Natural-colored Cu-bearing plagioclase feldspar (andesine) has been mined from an alluvial secondary deposit near the Tibetan city of Xigazê since 2006. The gem-quality feldspar is derived from Quaternary-Tertiary volcano-sediments related to Jurassic-Cretaceous volcanic rocks. Weathering and alluvial transport resulted in rounded crystal morphologies. Most of the crystals have an orangy red bodycolor, and a few have red and green bicolored zones. EPMA chemical analyses calculated an empirical chemical formula of (K0.03Na0.46-0.49)2Ca0.46-0.49Al1.43-1.48Si2.51-2.53O8. The ratio of albite:anorthite:orthoclase for this composition is Ab47-50:An47-50:Or3, which correlates to andesine or andesine-labradorite. The lattice parameters calculated from XRD measurements were a0=8.161-8.193Å, b0=12.849-12.919Å, c0=7.105-7.126Å, α=93.26°-93.74°, β=116.27°-116.38°, γ=89.93°-90.16°. Optical characterization by RI measurement (α=1.550-1.551, β=1.555-1.556, γ=1.560-1.561) correlated with andesine. Pale yellow non-Cu-bearing plagioclase feldspar (andesine) has been mined near the villages of Shuiquan and Haibouzi in Inner Mongolia. This material can be diffused with copper to produce a red color. Quantitative chemical analysis using EPMA and LA-ICP-MS showed that andesine from these two regions had a similar range of major and trace elements, but they could be differentiated by plotting Ba/Sr vs. Ba/Li ratios. In general, the Tibetan andesine had a lower Ba/Li ratio than Inner Mongolian andesine.

Download this issue.
(andesine-mines-Tibet-Inner-Mongolia.pdf)

Special Issue on Red Feldspar

GIA Research

A compilation of ongoing research into red feldspar and its sources around the world, as well as the detection of various color treatment processes.

Download this issue.
(http://www.gia.edu/research-resources/news-from-research/special-issue-homepage.html)