K. R. Ludwig and R. I. Grauch
Coexisting coffinite and uraninite in some sandstone-host uranium ores of Wyoming
Economic Geology and the Bulletin of the Society of Economic Geologists (April 1980), 75(2):296-302

Abstract:
Coexisting coffinite and uraninite appear to be very common in high-grade, sandstone-host uranium ores of Wyoming. The two minerals commonly occur as intimate intergrowths, with no marked time break between their formation. Uraniferous material with variable reflectance, intermediate between that of pure coffinite and uraninite, is commonly present and probably consists of submicroscopic mixtures of uraninite and coffinite or is the result of variations in the chemical composition of one or both of these minerals. Semiquantitative microprobe analyses always show the presence of major Ca and Si in both the uraninite and coffinite but no other major metals besides U. The Ca is not strongly fractionated between the two minerals, and the Si is possibly in the structure of uraninite as well as that of coffinite.