How to identify Labradorite

To correctly identify Labradorite (Plagioclase Feldspar (Na,Ca)(Al,Si)4O8), 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 black in ordinary light. Exhibits strong labradorescence (iridescence) with flashes of blue, green, yellow, orange, red, and purple when light strikes it at certain 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 intergrowths.

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, often twinned (polysynthetic twinning, visible as fine parallel lines on cleavage surfaces).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect in two directions, intersecting at approximately 94 degrees (basal {001} and pinacoidal {010}). A third, less perfect cleavage may be present.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Mafic igneous rocks (gabbro, basalt, norite, anorthosite), some metamorphic rocks, and rarely in detrital sediments.

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, black; exhibits labradorescence (iridescent play of colors)

Luster

Vitreous to pearly

Transparency

Translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions ({001} and {010}) at approximately 94 degrees

Composition

Sodium calcium aluminum silicate (Na,Ca)(Al,Si)4O8, specifically an intermediate member of the plagioclase solid solution series (An50-An70)

Physical characteristics

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

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