specifically, 90 degrees and 30 minutes, or 90.5 degrees). One cleavage is perfect ({001}), and the other is good ({010}).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in felsic igneous rocks (granite, syenite, rhyolite), high-grade metamorphic rocks (gneiss, schist), and pegmatites. Also found in some hydrothermal veins and as detrital grains in sedimentary rocks.

Key Facts

Hardness

6 to 6.5 on the Mohs scale

Specific Gravity

2.55 to 2.63 g/cm³

Crystal System

Monoclinic

Color

White, pink, salmon-orange, reddish, gray, yellow, colorless

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect on {001}, good on {010}, intersecting at 90.5 degrees

Composition

Potassium aluminum silicate (KAlSi3O8)

Physical characteristics

• Crystal Habit: Prismatic, tabular, anhedral to euhedral grains, massive. Often twinned (Carlsbad, Baveno, Manebach).
• Cleavage Type: Two distinct cleavages, one perfect ({001}) and one good ({010}), intersecting at approximately 90 degrees (90°30').
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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