How to identify Potassium Feldspar

To correctly identify Potassium Feldspar (K-feldspar), 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, red, or gray. Green varieties (amazonite) also exist. Color is often a key indicator, especially the characteristic salmon-pink of orthoclase.

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

   Generally smooth on cleavage surfaces, rougher on fracture surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Often forms euhedral to subhedral crystals, commonly prismatic or tabular. Twinning (Carlsbad, Baveno, Manebach) is common and can be diagnostic. Microcline often shows characteristic 'gridiron' twinning under a petrographic microscope.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavage planes intersecting at or near 90 degrees (typically 90 degrees for orthoclase and sanidine, slightly less than 90 degrees for microcline). This is a critical diagnostic feature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in felsic igneous rocks (granite, granodiorite, syenite, rhyolite, trachyte), pegmatites, high-grade metamorphic rocks (gneiss, schist), and as a detrital component in arkosic sandstones.

Key Facts

Hardness

6-6.5 (Mohs scale)

Specific Gravity

2.55-2.63 g/cm³

Crystal System

Monoclinic (Orthoclase, Sanidine), Triclinic (Microcline)

Color

White, pink, salmon-orange, red, green (amazonite), gray

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Two distinct cleavages, one perfect {001} and one good {010}, intersecting at or near 90 degrees.

Composition

KAlSi3O8 (Potassium Aluminum Silicate)

Physical characteristics

• Crystal Habit: Prismatic, tabular, anhedral masses; often twinned (Carlsbad, Baveno, Manebach). Microcline can show 'gridiron' twinning.
• Cleavage Type: Perfect on {001}, good on {010}, intersecting at 90° (orthoclase, sanidine) or slightly less than 90° (microcline).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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