|The MP2 Research Group Abstracts|
In 1998-2000, alluvial deposits in the Ibadan area in western Nigeria provided a major source of gem tourmaline. A suite of 50 faceted stones from this region were analyzed by electron microprobe. The stones were loaned by Bill Larson (Pala International, Fallbrook, California) and Bill Barker (Barker & Co., Scottsdale, Arizona), and ranged in color from near colorless to pink, to pinkish orange to purplish red, red-orange, pale yellow to yellowish green and dark green, bluish green, and grayish blue. Seven of the stones were bicolored in pink/yellowish green, pink/near colorless, purplish red/near colorless, and pink/grayish blue. A second set of nine blue Nigerian faceted tourmalines from Philip Zahm Gemstones (Aptos, California) were also analyzed.
|The results show that the majority of the stones are elbaite and liddicoatite in about equal proportions (Fig. 1). Notably, portions of two stones plot in the X-site-vacant field. These are the first confirmed samples of faceted rossmanite and the first reported rossmanite from Nigeria.|
Fe, Mn, and Ti are the principal chromophoric elements in these tourmalines; however Fe content is the dominant factor in determining color. As Y-site Fe increases, the colors change from pink/red to green and blue. The near-colorless to pink to red stones contain none to low quantities of Fe. Green and blue stones have higher Fe contents, a broad range of Mn, and in some cases higher Ti contents. Blue stones have the highest Fe content, a broad range of Mn and low Ti contents. Notably, no correlation between Mn content and the intensity of the pink/red color is observed (Fig. 2), suggesting that natural radiation which oxidizes Mn2+ to Mn3+ may be a critical factor in producing pink to red Li-Al tourmaline.
|Figure 2: Near-colorless to pink to red color intensity vs. Y-site Mn (apfu).|
Last Updated: August 22, 2006
Webmaster: Raymond Sprague
For more E-Mail information & Inquiry: Dr. William Simmons
Copyright - ©2006 Raymond Sprague - All Rights Reserved of layout, design & revisions
Copyright - ©2006 MP2 Group- All Rights Reserved of Information, Name & Logo's