How to identify Labradorite

To correctly identify Labradorite (Labradorite (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, often with a dull appearance until light strikes it at a specific angle, revealing the iridescent labradorescence (blues, greens, yellows, reds, purples).

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 tabular crystals within host rocks. The internal texture responsible for labradorescence is microscopic exsolution lamellae.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually anhedral to subhedral grains in igneous rocks; can form tabular to blocky crystals, often twinned (polysynthetic twinning visible on cleavage surfaces as fine parallel lines).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions at nearly 90 degrees (86 degrees and 94 degrees), characteristic of feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mafic igneous rocks (gabbro, basalt, anorthosite), some intermediate igneous rocks (diorite), and certain metamorphic rocks derived from these protoliths. Also found in meteorites.

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

Transparent to translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions (pinacoidal) at 86° and 94°

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)(Al,Si)AlSi2O8 with 50-70% anorthite (CaAl2Si2O8) and 30-50% albite (NaAlSi3O8) end-members.

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 fresh surfaces, pearly on cleavage planes.

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