How to identify Feldspar

To correctly identify Feldspar (Feldspar group), 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, yellow, to brown. Plagioclase feldspars are typically white to gray, while alkali feldspars can be white, pink, or reddish.

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 crystals within igneous and metamorphic rocks.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Often tabular, prismatic, or blocky crystals. Plagioclase often exhibits polysynthetic twinning (fine parallel striations on cleavage surfaces), which is a key diagnostic feature.

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). This is a crucial identifying characteristic.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in virtually all types of igneous rocks (e.g., granite, basalt, gabbro, rhyolite), metamorphic rocks (e.g., gneiss, schist), and as detrital grains in sedimentary rocks (e.g., arkose sandstone).

Key Facts

Hardness

6 - 6.5 (Mohs scale)

Specific Gravity

2.55 - 2.76 g/cm³

Crystal System

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

Color

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

Physical characteristics

• Crystal Habit: Tabular, prismatic, blocky, massive, granular
• Cleavage Type: Perfect to good in two directions, intersecting at 90° (orthoclase, microcline) or 86-94° (plagioclase)
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to pearly on cleavage surfaces

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