How to identify Labradorite

To correctly identify Labradorite (Labradorite (a variety of Anorthite, 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 gray, dark gray, greenish-gray, or brownish-gray. The most distinctive feature is the iridescent play of colors (labradorescence) in blues, greens, yellows, oranges, reds, and purples.

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 submicroscopic lamellar twinning/exsolution.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually occurs as anhedral to subhedral grains in igneous rocks, or as tabular to blocky crystals. Crystals are triclinic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 94 degrees (near 90 degrees), and good in a third direction. This produces characteristic step-like fractures.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mafic igneous rocks (gabbro, basalt, norite, anorthosite), and some high-grade metamorphic rocks. It is a primary constituent of anorthosite bodies.

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, brownish-gray, with iridescent flashes of blue, green, yellow, orange, red, purple (labradorescence)

Luster

Vitreous to pearly

Transparency

Transparent to translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions ({001} and {010}) intersecting at approximately 94 degrees, good in a third direction ({110})

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)(Al,Si)4O8 with An50-An70 (50-70% anorthite, 30-50% albite)

Physical characteristics

• Crystal Habit: Typically massive, granular, or in tabular to blocky crystals. Often twinned (polysynthetic twinning).
• Cleavage Type: Perfect basal ({001}) and perfect pinacoidal ({010}), intersecting at 93°12' to 94°10'. Good prismatic ({110}).
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to pearly on cleavage surfaces

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