Special Report from October 2009 Newsletter

A Mystery of the Orient

Tibetan red feldspar plays peek-a-boo

Conclusive and less conclusive information regarding the issue of red feldspar treatment was released September 9 by several researchers under the auspices of the Gemological Institute of America. Although some of the information has been reported before in bits and snatches, “Special Issue on Red Feldspar” collects it in one place.

At issue is whether feldspar from China has been enhanced to achieve a red color. While colored gemstone aficionados suffering from attention deficit disorder will once again have their patience stretched thin, the issues surrounding red feldspar (aka andesine-labradorite) actually may be an example of scientific rigor at its best: the structured examination of evidence, replication of methodology and results, and deductions that don’t presume too much.

From the source. A selection of rough andesine feldspar, 2.4–34.42 carats, collected from the Bainang mine in Tibet. Ahmadjan Abduriyim explained that they “displayed rounded shapes and a water worn appearance.” (Photo: M. Kobayashi, courtesy GAAJ-ZENHOKYO Laboratory)

What follows is not a comprehensive digest of the individual reports, but rather a taste of each; anyone interested will want to look at the full special issue. Briefly, feldspar specimens from a mine in Tibet, roughly 100 km. north of the Bhutan border, and from two more in central Inner Mongolia, were personally collected by Ahmadjan Abduriyim of Gemmological Association of All Japan (GAAJ)-ZENHOKYO Laboratory, in October and November of 2008. According to an introductory report subtitled “A Puzzle to Fix,” during his visit to Inner Mongolia, Abduriyim learned of a “new technique” to diffuse copper into colorless material, resulting in red. Prior to that collection, John Emmett and Troy Douthit, of Crystal Chemistry, already had demonstrated that copper could diffuse into colorless plagioclase feldspars from four localities in North America and Mongolia. And while the phenomenology is still under investigation, according to the Crystal Chemistry report included in GIA’s special issue, “the diffusion is rapid enough that an economic commercial process could have been easily developed.”

Samples treated by Crystal Chemistry were forwarded to Kamolwan Thirangoon of GIA’s Bangkok lab, who analyzed both heated and diffused material. These samples were compared with Tibetan and Mongolian material received from Abduriyim. While locality was easily delineated from trace element chemistry for the Crystal Chemistry samples, Abduriyim’s material had chemistry too similar to separate the Tibetan from the Mongolian. Copper concentration could not be used by the lab to differentiate between purportedly natural material and samples known to be diffused in the lab. More study needed.

Shane McClure of GIA’s Carlsbad lab concluded that material from Mongolia probably was colorless originally, but that “the possibility exists that a Tibetan mine is producing red and green andesine feldspar.” Like Thirangoon, McClure found that the two localities could not be separated chemically. Nevertheless, in his report McClure includes a fascinating mention of a colorless zone that follows the surface of rough material—a zone that is present in both natural and treated samples. Counterintuitively, “in the untreated stones the [colorless] zone contains little or no copper, and in the treated stones there is significant copper content” in the colorless zone. More study needed.

A little something extra. Fused fragments and glass found in surface depressions are indicators of possible high-temperature heating required for diffusion. If the glass analyzes as un-volcanic, suspicion is raised. The example above is from a rough andesine feldspar from Inner Mongolia, included in George Rossman’s interim report. Material from Tibet also displayed fused fragments and glassy material. Hmm… (Photo: George R. Rossman, California Institute of Technology)

Caltech professor George Rossman’s interim report of an ongoing study includes two graphical representations of chemical fingerprinting, demonstrating the challenge of separating feldspars from Tibet, China, and the Democratic Republic of the Congo. Rossman shows that (re)heating red material from Tibet and Mongolia releases almost no radiogenic argon, in contrast with yellow material that has not been heated. And glassy material found in depressions from purportedly natural red Tibetan material “was rich in potassium and enriched in copper”—an indication of diffusion enhancement—“just like that found in the treated red feldspar from Inner Mongolia.” That the “natural” red material was obtained from a mine visit contradicts the lab results. More study needed.

Abduriyim’s study was successful in separating the Tibetan from the Inner Mongolian material using two trace element combos: Ba/Sr and Ba/Li. A graph included in his report shows clear delineation, and other fingerprinting methods resulted in similar distinctions. A full range of observation, testing, and analysis was performed, and Abduriyim’s report—as are those of the other researchers—is filled with photomicrographs to illustrate the text.

Abduriyim begins his report with a detailed account of the mining at Tibet’s Baining mine and his encounter with his samples. Nevertheless, in an August 21 meeting at GIA Carlsbad, the researchers came to a conclusion: More study needed. The stakeholders agreed to a new expedition to the Tibetan mine—and other mines in the area—with old and new participants being involved. Video of Abduriyim’s visit a year ago was posted last month on the GAAJ website. [back to top]
 

East African Sunstone…

Earlier this week Robert James of the International School of Gemology sent out a newsletter detailing lab analysis of another sort of feldspar: faceted “East African Ruby Sunstone and Emerald Sunstone.” The gemstones, purchased by ISG on the open market, were set in rings and marketed as untreated, but under immersion revealed a coating of some sort. Some of the coating actually broke away from the stones when they were removed from their settings, having been glued to the prongs. [back to top]