Abstract

Coconinoite, named for Coconino County, Arizona, is a new uranium mineral with the probable formula

Fe2Al2(UO2)2(PO4)4(SO4)(OH)2·20H2O.

It has been found in the oxidized zone of uranium deposits in Utah and Arizona and occurrences have been documented from Wyoming, New Hampshire, and from two unknown localities. Coconinoite is light creamy yellow, nonfluoréscent, and soft. It forms aggregates of microcrystalline lathlike to platy grains which have a maximum observed length of 20 microns. Measured specific gravity is 2.70 and specific gravity calculated from the Gladstone and Dale rule is 2.68. It is biaxial (−) with α = 1.550 ± 0.005, β = 1.588 ± 0.005, γ = 1.590 ± 0.005, 2V about 40°; the pleochroism is α = colorless, β = γ = pale yellow. Although the mineral is too fine grained for single-crystal x-ray study, monoclinic symmetry is suggested by extinction angles. The d-spacings of the strongest lines are 11.05 Å (VVS), 5.52(S), 3.28(M), 3.67(M) and 5.61 (M). The formula is based on microchemical analyses of samples from Utah and Arizona. The sample from the Blackwater No. 4 Mine, Apache County, Arizona, contained (in weight per cent) Fe2O3 = 9.7, Al2O3 = 6.6, UO3 = 34.9, P2O5 = 18.3, SO3 = 5.4, H2O − = 18.2, H2O + = 5.8, acid insol. = 0.5, CaO = 0.1, CO2 = <0.1, Na2O = <0.1, Total = 99.5. The sample from the Jomac Mine, White Canyon, San Juan County, Utah, contained (in weight per cent) Fe2O3 = 10.3, Al203 = 6.5, UO3 = 34.2, P2O5 = 17.7, SO3 = 6.1, H2O − = 18.8, H2O + = 4.6, acid insol. = 1.0, Ca0 = < 0.1, CO2 = 0.7, Na2O = 0.1, Total = 100.0.

Differential thermal analysis study of coconinoite shows a large endothermic peak between 150° and 250° C. which corresponds to the water loss and a smaller endothermic peak near 800° C. which corresponds to the loss of SO2.

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