129-15 Colorful Colorado: Photochromic reactions and crystal lattice modifications in Colorado mineral samples

Abstract:

Several minerals have been known to express a color changing property when exposed to sunlight. The term for this reaction is photochromism. The type of light that usually prompts this color response is ultraviolet (UV) light, since UV radiation can lead to permanent alterations to the crystalline structure in certain mineral samples. Color changes in minerals commonly found in Colorado were examined through multiple tests to determine if certain wavelengths of light produce a faster or more vibrant change in the hue of a sample. The minerals collected for this study include amethyst, barite, celestine, smoky quartz, and fluorite. These samples were exposed to infrared, shortwave UV, longwave UV, and natural sunlight for up to ten hours. Infrared radiation (700–800 nm) produced no observable color change in any sample. In contrast, fluorite and barite exhibited distinct color changes when exposed to shortwave UV light (254 nm), longwave UV light (365 nm), and sunlight. Amethyst, celestine, and smoky quartz showed no visible response under any of the tested conditions. X-ray fluorescence (XRF) analysis was conducted on both altered and unaltered samples of fluorite and barite to further examine the origin of the color changing reaction. The XRF data indicates that samples that contained trace elemental impurities exhibited a change in color. This demonstrates that the existence of color centers in the crystal lattice is associated with trace impurities found in the samples that changed color. 

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