Labradorite: The feldspar mineral Labradorite information and pictures

About

Labradorite is an unusual mineral. It can display a beautiful iridescent play of colors, caused by internal fractures in the mineral that reflect light back and forth, dispersing it into different colors. This effect, known as labradorescence, gives Labradorite its appeal and fame. Specimens sold to collectors are usually polished or sliced by dealers to fully bring out this effect. Sliced slabs are sometimes sold by dealers in water, which enhances the effect.

Labradorite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. Labradorite is an intermediary member of this series. Labradorite is considered by some authorities as a variety of Anorthite rather then a separate mineral. The acclaimed Dana's System of Mineralogy lists Labradorite as an individual mineral, whereas the IMA does not recognize it as individual mineral species, but rather a sodium-rich variety of Anorthite.

For additional information, see the gemstone section on Labradorite.

Identification

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 Schiller effect with dark reds, orange, yellow, blues and green color flashes.
Streak: White
Hardness: 6–6.5
Crystal System: Triclinic
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
Transparency: Transparent to translucent
Specific Gravity: 2.69–2.72
Group: Silicates; Tectosilicates; Feldspar Group
Rock Type: Igneous, Metamorphic

Rankings

Popularity (1–4): 2
Prevalence (1–3): 2
Demand (1–3): 1

Crystal Forms & 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.

Striking Features

Play of color, hardness, cleavage, and crystal forms

Environment

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

Varieties

✓ common · ✗ uncommon

Spectrolite— Variety of Labradorite from Finland that displays a schiller color effect with intense colors such as dark red, orange, yellow, blue, and green.

Uses

Labradorite is a popular mineral, and it makes a unique gemstone. It is cut and polished into cabochons and beads, and occasionally as other facets. Some exquisite ornaments are carved out of large Labradorite chunks.

Noteworthy Localities

Perhaps the most colorful Labradorite comes from Finland at the Ylamaa Quarries, Lappeenranta. Two other outstanding localities are Golovinskoye, Zhytomyr, Ukraine; and the Antsohamamy Quarry, Tulear Province, Madagascar. Good Labradorite also comes from Black Hill, New South Wales, Australia.

Some of the most important Labradorite deposits are in Canada in Labrador, at Nain and Tabor Island. In the U.S., the Adirondack Mountains of upstate New York have produced colorful Labradorite, especially at Saranac Lake. Franklin Co.; Blue Ridge Road, North Hudson, Essex Co.; and Roaring Brook Falls, Keene Valley, Essex Co. A transparent form of Labradorite comes from Millard Co., Utah; and from the Woodward Ranch, near Alpine, Brewster Co., Texas.

Common Mineral Associations

Quartz, Muscovite, Biotite, Hornblende, Augite

Distinguishing Similar Minerals

Due to its unique color effect, Labradorite is easily distinguished from all minerals. However, specimens that don't exhibit labradorescence may be confused with many minerals, especially other feldspars.