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Labradorite

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Labradorite

Chemical Composition :  (Ca,Na)Al(Al,Si)Si2O8 Calcium Sodium Aluminum Silicate

Hardness : 6.0 – 6.5

Specific Gravity (Density) : 2.68 – 2.72 (g/cm3)

Refractive Index : 1.554 – 1.573 Biaxial ( + )

Labradorite is a variety of Anorthite which is a member of the plagioclase Feldspars of the Feldspar Group of minerals that includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. Although Labradorite is a variety of Anorthite, it is actually a mixture of Albite and Anorthite with a ratio ranging from 30 : 70 to 50 : 50. It is an intermediate member of the Albite-Anorthite Series.

Labradorite is a plagioclase feldspar variation and is a mixture of Albite and Anorithe, with a small percentage of Orthoclase. Chemical analysis shows that Labradorite always has an Anorithe contribution of 44 to 61 percent, which in mineralogy is referred to as the ”Bøggild Range”. Within the material, the twinned Anorithe and Albite crystals are aligned in parallel lamellae of varying thickness, usually according to the albite law.

Labradorite is a member of the Plagioclase Feldspars of the Feldspar Group of minerals that also includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. Labradorite may be best known for the opaque variety with a color play of iridescent colors that includes blues, greens, gold, orange, yellow and purple. The transparent variety ranges in color from colorless to yellow. It occassionally has inclusions of small to microscopic particles of colloidal copper which creates the effect of schiller. Schiller is the reflection of light off the copper particles suspended in the gem. This type of Labradorite with schiller is called Sunstone.

Labradorite was named in 1780 by Abraham Gottlob Werner (1749-1817) from the occurrence at Ford Harbour, Paul Island, Labrador, Canada.
Labradorite is named after its location of discovery on the Isle of Paul, near Nain, Labrador, Canada. It was discovered there in 1770 by a Moravian missionary.

Several mines in Oregon produce transparent orange, yellow, red, blue, green, and clear labradorite without labradorescence. These can be cut into very nice faceted stones. Some of this material has platy inclusions of copper in a common alignment that can produce an aventurescent flash when played in the light. These materials are marketed under the name “Oregon Sunstone” and have attracted a strong following from local designers and the tourist trade.

Ove Balthasar Bøggild (1924), “On the Labradorization of the Feldspars” Labradorite can display an iridescent optical effect, (or schiller) known as labradorescence.

Ove Balthasar Bøggild (1872–1956) was Professor of Mineralogy at the Mineralogical Museum of the University of Copenhagen, Denmark.

The term labradorization was first proposed in 1924 by Ove Balthasar Bøggild (1872–1956) in his book “On the Labradorization of the Feldspars”. He defined labradorization as “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.” His term labradorization later became labradorescence, a current gemological term.

the light entering the material and returned from its interior is not a true optical reflection but is a diffusoin of light spread over a range of angles.

reflection and refraction on alternating Albite and Anorithe layers. The reflected light waves can constructively or destructively interfere with each other. The reason for this colour play is that the material is made up from repeated, microscopically thin twinned crystal lamellae, i.e. thin layers. Such twinned structures can appear when two types of crystal with similar structure inter-grow, and both end up sharing the same crystal lattice. The two different crystals form alternating, parallel layers that are approximately 50 to 100 nm thick. On each of these lamellae, light is partly reflected and partly refracted – which, together with the thinness of the layers, can cause significant interference colours. In contrast to most other materials that exhibit this effect, though, the interference colours are highly directional in this case; this is one of the main characteristics of labradorescence.

As stated earlier, the phenomenon of Labradorite is not limited to one particular reflection colour. In fact, different zones of colour can usually be seen in a single stone; it is rare to find stones with completely monochromatic reflection patterns. The reason for this is that the individual lamellae do not grow regularly, but that the overall chemical composition of the mineral, and therefore also the thickness of the individual lamellae, usually changes slightly throughout the material.

Labradorite often shows smooth transitions between colour zones, while Spectrolite tends to have sharp edges between them. However, no general rules can be given.

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

One remarkable property of Labradorescent minerals is that their iridiscence colour hue can be predicted from their chemical composition alone.

labradoresence a play of colors or colored reflections exhibited especially by labradorite and caused by internal structures that selectively reflect only certain colors

an iridescent play of colors similar to adularescence. This labradorescence, or schiller effect, is the result of light diffraction within the lamellar intergrowths – fine, adjacent layers of the separate materials (lamellae)

Labradorescent material is most often cut into cabochons. The labradorescence phenomenon is best exhibited when the base of the cabochon is parallel to the layers in the material that produce the labradorescent flash. Careful study of the material is required so that the finished stone will be oriented to produce a full “face up color.” If the stone is cut at any other angle, the layers that produce the labradorescence will be inclined when the stone is viewed from directly above. This will yield a labradorsecent flash that will appear to be off-center.

Some specimens of sunstone are labradorite. Sunstone is a plagioclase gemstone in which tiny platelets of copper or another mineral are arranged in a common orientation. These platelets produce a reflective flash when incident light enters the stone at a proper angle relative to the angle of observation.

Labradorite is the only mineral in the plagioclase series that exhibits strong labradorescence.

Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors as shown in the photographs above and at right. Labradorite is so well known for these spectacular displays of color that the phenomenon is known as “labradorescence.

Labradorescence is not a display of colors reflected from the surface of a specimen. Instead, light enters the stone, strikes a twinning surface within the stone, and reflects from it. The color seen by the observer is the color of light reflected from that twinning surface. Different twinning surfaces within the stone reflect different colors of light. Light reflecting from different twinning surfaces in various parts of the stone can give the stone a multi-colored appearance.

Originally found at Ford Harbour, Paul Island, near Nain, off the east coast of Labrador, Canada.

Lynx Eye is a variety of labradorite with green iridescence.

Labradorite may be best known for the opaque variety with a color play of iridescent colors that includes blues, greens, gold, orange, yellow and purple. The transparent variety ranges in color from colorless to yellow. It occassionally has inclusions of small to microscopic particles of colloidal copper which creates the effect of schiller. Schiller is the reflection of light off the copper particles suspended in the gem. This type of Labradorite with schiller is called Sunstone.

Labradorite is named after the location of its first discovery; Labrador, Canada. Plagioclase is from the Greek meaning oblique cleavage. Feldspar is from the Swedish feldt + spat meaning that it was found in fields overlying granite.

Labradorite distribution: widespread. From Ford Harbour, Pauls Island, Labrador, Newfoundland; at Lake St. John, Quebec; and elsewhere in Canada. In the USA, especially in northern New York, forming the Adirondack Mountains; crystals from Sagebrush Flat, about 37 km north of Plush, Lake County, Oregon; atop the San Marcos Mountains, San Diego County, and in the western San Gabriel Mountains, Los Angeles County, California. Abundant gem crystals in the Pinacate volcanic field, Sonora, Mexico. At Vesuvius, Campania, and on Mt. Etna, Sicily, Italy. From Ylämaa, near Lappeenranta, Finland. In the Langesundsfjord- Larvik-Tvedalen area, Norway. On Surtsey Island, south of Iceland.

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