How to identify Potassium Feldspar

To correctly identify Potassium Feldspar (Orthoclase or Microcline (KAlSi3O8)), 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, cream, pink, salmon-orange, or reddish-brown. Microcline can be green (amazonite). Sanidine is often colorless or white.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on cleavage surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically massive, granular, or in well-formed euhedral to subhedral crystals. Can exhibit perthitic exsolution textures (intergrowths with albite).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthoclase and Sanidine: Monoclinic, often forming prismatic or tabular crystals. Microcline: Triclinic, often forming blocky or tabular crystals. Twinning (Carlsbad, Baveno, Manebach) is common, especially in orthoclase. Microcline often shows characteristic 'grid' or 'tartan' twinning under a polarizing microscope.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavage directions intersecting at or near 90 degrees (90 degrees for orthoclase, sanidine; slightly less than 90 degrees for microcline).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Abundant in felsic igneous rocks (granite, granodiorite, rhyolite, syenite), pegmatites, and some metamorphic rocks (gneiss, schist). Also found as detrital grains in arkosic sandstones and other sedimentary rocks. Sanidine is characteristic of rapidly cooled volcanic rocks (rhyolite, trachyte). Adularia occurs in low-temperature hydrothermal veins.

Key Facts

Hardness

6 to 6.5 (Mohs scale)

Specific Gravity

2.55 to 2.63 g/cm³

Crystal System

Orthoclase/Sanidine: Monoclinic; Microcline: Triclinic

Color

White, cream, pink, salmon-orange, reddish-brown, green (amazonite), colorless (sanidine)

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions, intersecting at 90 degrees (orthoclase/sanidine) or slightly less than 90 degrees (microcline)

Composition

Potassium aluminum silicate (KAlSi3O8)

Physical characteristics

• Crystal Habit: Prismatic, tabular, blocky, massive, granular. Often twinned (Carlsbad, Baveno, Manebach). Microcline can show 'grid' twinning.
• Cleavage Type: Perfect {001} and good {010} cleavage, intersecting at 90 degrees (orthoclase/sanidine) or 89°30' (microcline).
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces

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