Micro-World
Gems & Gemology, Winter 2017, Vol. 53, No. 4

New Phenomenal Feldspar from North Carolina with Iridescent Inclusions

Stephen Challener, Nathan Renfro, and Ziyin Sun
Brown and black exsolution platelets of hematite and ilmenite in feldspar show colorful thin-film interference colors.
Brown and black exsolution platelets of hematite and ilmenite in feldspar (left) show colorful thin-film interference colors in oblique fiber-optic illumination (right). Photomicrographs by Nathan Renfro; fields of view 3.55 mm.

Several varieties of aventurescent feldspar are well known in the gem trade. Of these, one of the most remarkable is orthoclase from the Hart’s Range area in Australia. This unique material showcases exsolution platelets of hematite and ilmenite that display thin-film interference colors along the interface between the exsolution product and the feldspar host. Because of its array of colors and the crosshatch pattern of exsolution platelets, the material’s trade name is “rainbow lattice sunstone.”

The authors recently examined samples of potassium feldspar from a relatively obscure source (see above) that displayed a phenomenon remarkably similar to Australian rainbow lattice sunstone. This new material is reported to be from the Statesville area of North Carolina and contains brown and black exsolution platelets of hematite and ilmenite showing thin-film interference colors along the interface with the host feldspar. This deposit was first reported by George Frederick Kunz (Gems and Precious Stones of North America, The Scientific Publishing Co., New York, 1890, p. 164). Chemical analysis of the feldspar was performed using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), and the results were consistent with potassium feldspar.

Local prospector Ryan Underwood first found samples of this material as loose crystal fragments and traced them back to their source rock, a metamorphic biotite gneiss. The crystals are brittle, apparently as a result of metamorphic stresses, and tend to cleave during cutting. As such, material suitable for cutting is rare and finished stones tend to be small (though some large multicrystalline specimens up to 30 cm in diameter have been recovered). Base colors include blue-gray, tan, and a pink color apparently induced by additional fine hematite inclusions. 

Richard T. Liddicoat Gemological Library

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