Coffinite is a uranium-bearing silicate mineral with formula: U(SiO4)1−x(OH)4x. It occurs as black incrustations, dark to pale-brown in thin section. It has a grayish-black streak. It has a brittle to conchoidal fracture. The hardness of coffinite is between 5 and 6. It was first described in 1954 for an occurrence at the La Sal No. 2 Mine, Beaver Mesa, Mesa County, Colorado, US, and named for American geologist Reuben Clare Coffin (1886–1972). It has widespread global occurrence in Colorado Plateau-type uranium ore deposits of uranium and vanadium. It replaces organic matter in sandstone and in hydrothermal vein type deposits. It occurs in association with uraninite, thorite, pyrite, marcasite, roscoelite, clay minerals and amorphous organic matter. Composition Coffinite's chemical formula is U(SiO4)1−x(OH)4x. X-ray powder patterns from samples of coffinite allowed geologists to classify it as a new mineral in 1955. A comparison to the x-ray powder pattern of zircon (ZrSiO4) and thorite (ThSiO4) was the basis for this classification. Preliminary chemical analysis indicated that the uranous silicate exhibited hydroxyl substitution. The results of Sherwood's preliminary chemical analysis were based on samples from three locations. Hydroxyl bonds and silicon-oxygen bonds also proved to exist after infrared absorption spectral analyses were performed. The hydroxyl substitution occurs as (OH)44− for (SiO4)4−. The hydroxyl constituent in coffinite later proved to be nonessential in the formation of a stable synthetic mineral. Recent electron microprobe analysis of the submicroscopic crystals uncovered an abundance of calcium, yttrium, phosphorus, and minimal lead substitutions along with traces of other rare earth elements. Crystal structure Coffinite is isostructural with the orthosilicates zircon (ZrSiO4) and thorite (ThSiO4). Stieff et al. analyzed coffinite using the x-ray powder diffraction technique and determined that it has a tetragonal structure. Occurring naturally with U4+ cations, the UO8 triangular dodecahedra coordinate with edge-sharing, alternating SiO4 tetrahedra in chains along the c-axis. The central uranium site of coffinite is surrounded by eight SiO4 tetrahedra. The lattice dimensions of naturally-occurring and synthetic coffinite are similar, with a naturally-occurring sample from Arrowhead Mine, Mesa County, Colorado having a=6.93kx, c=6.30kx, and a sample synthesized by Hoekstra and Fuchs having a=6.977kx and c=6.307kx. Physical properties Initial examination of coffinite by Stieff et al. described the mineral as black in color with an adamantine luster, indistinguishable from uraninite (UO2). Additionally, the discoverers reported that although no cleavage is seen in coffinite, it does exhibit subconchoidal fracturing and is very fine grained. Initial samples