Labradorite

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Labradorite is a type of plagioclase feldspar silicate-mineral consisting of between 30-50% Albite and 50-70% Anorthite. Its chemical position in the family of feldspar minerals is shown on the diagram below (Fig. 1). Under the diagram is a picture of a labradorite cabochon showing labradorescence (Fig. 2). Other common gemstone labradorite varieties are Oregon-sunstone, golden-labradorite, and rainbow-moonstone (Newman, Ref-1).

 Fig. 1: Feldspar Mineral Varieties 

Fig. 2: Labradorite Cabochon

Labradorite has a chemical formula of (Ca,Na)[Al(Al,Si)Si2O8], a hardness of 6-6.5, and a specific gravity of 2.70-2.74.

“Labradorite, Canada’s first described and most celebrated gemstone, flashes with the blues, greens, gold and reds that the Indians described as “fire”. This calcium-rich feldspar appears almost black, displaying its brilliant color’s only when caught in light from a specific direction. Although the earth’s crust contains a lot of feldspar, one rarely finds this particular species of the group. Labradorite no longer comes just from Labrador, other notable localities have now been discovered in Oregon, Finland and Madagascar”. (Grice, Ref-2)

Labradorite shows labradorescence - a shiller effect in lustrous metallic tints, often blue and green, and sometimes the complete spectrum. This effect is caused by interference of light from lattice distortions resulting from alternating microscopic exsolution lamellae of high- and low-calcium plagioclase phases. (Gem-Dat, Ref-3)

“Labradorite is truly a fascinatingly beautiful mineral. It’s a mineral whose charm is not fully noticed and may be overlooked if not viewed from the proper position. Generally a dull, dark looking mineral with no special virtue until the colorful shiller is observed glowing on the surface. Labradorite can produce a 

colorful play of light across cleavage planes and in sliced sections called labradorescence. The usually intense colors range from typical blues and violets through greens, yellows and oranges. Some rare specimens display all these colors simultaneously.

The color display is from lamellar intergrowths inside the crystal. These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus a separating and layering of these two phases. The resulting color effect is caused by a ray of light entering a layer and being refracted back and forth by deeper layers. This refracted ray is slowed by the extra travel through the layers and mixes with other rays to produce a light ray coming out that has a different wavelength than when it went in. The wavelength could correspond to the wavelength of a particular color, such as blue. The effect depends on the thickness and orientation of the layers. If the layers are too thick or too thin no color shiller is seen. Also if the viewer does not observe from the precise angle or if light is not supplied from the proper angle then no color shiller is seen. The labradorescence is truly a one-of-a-kind mineralogical experience and must be observed in person in order to truly appreciate its beauty.” (MinGal, Ref-4)

World Labradorite Occurrences:

“Labradorite is a common constituent of anorthosites, norites, basalts and gabbros, as well as other igneous rocks. In the metamorphic environment it occurs in gneisses derived from basic rocks. It is found in Canada (Tabor Island, Nain area of Labrador, Newfoundland Province – extensive rock-like masses, Madagascar (striped labradorite from Berily, the bands due to twinning showed exsolution labradorescence). Blue “flashing” labradorite from India is reported to show a similar effect as the light source relative to the stone is moved. Blue labradorescence twinning bands become dark and vice versa. Found in Tanzania, Mexico, Russia, Brazil, and widely distributed throughout the USA (in addition to the commercial Oregon deposits of faceting-grade labradorite (see sunstone). Gem-quality material has been recovered from localities in Arizona, California – Mode County (sunstone), New Mexico Nevada and Utah – Sunstone Knoll, Millard County - yellow transparent; RI 1.565-1.572; SG 2.68 (similar material comes from Mexico and Australia, Hogarth Range, New South Wales, and near Springsure, Queensland - pale yellow transparent RI 1.556-1.564; SG 2.695). Anorthosite rocks host iridescent labradorite at the very large Golovinskoe deposit in the Volyn district which is part of the western Ukraine shield and at Dzhugdzhurskoe in the east of the Aldan shield, Russia.” (O’Donoghue, Ref-5) 

Sunstone Labradorite:

“The name "sunstone" is used for specimens of translucent to transparent feldspar that produce bright metallic flashes when light interacts with tiny plate-like mineral inclusions within the stone. These mineral inclusions usually have a common orientation, and light entering the stone reflects from them at a common angle. This produces a flash of light in the eye of the observer who views them at the proper angle. This optical phenomenon is known as aventurescence." (GeoGem, Ref-12)

“The first materials to be called "sunstone" because of their aventurescence were specimens of oligoclase, a plagioclasefeldspar. As other types of feldspar with a strong aventurescence were discovered, the name was also applied to them. Labradorite feldspar (another plagioclase) and orthoclase feldspar have both been found with strong aventurescence. The aventurescent flash of light produced by a sunstone can be observed by three different actions:

