showed a brittle texture and a hardness between 5 and 6, with a specific gravity of 5.1. Later samples from Woodrow Mine in New Mexico collected by Moench showed fibrous internal structure and exceptional crystallization. A polished thin section of coffinite has a brown color and shows anisotropic transmission of light. Optical analysis yielded a refractive index of about 1.74. Geological occurrence Coffinite was first discovered in sedimentary uranium deposits in the Colorado Plateau region, but has also been discovered in sedimentary uranium deposits and hydrothermal veins in many other locations. Samples of coffinite from the Colorado Plateau were found with black fine-grained low-valence vanadium minerals, uraninite and finely dispersed black organic material. Other materials associated with later finds from the same region were clay and quartz. In vein deposits of the Copper King Mine in Colorado, coffinite was also found to occur with uraninite and pitchblende. Coffinite is metastable compared to uraninite and quartz, thus formation of coffinite requires a uranium source in reducing conditions, as evidenced by the associated presence of low-valence vanadium minerals. Silica-rich solution provides such a reducing condition in cases where coffinite results as an alteration product of uraninite. Hansley and Fitzpatrick also noted that the brownish color of their coffinite samples was caused by organic material, leading them to conclude that coffinite can also form in low temperature conditions if organic carbon is present. This finding is consistent with the coffinite samples of the Colorado Plateau, which included fossilized wood. In China, coffinite can be found in granite in addition to sandstone. Hansley and Fitzpatrick concluded that coarse-grained coffinite most likely forms in high temperature environments. Coffinite and uraninite precipitate inside brecciated and fractured regions of altered granite at pressures between 500 and 800 bars and temperatures at 126 to 178 °C. Special characteristics A large percentage of the Earth's uranium supply is contained in coffinite deposits, which is significant because of uranium's use in nuclear energy. Sedimentary deposits contain the most radioactive samples, as evidenced by the intensely radioactive coffinite found in the Colorado Plateau. Researchers at Harvard University, the United States Geological Survey (USGS), and several other institutions attempted unsuccessfully to synthesize coffinite in the mid-1950s after its initial discovery. In 1956, Hoekstra and Fuchs managed to create stable samples of synthetic coffinite. All of this research was conducted for the United States Atomic Energy Commission. References Uranium(IV) minerals Nesosilicates Tetragonal minerals