How to identify Labradorite

To correctly identify Labradorite (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, black, or greenish-gray, with iridescent flashes of blue, green, yellow, orange, red, and purple (labradorescence).

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

   Typically massive, granular, or in tabular crystals within host rocks. The internal texture responsible for labradorescence is submicroscopic lamellar twinning.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Usually anhedral to subhedral grains in igneous rocks; can form tabular to blocky crystals, often twinned (polysynthetic twinning visible on cleavage surfaces as fine parallel lines).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavages at nearly 90 degrees (86° and 94°), typically perfect to good.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mafic igneous rocks (gabbro, basalt, anorthosite), some metamorphic rocks, and detrital 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, black, greenish-gray, with iridescent blue, green, yellow, orange, red, purple flashes

Luster

Vitreous to pearly

Transparency

Transparent to translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions at nearly 90 degrees (86° and 94°)

Composition

(Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)Al(Si,Al)Si2O8, 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; often twinned (polysynthetic twinning).
• Cleavage Type: Perfect in two directions, forming angles of approximately 86° and 94°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to pearly on cleavage surfaces.

Explore Labradorite