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Uranium dioxide (UO) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO, the mineralogical form of UO called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO specimens: synthetic single-crystal UO, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO under a variety of conditions.