1. moving the stone in the light

2. moving the position of the light

3. moving the eye of the observer

Sunstone is also known as "heliolite" and more commonly "aventurescent feldspar." It is cut into cabochons, beads, and small sculptures. The most transparent pieces are used to produce faceted stones. (GeoGem, Ref-12)  

“Sunstone is popular with innovative jewelry designers and is especially popular in the geographic areas where it is commercially mined. It is not a gemstone that is seen in every jewelry store, and many jewelry buyers have never witnessed its aventurescence. However, once a person is given a demonstration of sunstone’s aventurescence, they often want to try it and are fascinated. It is a gemstonethat sells best if the jeweler takes a moment to educate the buyer about aventurescence.” (GeoGem, Ref-12)

“Aventurescent feldspar has been found in Australia, Canada, China, Congo, India, Mexico, Norway, Russia, Sri Lanka, Tanzania, the United States (Oregon, New York, Virginia, Pennsylvania) and other localities. The most famous sunstone deposits in the United States are located in Oregon. A few of the sunstone deposits in Oregon are large enough to sustain mining operations. They are found in certain basalt flows in Lake County and Harney County. There the sunstone occurs as phenocrysts within the basalt. Some sunstone is produced from the weathered zone above the basalt flows, and some is produced from the basalt.” (GeoGem, Ref-12)

Labradorite: var, Oregon Sunstone:

“Oregon sunstone, the official state gemstone since 1987, is natural copper-bearing labradorite feldspar. Some examples exhibit red to green dichroism and fine specimens larger than 5 ct can sell for more than $1,000 per carat. Feldspars are silicate minerals that contain variable amounts of sodium (Na), potassium (K), and calcium (Ca). Labradorite belongs to the plagioclase feldspar series, which forms a solid solution between anorthite (CaAl2Si2O8) and albite (NaAlSi3O8). Oregon sunstone, which contains substantially more calcium than sodium, has a composition of about 70% anorthite and 30% albite, or An70/Ab30.” (GIA, Ref-6) 

Fig-3: This superb 2.85 ct sunstone from Sunstone Butte displays the gem’s most valued attributes: a blend of green and red bodycolor, with reflective spangles of native copper glittering in the interior. (GIA, Ref-6)

Fig-4: These three gems from Sunstone Butte illustrate the range of colors the mine produces.

Fig-5: This large, blocky labradorite feldspar crystal is from the Sunstone Butte mine 

Rainbow Lattice Sunstone (very rare):

Rainbow lattice sunstone (Fig-6) is a type of feldsparwhich is predominantly moonstone that is made up of 75% orthoclase and 25% albite. The inclusions (internal features) are referred to as: the result of crystalographically oriented exsolution crystals within the feldspar mass, these are hematite and ilmenite. Hematite (small mainly yellow to deep orange platelets which can be hexagonal shape) are generally in one plane within the feldspar. This effect is called aventurescence or "sunstone effect" which gives some of the gems an orange glow. Ilmenitecreates the lattice effect. This forms as very thin blades that occur in one plane at different levels. These blades orientate in different levels by a process known as lamellar twinning, and also display “Sagentic twinning”, which forms the lattice pattern. The ilmenite inclusions in many cases have oxidized or altered through geochemical processes to give the iridescence or rainbow effect across the lattice patterning. The ilmenite that has no alteration remains black with a metallic sheen. The ilmenite forms equilateral triangles, and the lattice pattern has triangular terminations. It was first discovered in late 1985 by Darren Arthur and Sonny Mason (deceased). It was identified at the GIA and declared a new gem variety in 1989. It is located at what is now known as the “Rainbow Serpent Mine” in the Harts Range Northeast of Alice Springs, Northern Territory, Australia. (Wikipedia, Ref-13)

Fig-5: This large, blocky labradorite feldspar crystal is from the Sunstone Butte mine 

Rainbow Lattice Sunstone (very rare):

Rainbow lattice sunstone (Fig-6) is a type of feldsparwhich is predominantly moonstone that is made up of 75% orthoclase and 25% albite. The inclusions (internal features) are referred to as: the result of crystalographically oriented exsolution crystals within the feldspar mass, these are hematite and ilmenite. Hematite (small mainly yellow to deep orange platelets which can be hexagonal shape) are generally in one plane within the feldspar. This effect is called aventurescence or "sunstone effect" which gives some of the gems an orange glow. Ilmenitecreates the lattice effect. This forms as very thin blades that occur in one plane at different levels. These blades orientate in different levels by a process known as lamellar twinning, and also display “Sagentic twinning”, which forms the lattice pattern. The ilmenite inclusions in many cases have oxidized or altered through geochemical processes to give the iridescence or rainbow effect across the lattice patterning. The ilmenite that has no alteration remains black with a metallic sheen. The ilmenite forms equilateral triangles, and the lattice pattern has triangular terminations. It was first discovered in late 1985 by Darren Arthur and Sonny Mason (deceased). It was identified at the GIA and declared a new gem variety in 1989. It is located at what is now known as the “Rainbow Serpent Mine” in the Harts Range Northeast of Alice Springs, Northern Territory, Australia. (Wikipedia, Ref-13)   for rainbow moonstone being referred to as "moonstone" is due to its adularescence, which according to some deems it worthy of the name. However, the adularescence of rainbow moonstone is caused by the same phenomenon as labradorite (reflection from twinning planes), where the true orthoclase moonstone gets its unusual adularescence from albite inclusions. This, and rainbow moonstone's composition set it apart from true, orthoclase moonstone. Some refer to rainbow moonstone as "labradorite moonstone" to distinguish it from orthoclase moonstone, but neither one of these is technically sound. Yet, the name, "rainbow moonstone" has stuck and some people prefer rainbow moonstone to regular moonstone.

