Coffinite

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 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.

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)4 for (SiO4). 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.