How to identify Feldspar

To correctly identify Feldspar (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

   Highly variable, ranging from white, cream, pink, red, green, gray, to black. Often translucent to opaque.

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

   Smooth and polished due to tumbling. Untumbled, it exhibits distinct cleavage planes.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Tumbled feldspar will not show original crystal forms. In its natural state, feldspar typically forms blocky, tabular, or prismatic crystals. Twinning (e.g., Carlsbad, Albite, Pericline) is common.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavage directions intersecting at or near 90 degrees (orthoclase, microcline) or slightly less than 90 degrees (plagioclase). This is a key diagnostic feature, even in tumbled specimens where cleavage planes might be visible as internal reflections or subtle linear features.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in virtually all types of igneous rocks (granite, basalt, gabbro, diorite), metamorphic rocks (gneiss, schist), and as a major component of many sedimentary rocks (arkose sandstone).

Key Facts

Hardness

6 to 6.5 (Mohs scale)

Specific Gravity

2.55 to 2.76 g/cm³ (varies with composition)

Crystal System

Monoclinic (Orthoclase, Sanidine) or Triclinic (Microcline, Plagioclase)

Color

White, pink, gray, green, red, brown, black; often translucent to opaque.

Luster

Vitreous to pearly

Transparency

Transparent to opaque

Fracture

Uneven to conchoidal

Cleavage

Two distinct cleavages, typically at or near 90 degrees.

Composition

Aluminosilicates of potassium, sodium, and calcium (KAlSi3O8 - Orthoclase/Microcline; NaAlSi3O8 - Albite; CaAl2Si2O8 - Anorthite)

Physical characteristics

• Crystal Habit: Blocky, tabular, prismatic; often twinned. Tumbled specimens are rounded and anhedral.
• Cleavage Type: Perfect in two directions, typically at 90° (orthoclase, microcline) or 86° (plagioclase).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

Explore Feldspar