The Romans admired moonstone and believed that it came from the rays of the moon made solid. Moonstones have been used as jewels for hundreds of years. They were admired by the ancient Romans and Greeks, and then were used in Art Nouveau jewelry. Moonstones later became popular during the hippie movement of the 1960s and the contemporary New Age spiritual movement.” Rainbow moonstone generally displays a blue or multi-colored sheen, which distinguishes it from other varieties of moonstone.” (GemSel, Ref-9)

Fig-7: Rainbow Moonstone

Rainbow moonstone is found all over the world, with significant deposits in Canada, Australia, India, Madagascar, Russia, and Sri Lanka, among others.

Fig-8: Golden Labradorite Rough

Woodward Ranch, Texas

Golden-Labradorite:

While Labradorite can be found in several locations around the world, the only known deposits for Golden Labradorite (Fig-8) are located in Mexico the United States (Texas), and New South Wales, Australia. Australian Golden-Labradorite was used in the “Cub of David” design as described in (GemJew, Ref-7)

 Fig-9: Golden Labradorite Pendant 

 Spectrolite (Fig-10):“Some rare specimens of labradorite display a full spectrum of colors, not only violet, blue and green; but also yellow, orange and red. These rare specimens have been given the name spectrolite. The name was introduced by the Finnish geologist Aarne Laitakari (1890-1975), former director of the Geological Survey of Finland. Laitakari's son, Pekka, discovered the first spectrolite deposit at Ylämaa in South-Eastern Finland near the Russian border while building the Salpa Line fortifications there in 1940. The quarrying of spectrolite began after the Second World War and has become a significant local industry. In 1973, the first workshop in Ylämaa began cutting and polishing spectrolite for gemstones and jewelry.” (GemSel, Ref-9) 

 Gem Feldspar Local Occurrences: Spencer, Idaho:“ In Clark County Ream (1989) describes gem quality labradorite crystals in the volcanic debris of Cinder Butte, located about 18 mi (28 km) southeast of Spencer. The crystals are between 0.5 and 1.25 in. (1-3 cm) across, and broken fragments are common among the debris. I (Sinkankas) was shown a number of iron oxide-coated tabular crystals that had been found by Geary Murdock, which may be from the same locality. The late Lee Horensky (San Diego) faceted several fine flawless gems of straw-yellow labradorite from such crystals. The coating of iron oxides is so thick and tenacious that scraping or grinding is needed to reveal the presence of the facet-grade material underneath.” (Sinkankas, Ref-10)Field Guides to the Cinder-Butte Labrador site can be found in books by Ream, 2012 (Ref-8) and Romaine, 2010 (Ref-11). From Hamilton to Cinder Butte is about 230 mi, and it takes about 4 hours of driving time to get there.
Beaverhead Co., MT: Moonstone, of an unknown variety, has been reported by Tim Hoff (Butte Rock Club) on the Anderson Range east of Dillon. The specific location is about ¼ mile north of the abundant range buildings along the Sweetwater road which goes between Dillon and the Ruby. The “Bitterroot Gem & Mineral Society” (BGMS) has searched this area in the last couple of years, and members have found several moonstone feldspar crystal fragments. It is probably worth further exploration, as BGMS members also found Garnet Crystals and Hydroglossular in the same area.

References:

1. Newman, Renee, “Exotic Gems Vol. 1”, 2014

2. Grice, Joel D., “Famous Mineral Localities of Canada”, 1989

3. GemDat, 10/17, https://www.gemdat.org/gem-2308.html

4. MinGal, 10/07, http://www.galleries.com/Labradorite

5. O’Donoghue, Michael, “Gems”, sixth edition, 2016

6. GIA, https://www.gia.edu/gems-gemology/fa13-oregon-sunstone-pay

7. GemJew, “Gems & Jewelry”, January 2009

8. Ream, Lanny, “Gem Trails of Idaho & Western Montana”, 2012

9. GemSel, https://www.gemselect.com

10. Sinkankas, John, “Gemstones of North America, Volume III”, 1997 

11. Romaine, Garret, “Rock Hounding Idaho, A Guide to 99 of the State’s Best  Rockhounding Sites”, 2010 

12. GeoGem, http://geology.com/gemstones/sunstone/ 

13. Wikipedia, https://en.wikipedia.org/wiki/Rainbow_lattice_sunstone