How to identify Labradorite

To correctly identify Labradorite (Labradorite (a variety of 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, black, or greenish-gray, but displays a strong iridescent play of colors (labradorescence) when light strikes it at certain angles. The colors can include blue, green, yellow, orange, red, and purple.

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 structure responsible for labradorescence is submicroscopic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually occurs as anhedral to subhedral grains in igneous rocks. When euhedral, it forms tabular to prismatic crystals, typically triclinic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, nearly at right angles (approximately 86 degrees and 94 degrees), characteristic of plagioclase feldspars. This can be observed as flat, reflective surfaces.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

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

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, or greenish-gray, with characteristic iridescent play of colors (labradorescence)

Luster

Vitreous to pearly

Transparency

Translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions (pinacoidal), nearly at right angles (86° and 94°)

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically An50-An70 (50-70% anorthite, 30-50% albite)

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Twinning (e.g., Albite twinning) is common but often microscopic.
• Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at approximately 86° and 94°.
• Fracture Type: Uneven to conchoidal, particularly in massive specimens.
• Tenacity: Brittle
• Luster Type: Vitreous on fresh surfaces, pearly on cleavage planes.

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