How to identify Labradorite

To correctly identify Labradorite (Labradorite (a plagioclase feldspar mineral)), 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, black, or greenish-gray, but displays a strong iridescent play of colors (labradorescence) including blue, green, yellow, orange, red, and purple when light hits it at specific angles.

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. The internal texture responsible for labradorescence is microscopic lamellar twinning.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually occurs as anhedral to subhedral grains in igneous rocks, or as tabular, prismatic crystals. Twinning (polysynthetic albite twinning) is common and often visible under magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 94 degrees (near 90 degrees), characteristic of plagioclase feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in mafic igneous rocks (gabbro, basalt, anorthosite, norite, diorite) and some high-grade metamorphic rocks. It is a primary constituent of anorthosite massifs.

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, black, greenish-gray, with strong iridescent play of colors (labradorescence)

Luster

Vitreous to pearly

Transparency

Transparent to translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions (pinacoidal), intersecting at approximately 94 degrees

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Ca0.5-0.7Na0.5-0.3)Al(Si1.5-1.3)O8

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Often twinned (polysynthetic twinning).
• Cleavage Type: Perfect on {001} and {010}, intersecting at 93°20' to 94°10'.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

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