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Making sense of andesine/labradorite
By Mary L. Johnson, Ph.D.
January 12, 2009
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| Mary Johnson has a doctorate in mineralogy and crystallography and is a former research and development manager at the Gemological Institute of America. Got gemological questions? E-mail them to national jeweler100@yahoo.com |
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Earlier this fall, Jewelry Television settled a lawsuit filed by a consumer who purchased stones described as green or red andesine/labradorite, which the consumer claims was "fraudulently advertised and misrepresented" as an all-natural stone. Jewelry Television claimed it sent the stones to various laboratories and believed they were all-natural. What should jewelers know about orange-red or green andesine/labradorite feldspar?
What is andesine/labradorite?
Andesine/labradorite is an intermediate plagioclase feldspar. Feldspar is the most common mineral group in the earth's crust. The three main end-members are: potassium feldspar (orthoclase, microcline), KAlSi3O8; sodium feldspar (albite, monalbite), NaAlSi3O8; and calcium feldspar (anorthite), CaAl2Si2O8. The solid-solution series between albite and anorthite is called the plagioclase series.
If the feldspar group were described today, anything with more albite than anorthite would probably be called albite, and anything with more anorthite would be anorthite. However, since the group was described many years ago, there are several intermediate species, defined by how much anorthite (An) or albite (Ab) they have: Albite: An0Ab100 to An10Ab90; Oligoclase: An10Ab90 to An30Ab70; Andesine: An30Ab70 to An50Ab50; Labradorite: An50Ab50 to An70Ab30; Bytownite: An70Ab30 to An90Ab10; Anorthite: An90Ab10 to An100Ab0.
Why keep all these names? Different plagioclase feldspars tell petrologists the likely source types of igneous rocks. The feldspars in question have compositions at the andesine/labradorite border: some are andesines, and some are labradorites.
The original red or green copper-colored "sunstones" are higher-calcium labradorites (An65-75Ab25-35) from Oregon. This is the only well-verified source of such material.
Where is gem andesine/labradorite from?
It's not clear. Sources mentioned include the Congo (in 2002), the Chinese-Mongolian border (in 2004) and Tibet. The Congolese locality has not been studied; that material may be Chinese andesine/labradorite. The Web site TibetanSun.com shows images of a Tibetan mine that supposedly yields red rough. The Tibetan material needs further investigation. Feldspars stated to be from the Congo, China, Mongolia and Tibet all have similar compositions (about An47-52Ab48-53).
According to trade press reports, a mine in Mongolia produced andesine, but the yellow stones needed treatment to produce other colors.
What causes the color?
I asked this question of George Rossman, professor of mineralogy at Caltech, who previously studied Oregon sunstone labradorite with another Ph.D., Anne Hofmeister. In Oregon labradorite, both green and orange-red colors are due to the presence of copper. Green samples are highly pleochroic, green in one direction and orange in another; the orange-red material is only slightly pleochroic. The orange material contains extremely tiny particles of colloidal copper (copper as Cu0); yellow feldspar from Oregon contains copper in the Cu1+ oxidation state; and green material, often found as rims between the yellow and red feldspar, has copper atoms in both oxidation states. Neighboring copper atoms can share an electron; sharing causes the green color and its strong pleochroism.
Rossman has been examining rough and fashioned andesine/labradorites stated to be from the Mongolia/Chinese border (also some "from Tibet"). Yellow rough from Mongolia/China contains no copper, but copper is detected in red and green feldspars, and their color and pleochroism resemble those of Oregon labradorites. Green cores or green/red splotchy zoning are commonly observed in "Chinese" feldspars, unlike the yellow-green-red zoning in some Oregon labradorite.
Is gem andesine/labradorite treated? If so, how?
We lack evidence for material claimed to be from Tibet or the Congo. Most orange-red and green andesine/labradorite on the market apparently comes from the China-Mongolia border region; that material has been heated. According to Rossman, the temperatures needed are too high to be due to faceting alone.
Orange and green samples have more copper than the yellow rough; and some orange rough was fused to a foreign material with elevated copper contents. (This foreign material should not be confused with polishing compound.) So it seems that the Chinese/Mongolian material has been heat-treated with a copper-containing material, leading to copper diffusing into the samples. Dr. Rossman calls it "duplicating what has been done naturally in Oregon."
Much work on andesine/labradorite is reportedly being performed by the Japan Germany Gemmological Laboratory. The feldspar is supposedly treated in China in three month-long stages, polishing off the "rind" that limits copper diffusion between heatings.
How should andesine/labradorite be described?
It's too soon to say. Orange-red and green andesine/labradorite from all sources may not be diffusion-treated, although apparently much of it is. We need to visit the sources of these feldspars and determine methods for distinguishing naturally colored feldspars (such as the Oregon material) from treated ones. Many laboratories are working on these feldspars; new results arrive continually. Ted Themelis of GemLab in Bangkok specializes in gemstone treatment and is expected to have more information in time for the Tucson gem shows in February.
Editor's note: Mary Johnson's "Gemological Answers" column first appeared in the January 2009 print edition of National Jeweler, in the Your Store section.
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