TEM Investigation of Micro- and Nano–Precipitates in Oregon Sunstones: Genesis of and Mechanisms for Coloration, Pleochroism, and Dichroism in Gem-Quality Feldspars

Pleochroism and copper schiller is extremely rare in feldspars. This study images crystallographically oriented nanoscale copper platelets in gem-quality labradorite phenocrysts from Lake County, Oregon, to assess mechanisms for color and pleochroism in feldspars.

This schiller is evident in sporadically distributed labradorite phenocrysts of a mid-Miocene basaltic lava flow from the Dust Devil Mine. These crystals are transparent clear with copper schiller or zoned transparent red or red/green (displaying a “watermelon” effect) with varying concentrations of copper, some with visible schiller. The red/green labradorite crystals are atypically pleochroic and dichroic. The copper is shown to be native copper platelets in prior studies. These earlier investigations demonstrate copper concentrations, by Mossbauer spectroscopy, of 80-300 ppm in transparent schiller-bearing crystals, followed by a successive decrease in red/green and pale-yellow crystals.

Macroscopic copper inclusions appear to be oriented on crystallographic planes (010) and (001). Pyroxene lamellae also occur in conjunction with some copper platelets. Using High Resolution Transmission Electron Microscope (HRTEM) we focus on exact crystallographic orientation of the platelets. Although previous researchers have been unable to image the inclusions due to difficulty in TEM sample preparation and scarcity of nanoscale inclusions in samples, our first promising TEM data, combined with Electron Dispersive X-ray Spectroscopy (EDX) spectra, demonstrate localized copper regions. Based on this data, we propose that crystals with red color plus schiller and green color in transmitted light are a result of ~50 nm thick copper nano-platelets parallel to (010) in the labradorite lattice. Our results from the labradorite phenocrysts with copper schiller may have potential implications in designing new ceramic materials with dichroism and multiple colors.