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Labradorite Mineral Facts:

Chemical Formula: 1NaAlSi308 to 3CaAl2Si208
Intermediate between albite and anorthite corresponding chiefly to one quarter albite and three quarters anorthite.

Colors: Grey, brown, green, reddish, rarely colorless.
Streak nearly colorless. Sometimes shows a rich play of iridescent colors, blue, gold and green generally predominating. This play of color is not always seen.

Hardness: 6

Density: 2.73

Cleavage: The usual feldspar cleavage; perfect parallel to basal plane, less perfect parallel to brachypinacoid.

Crystallography: Triclinic
Well formed crystals are uncommon; when crystals are found they are usually in thin tabular crystals flattened parallel to the brachypinacoid. The angle between basal plane and the  brachypinacoid is a little more than 86 degrees. Labradorite usually occurs in cleavable crystalline masses.

Luster:Vitreous luster that is nearly pearly on the cleavage planes. It is transparent to opaque. 

Labradorite Feldspar

Labradorite Feldspar

Composition, Structure and Associated Minerals:
Labradorite, bytownite and anorthite are commonly found in the more basic igneous rocks. The most brilliantly colored plagioclases are some forms of labradorite, which, on cleavage surfaces, show a great display of yellow, green, red, purple and blue flashes in reflected light. The cause of the play of colors is not exactly known, but it is probably due twinning planes and the presence of numerous very tiny parallel acicular inclusions.

The twins in Labradorite are like those of albite, but the twins are usually in broad lamellae as opposed to the finer lamellae of albite. In weathering it alters to epidote, and in some instances into scapolite, and very commonly into the mixture known as saussurite, which is an aggregate containing zoisite or garnet as its most important component.

Identification and Diagnostics
Before the blowpipe it fuses rather more easily than orthoclase or oligoclase to a colorless glass. Powdered labradorite is soluble in hot hydrochloric acid. To be told from albite only by a test for calcium. Briefly, the test is made as follows: Fuse powdered mineral with sodium carbonate; dissolve fusion in hydrochloric acid and evaporate to dryness, moisten residue with water and a little nitric acid, boil and then filter off insoluble silica; to filtrate add ammonium hydroxide in excess, filter off precipitate of aluminum hydroxide; in filtrate get precipitate of calcium oxalate upon addition of ammonium oxalate. To be positively distinguished from andesine and oligoclase only by a chemical analysis or an optical examination.

Decorative Labradorite Feldspar with play of color

Decorative Labradorite Feldspar with play of color


Occurrence, Localities and Origins:
Occurs as a primary constituent of basic igneous rocks gabbro, dolerite, and basalt. Its occurrence is like albite, but more commonly in the darker colored basic igneous rocks, and usually associated with pyroxene or amphibole. Found on the coast of Labrador in large amounts, associated with hypersthene and magnetite, and when polished showing a fine iridescent play of colors. This beautiful play of colors is due in part to the intimate twinning structure, and in part to inclusions. Labradorite crystals occur at Visegrad, Hungary; and at Mt. Aetna, Italy; and beautiful cleavage pieces come from Labrador, where it forms one of the constituents of a coarse-grained igneous rock.

Used as an ornamental stone and to a limited extent as a gemstone. Labradorite and andesine-labradorite often show a peculiar sheen or "schillerization" due to microscopic linear inclusions which show a parallel arrangement. Feldspar of this kind was first found in the coarse gabbros of Labrador, hence the name. Similar material occurs also in Russia and in the United States and is sometimes used as a gemstone. It can be obtained in large pieces at and near Nam, a Moravian mission station on the coast of Labrador, and these yield slabs showing a fine play of colors when suitably cut and polished. In spite of its beauty, its ready cleavage and brittleness mitigate against its use in a large way as an ordinary gemstone.  It is used as a decorative stone material.

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