Crystal Black Labradorite

Chemical Formula

(Ca,Na)Al_1-2Si_3-2O₈

Composition

Sodium calcium aluminum silicate. The ratio of sodium to calcium is about 7:3. The amount of aluminum atoms are between 1 and 2, and the amount of silicon atoms are between 3 and 2.

In the Plagioclase Feldspar series, Labradorite contains between 30 and 50 percent Albite (Ab), and between 50 and 70 percent Anorthite (An).

Color

White, gray, light blue, light green, pale orange-red, black, usually with a strong multicolored display of purple, blue, and green schillers. A variety from Finland known as Spectrolite, shows the Schillereffect with dark reds, orange, yellow, blues and green color flashes.

Streak

White

Hardness

6 - 6.5

Crystal System

Triclinic

Crystal Forms 
and Aggregates

Labradorite rarely forms in crystals. When it does, they are generally tabular and often twinned. Most commonly occurs massive , grainy , as elongated fragments, as chunky masses, and rounded.

Transparency

Transparent to translucent

Specific Gravity

2.69 - 2.72

Luster

Vitreous to pearly

Cleavage

2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.

Fracture

Conchoidal to uneven

Tenacity

Brittle

In Group

Silicates; Tectosilicates; Feldspar Group

Striking Features

Play of color, hardness, cleavage, and crystal forms

Environment

In igneous environments in diabase, and in contact metamorphic rocks.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

2

Prevalence (1-3)

2

Demand (1-3)

1