Coffinite is the dominant ore mineral in the tabular vanadium-uranium ores of the Tony M mine in the Henry Mountains Mineral Belt of the Colorado Plateau physiographic province of the United States. This ore body formed at a density-stratified solution interface between uranyl ion bearing-meteoric water and a saline fluid which was locally reducing. Uranium localization at this solution interface occurred by adsorption onto the surfaces of detrital minerals. This adsorption was related to the pH difference between the two fluids. New experimental evidence is presented which shows that adsorption facilitated the reduction of uranium to U(IV). This adsorbed, reduced uranium bonded with aqueous silica in the ore zone to form coffinite. Evidence is presented that in order to stabilize coffinite over uraninite, this silica was highly concentrated in the ore-forming solution, and the dominant form of this silica was as a monomeric species.

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