Labradorite

Labradorite ((Ca, Na)(Al, Si)4O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect (schiller).

Labradorite is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573 and twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which are nearly at right angles and are more obvious, being of good to perfect quality (while the third direction is poor). It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

Labradorite

General

Category Tectosilicate minerals, feldspar group, plagioclase series

Formula (Ca,Na)(Al,Si)4O8, where Na is 30-50% & Ca is 50–70%

Crystal system Triclinic

Crystal class Pinacoidal (1)

(same H-M symbol)

Unit cell a = 8.155 Å, b = 12.84 Å

c = 10.16 Å; α = 93.5°

β = 116.25°, γ = 89.133°; Z = 6

Identification

Color Gray, gray-white, brown, greenish, pale green, blue, orange, pink, yellow, colorless

Crystal habit Crystals typically thin and tabular, rhombic in cross section, striated; massive

Twinning Common by albite, pericline, Carlsbad, Baveno, or Manebach twin laws

Cleavage Perfect on {001}, less perfect on {010}, intersecting at near 90°; distinct on {110}

Fracture Uneven to conchoidal

Mohs scale hardness 6–6.5

Luster Vitreous to pearly on cleavages

Streak White

Diaphaneity Translucent to transparent

Specific gravity 2.68 to 2.72

Optical properties Biaxial (+)

Refractive index nα = 1.554–1.563

nβ = 1.559–1.568

nγ = 1.562–1.573

Birefringence δ = 0.008–0.010

2V angle Measured: 85°

Dispersion None

Other characteristics Labradorescence (iridescence, schiller optical effect)

Etymology and history

Labradorite was named after its first discovery, the Labrador Peninsula, where it was first discovered in 1770 by the Czech missionary Father Adolf.

Spectrolite was first described in 1896 in Max Bauer 's book "Gemstone Science." However, it received its name from the Finnish geologist and mineralogist Aarne Laitakari because of its iridescence in all spectral colors. In 1924, Ove Balthasar Bøggild described the occurrence of thin segregation lamellae as the cause of the iridescent play of colors. 

Special features

Labradorite's striking iridescent play of colors with a metallic sheen, known as labradorescence (verb: to labradorize), is caused by interference and reflection of light at the submicroscopic lamellae. This shimmer is predominantly blue, violet, and green, but occasionally also found in other colors. The term labradorescence was coined by Ove Balthasar Bøggild, who defined it (labradorization) as follows:

Labradorization is the peculiar reflection of the light from submicroscopical planes orientated in one direction (rarely in two directions); these planes have never such a position that they can be expressed by simple indices, and they are not directly visible under the microscope.

Contributions to the understanding of the origin and cause of the effect were made by Robert Strutt, 4th Baron Rayleigh (1923), and by Bøggild (1924).

The cause of this optical phenomenon is phase exsolution lamellar structure, occurring in the Bøggild miscibility gap. The effect is visible when the lamellar separation is between 128 and 252 nm (5.0×10−6 and 9.9×10−6 in); the lamellae are not necessarily parallel; and the lamellar structure is found to lack long range order.

The lamellar separation only occurs in plagioclases of a certain composition; those of calcic labradorite (50–70% anorthite) and bytownite (formula: (Ca0.7-0.9,Na0.3-0.1)[Al(Al,Si)Si2O8], i.e., with an anorthite content of ~70 to 90%) particularly exemplify this. Another requirement for the lamellar separation is a very slow cooling of the rock containing the plagioclase. Slow cooling is required to allow the Ca, Na, Si, and Al ions to diffuse through the plagioclase and produce the lamellar separation. Therefore, not all labradorites exhibit labradorescence (they might not have the correct composition, cooled too quickly, or both), and not all plagioclases that exhibit labradorescence are labradorites (they may be bytownite).

Some gemstone varieties of labradorite exhibiting a high degree of labradorescence are called spectrolite.

Varieties

A labradorite from Ylämaa in Finland is called a spectrolite and is characterized by the full color spectrum of labradorescence. 

Madagascar moonstone or rainbow moonstone, on the other hand, is the trade name for a white, almost transparent labradorite with strong blue surface iridescence, which is often used as an imitation for the real moonstone. 

Occurrence

The geological type area for labradorite is Paul's Island near the town of Nain in Labrador, Canada. It has also been reported in Poland, Norway, Finland and various other locations worldwide, with notable distribution in Madagascar, China, Australia, Slovakia and the United States.

Labradorite occurs in mafic igneous rocks and is the feldspar variety most common in basalt and gabbro. The uncommon anorthosite bodies are composed almost entirely of labradorite. It also is found in metamorphic amphibolites and as a detrital component of some sediments. Common mineral associates in igneous rocks include olivine, pyroxenes, amphiboles and magnetite.

Canada

Newfoundland, Newfoundland and Labrador. More than 100 occurrences in this region which is that of the topotype.

United States

Lambtongue Mine (Baby Mine), Winona District, Josephine County, Oregon.

France

Suc de Monac, Saint-Pierre-Eynac, canton of Saint-Julien-Chapteuil, Haute-Loire.

Col de l’Escrinet, Plateau du Coiron, dep. of Ardèche, Rhône-Alpes.

La Fourque, Salau, Couflens, department of Ariège, Midi-Pyrénées.

Madagascar

Varieties: green, blue, white, pink

Ampanihy

Russia

Khavokiperskiye, Lower Tunguska Basin, Krasnoyarsk Krai, Eastern Siberia.

Ukraine

Holovyne, whose products were used, among other things, for Lenin's Mausoleum and the Moscow Metro.

Use as a gemstone

Labradorite's beautiful shimmer makes it a popular choice for gemstones and arts and crafts. However, its high sensitivity to heat, acids, alkalis, and electroplating baths makes it difficult to process. Even cleaning in an ultrasonic bath damages the stone. Furthermore, labradorite is softer than quartz and therefore susceptible to scratches from the ubiquitous dust, which usually contains fine quartz grains, causing the stone's surface to dull over time. This is especially true for mineral flooring containing labradorite.

Other uses

Labradorite is also used as a purification mineral in the production of luxury vodkas, particularly Polish vodkas.

Labradorite floor tiles are used in the Nunatsiavut Assembly building in Hopedale, Labrador.

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