How to identify Labradorite

To correctly identify Labradorite (Plagioclase Feldspar (Labradorite variety)), 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, and red (labradorescence).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically 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, but can form tabular, prismatic crystals. Twinning (polysynthetic) is common and often visible as fine parallel lines on cleavage surfaces.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mafic igneous rocks (basalt, gabbro, anorthosite), some 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 characteristic iridescent blue, green, yellow, and red flashes.

Luster

Vitreous to pearly.

Transparency

Translucent to opaque.

Fracture

Uneven to conchoidal.

Cleavage

Perfect in two directions, intersecting at approximately 86° and 94°.

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)(Al,Si)4O8 with 50-70% anorthite (CaAl2Si2O8) component.

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Polysynthetic twinning is common.
• Cleavage Type: Perfect basal {001} and good prismatic {010} cleavage, intersecting at nearly 90 degrees.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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