How to identify Labradorite

To correctly identify Labradorite (Labradorite), 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, grayish-white, or greenish-gray, but displays a strong iridescent play of colors (labradorescence) including blue, green, yellow, orange, red, and purple.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on cleavage surfaces, with a distinctive metallic schiller (labradorescence) when oriented correctly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically massive, granular, or in tabular crystals within igneous rocks.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually occurs as anhedral to subhedral grains, or as tabular to blocky crystals. Twinning (polysynthetic twinning) 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, nearly at right angles (approximately 94 degrees), characteristic of plagioclase feldspars.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in mafic igneous rocks such as gabbro, basalt, norite, and anorthosite. Also occurs in some metamorphic rocks and as detrital grains in sediments derived from these sources.

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, with iridescent play of colors (labradorescence)

Luster

Vitreous to pearly, with metallic schiller

Transparency

Transparent to translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

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

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Ca0.5-0.7Na0.3-0.5)Al1-1.5Si2.5-3O8, 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. Polysynthetic twinning is common.
• Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 94 degrees.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces; exhibits labradorescence (schiller effect).

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