Abstract
Gastunite is an alkali uranyl silicate with the composition (K, Na)2(UO2)3(Si2O5)4 · 8H2O. Chemical analysis, recalculated after deducting impurities, gives SiO2—31.2, UO3—53.8. K2O—4.6, Na2O—0.9, H2O—9.5, total 100.0 per cent. The proposed formula is supported by infra-red absorption data. X-ray study shows the mineral lo be orthorhombic, space group Pnna (W. Outerbridge), a0—14.24, b0—35.84, c0—14.20, calculated specific gravity 3.97 (3.96±0.03 measured). Indexed powder pattern shows the strongest lines to be 7.12 (10), 3.56 (9), 9.10 (8), 4.83 (7), 3.31 (7). Optically biaxial positive with a = 1.604, β = 1.610, γ = 1.621: 2V moderate, anomalous blue interference color. Habit: radiating acicular to fibrous aggregates. Hardness 2. Perfect (010) cleavage. Gastunite occurs as a secondary mineral in radial aggregates coating chalcedony at the Red Knob mine, Muggins Mountains, Yuma County, Arizona; and in vesicular cavities in welded tuff at the Mammoth prospect, Presidio County, Texas. Also present in dense fine-grained pseudomorphs after uraninite from the Easton area, Pennsylvania. Hydrothermal synthesis data for the potassium, potassium plus sodium, sodium, ammonium, and hvdronium analogues of gastunite are presented.
