How to identify Labradorite

To correctly identify Labradorite (Labradorite (a variety of plagioclase 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 gray, dark gray, greenish-gray, or brownish-gray in ordinary light. Displays iridescent flashes of blue, green, yellow, orange, red, and purple (labradorescence) when rotated.

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

   Often massive, granular, or in tabular crystals. Can exhibit polysynthetic twinning striations on cleavage surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Triclinic system, typically forming tabular to blocky crystals. Often found as anhedral to subhedral grains in igneous rocks.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at right angles (approximately 86 degrees and 94 degrees), characteristic of plagioclase feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in mafic igneous rocks such as anorthosite, gabbro, basalt, and norite. Also occurs in some high-grade metamorphic rocks.

Key Facts

Hardness

6 to 6.5 on the Mohs scale

Specific Gravity

2.68 to 2.72

Crystal System

Triclinic

Color

Gray, dark gray, greenish-gray, brownish-gray; exhibits labradorescence (iridescent play of colors)

Luster

Vitreous to pearly

Transparency

Translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions (pinacoidal), nearly at 90 degrees (86° and 94°)

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Ca0.5-0.7Na0.5-0.3)Al(Si1.5-1.7Al0.5-0.3)O8

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to blocky crystals. Often twinned (polysynthetic twinning).
• Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 86° and 94°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous on crystal faces, pearly on cleavage surfaces.

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