Author(s): Jacob Crump
Mentor(s): Jason Kaiser
Institution SUU
Variscite is a hydrated aluminum phosphate, AlPO₄ · 2H₂O, that occurs in hydrothermal deposits, where phosphate-rich water interacts with aluminum-rich rocks at or near the surface. In these deposits, variscite forms in nodules, veins, and crusts and is associated with minerals such as crandallite, wavellite, wardite, turquoise, and jasper. Typically, variscite is pale to emerald green, bluish green, colorless to white, or pale brown to yellow in color. The source of this color variation is poorly understood, though it has been suggested that octahedral chromium ions are responsible for the absorption bands that cause the green coloration of variscite and metavariscite (Calas et al., 2005). We have selected samples from Nevada, Idaho, Australia, Iran, and Utah, including samples from Clay Canyon, Amatrice Hill, and Snowville. These samples vary in green color, patterns, and associated minerals, even within the same localities. Samples were prepared using lapidary equipment to cut, thin, and polish each specimen. The samples were then analyzed using the Thermo Scientific Phenom ProX Scanning Electron Microscope (SEM) at Southern Utah University. The SEM produced high-resolution images highlighting the distribution of major and trace elements in each sample. The concentration and distribution of trace elements, such as chromium, were compared to the images of the samples to determine what might control the color variation. In the Siren Storm variscite from Nevada, darker green portions correlate with chromium concentrations equal to or greater than 0.4 wt%, while lighter green portions correlate with chromium concentrations equal to or less than 0.3 wt%. The Lucin variscite from Utah is a relatively homogeneous emerald green, yet the chromium concentrations vary between 0.1 and 0.4 wt%. These data suggest there may not be a strong correlation between chromium concentrations and color variation, as previously suggested. Other trace element concentrations, such as scandium, vanadium, and iron, also lack a consistent correlation with color distribution.