Geological Properties Of Labradorite
Geological Properties Of Labradorite, Geologic Occurrence of Labradorite
Labradorite is found in igneous, metamorphic, and sedimentary rocks. It most often occurs as a primary mineral in mafic igneous rocks such as basalt, gabbro, and norite. It is also found in anorthosite, an igneous rock in which labradorite can be the most abundant mineral. Labradorite occurs in gneiss that has been produced through the metamorphism of labradorite-bearing igneous rocks. It is also found in sediments and sedimentary rocks that are derived from the weathering of other rocks that contain labradorite.
Labradorite is a type of feldspar. Feldspar is found all over the world in all types of rock formation, igneous, metamorphic and sedimentary. Feldspar is made up of silica and aluminum and forms in a number of ways, including crystallization of magma or lava, being subject to high temperatures and great pressure below the earth’s surface or crushed together in sedimentary rock.
Labradorite is created when heat and pressure melt and mix various rocks and minerals. As the newly mixed materials slowly cool, the minerals separate into layers and this now, igneous rock, hardens with several inclusions. It is the layers that will later give the gemstone its shimmering color play.
This layering or lamellar separation only occurs with a particular mineral combination and so long as the rock cools over a very long time period. The wrong combination, or cooling too quickly, can create similar rocks but lack the shimmering play of color that identifies Labradorite gemstones.
Labradorite Discovery and History
Labradorite was known as fire stone or fire rock by the native Inuit people of Canada and was the subject of some legends and customs but it was not until 1770 that is was officially identified as Labradorite.
It was ‘discovered’ by Moravian missionaries on the Isle of Paul in Labrador (hence its name) who began to trade Labradorite gemstones with British merchants for vital supplies to ensure the mission’s survival.
The spectacular Spectrolite was discovered in Finland by accident when soldiers were making obstacles for invading Russian tanks. While blowing up rocks with dynamite they discovered this brightly colored version of Labradorite.
What Causes 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.
Properties of Labradorite
Labradorite is a mineral in the plagioclase series, and it shares many of the properties of plagioclase minerals. It has a Mohs hardness of about 6 to 6 1/2 and two distinct directions of cleavage that intersect at an angle of about 86 degrees or 94 degrees. Plagioclase minerals frequently exhibit twinning and striations on cleavage faces.
Geological Properties Of Labradorite
Labradorite is the only mineral in the plagioclase series that exhibits strong labradorescence; however, many specimens of labradorite do not exhibit the phenomenon. Without seeing labradorescence, distinguishing labradorite from other members of the plagioclase series can be difficult. The methods used for distinguishing them are x-ray diffraction, chemical analysis, optical tests, and specific gravity determinations on pure specimens.
Labradorite as a Gemstone
Labradorite has become a popular gemstone because of the unique iridescent play-of-color that many specimens exhibit. The quality, hue, and brilliance of the labradorescence varies from one specimen to another and within a single specimen. Stones with exceptional color are often given the name “spectrolite.”
Labradorite is rarely seen in mass-merchant jewelry. Instead it is most often used by designers and jewelers who do unique and custom work.
Many specimens of labradorite do not exhibit labradorescence. These materials can still produce beautiful gemstones because of their desirable color or other optical effects such as aventurescence.
Notable Labradorite Localities
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.
Labradorite with superb labradorescence is produced from a few deposits in Finland. The best of this material was given the name “spectrolite” by the director of the Geological Survey of Finland. Today, specimens of labradorite with exceptional labradorescence from other locations are frequently called “spectrolite.”
A significant amount of gray to black labradorite with good labradorescence is produced from locations in Madagascar and Russia. Small amounts of transparent labradorite with internal color flash are produced in India.
Geological Properties Of Labradorite
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.
Countries of Origin
Myanmar; Cameroon; Papua New Guinea; Paraguay; Portugal; Greece; Mongolia; El Salvador; Korea (the Republic of); Morocco; Unknown; Brazil; Chile; Nepal; Tonga; United States of America (the); Hungary; Japan; Ukraine; Taiwan (Province of China); India; New Zealand; Canada; Namibia; Finland; Italy; Peru; Ethiopia; Germany; Tanzania, United Republic Of; Czechia; Egypt; Madagascar; Thailand; Costa Rica; Saudi Arabia; Sweden; Pakistan; China; Russian Federation (the); Poland; Slovakia; France; Kyrgyzstan; United Kingdom of Great Britain and Northern Ireland (the); Dominican Republic (the); Spain; Cuba; Saint Lucia; Norway; Mexico; Israel; Greenland; Tajikistan; Indonesia.
Physical Properties of Labradorite
Color: Usually clear, white, or gray in reflected light. Labradorescent colors can include blue, green, yellow, orange, and red.
Streak: White
Luster: Vitreous, pearly on cleavage faces
Diaphaneity: Transparent to translucent
Cleavage Two directions of perfect cleavage intersecting at about 86 degrees
Mohs Hardness: 6 to 6.5
Specific Gravity: 2.68 to 2.72
Chemical Composition: (Na,Ca)(Al,Si)4O8 with Na (30-50%) and Ca (70-50%)
Crystal System: Triclinic
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