How to identify Labradorite

To correctly identify Labradorite (Labradorite (a 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 dark gray, black, or greenish-gray, with iridescent flashes of blue, green, yellow, orange, and red (labradorescence).

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 within host rocks.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually anhedral to subhedral grains in igneous rocks; can form tabular, prismatic crystals in pegmatites or vugs.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavages at nearly 90 degrees (86 degrees and 94 degrees), characteristic of feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in mafic igneous rocks such as gabbro, basalt, anorthosite, and norite. Also found in some high-grade metamorphic rocks and as detrital grains in sediments.

Key Facts

Hardness

6 to 6.5 on the Mohs scale

Specific Gravity

2.68 to 2.72

Crystal System

Triclinic

Color

Dark gray, black, or greenish-gray, with iridescent blue, green, yellow, orange, red flashes (labradorescence)

Luster

Vitreous to pearly

Transparency

Translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

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

Composition

Calcium-sodium aluminum silicate (Ca,Na)(Al,Si)AlSi2O8, with an anorthite content typically between An50 and An70 (50-70% anorthite, 30-50% albite).

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Often anhedral to subhedral.
• 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 to pearly on cleavage surfaces.

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