Samples of natural coffinite (USiO4·nH2O) from Grants uranium region, New Mexico were investigated in order to understand the alteration process of coffinite under reducing and oxidizing conditions. Alteration of the primary coffinite under reducing conditions was promoted by organic acids, and as a result, secondary coffinite precipitated. Subsequently oxidizing fluids altered the coffinite, and (Na,K)-boltwoodite [(Na,K)(UO 2)(SiO3OH)(H2O)1.5] and Jáchymovi te [(UO2)(SO4)(OH)14(H 2O)13] precipitated with no rare earth elements. Based on the charge balance calculation, we suggest that the amount of U6+ in the coffinite is less than 0.2 [apfu] and U6+ is accommodated in the structure via substitution: U4+ + Si4+ ≅ U 6+ + 2(OH)-. The high and variable totals for electron microprobe analyses indicate that H2O is not an essential component in coffinite structure. The U-Pb ages of coffinite formation vary from 36.6-0 Ma suggesting that the coffinite has precipitated continuously in this period and organic matter can preserve reducing conditions even when oxidizing conditions dominate.