How to identify Potassium Feldspar

To correctly identify Potassium Feldspar (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, or reddish-brown. Less commonly gray or green (e.g., amazonite, a variety of microcline).

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

   Often forms anhedral to subhedral grains in granular rocks. Can form euhedral crystals in pegmatites or volcanic rocks.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthoclase and Sanidine are monoclinic, often forming prismatic or tabular crystals. Microcline is triclinic, forming similar habits but with a slightly different cleavage angle. Adularia often forms rhombic or tabular crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavages intersecting at or very close to 90 degrees (90° for orthoclase/sanidine, 89°30' for microcline). This is a key distinguishing feature from plagioclase feldspar (which has cleavages at ~94°).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in felsic to intermediate igneous rocks (granite, granodiorite, syenite, rhyolite, dacite), high-grade metamorphic rocks (gneiss, schist), and some arkosic sandstones. Abundant in pegmatites.

Key Facts

Hardness

6 to 6.5 (Mohs scale)

Specific Gravity

2.55 to 2.63 g/cm³

Crystal System

Monoclinic (Orthoclase, Sanidine) or Triclinic (Microcline)

Color

White, pink, salmon-orange, reddish-brown; rarely gray or green (amazonite)

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions, intersecting at or near 90 degrees

Composition

Potassium aluminum silicate (KAlSi3O8)

Physical characteristics

• Crystal Habit: Prismatic, tabular, anhedral to subhedral grains; can form large euhedral crystals in pegmatites. Microcline often exhibits characteristic 'grid' or 'tartan' twinning under polarized light.
• Cleavage Type: Perfect basal {001} and good prismatic {010} cleavages, intersecting at 90° for monoclinic polymorphs and 89°30' for triclinic microcline.
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous on fresh surfaces, sometimes pearly on cleavage planes.

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