Gem-Quality Anorthoclase Feldspar from Southeast Vietnam
Le Ngoc Nang, Pham Minh Tien, Pham Minh, and Pham Trung Hieu
ABSTRACT
In southeast Vietnam, gem-quality anorthoclase is found as megacrysts in Cenozoic alkali basalt. The crystals are transparent to translucent with vitreous luster and can be near-colorless, yellow, or brownish gray. Gemological testing confirmed this mineral is alkali feldspar, but the data cannot specify the alkali feldspar subgroup (orthoclase, anorthoclase, microcline, or sanidine). Based on X-ray diffraction analysis and a feldspar composition ternary diagram created from energy-dispersive X-ray fluorescence analysis, this material was identified as anorthoclase. Energy-dispersive X-ray fluorescence determined the composition to be An3.50–8.02, Ab58.83–81.69, and Or11.94–36.96. The Raman spectra displayed differences when compared to those of other alkali feldspars such as sanidine, orthoclase, and microcline. Fourier-transform infrared spectra of this material are similar to those of anorthoclase from around the world and lack any distinctive features that would allow for its separation. Gem-quality anorthoclase from southeast Vietnam has commercial potential, as it is abundant and evenly distributed in the basaltic weathering products.