How to identify Feldspar

To correctly identify Feldspar (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8), 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, gray, pink, red, green, to brown. Alkali feldspars often exhibit pink to reddish hues (orthoclase, microcline), while plagioclase can be white, gray, or bluish-gray (labradorite).

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 blocky or tabular crystals, often with striations on plagioclase cleavage faces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic (orthoclase, sanidine) or triclinic (microcline, plagioclase series). Crystals are often prismatic, tabular, or anhedral masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavage planes intersecting at or near 90 degrees (orthoclase, microcline) or slightly less than 90 degrees (plagioclase).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in virtually all types of igneous rocks (granites, basalts, syenites, gabbros), metamorphic rocks (gneisses, schists), and some sedimentary rocks (arkoses, greywackes).

Key Facts

Hardness

6 - 6.5 (Mohs scale)

Specific Gravity

2.55 - 2.76

Crystal System

Monoclinic (orthoclase, sanidine) or Triclinic (microcline, plagioclase series)

Color

Highly variable: white, gray, pink, red, green, brown

Luster

Vitreous to pearly

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Two distinct cleavages, typically at or near 90 degrees

Composition

Aluminosilicates of potassium, sodium, and calcium (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8)

Physical characteristics

• Crystal Habit: Prismatic, tabular, blocky, anhedral masses; often twinned (e.g., Carlsbad, Albite, Pericline twins)
• Cleavage Type: Perfect in two directions, typically at 90 degrees (orthoclase) or slightly less (plagioclase)
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to pearly on cleavage surfaces

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