Calcium and Magnesium-bearing Sabugalite from the Tono Uranium Deposit, Central Japan

Calcium and magnesium-bearing sabugalite occurs as aggregations of yellowish platy crystals in veinlets or druses in conglomerate from the oxidized parts of the Tono uranium deposit, Central Japan. X-ray powder diffractometry of this mineral has reflections consistent with previous powder diffraction data of sabugalite. It is included in the monoclinic system with space group C2/m and calculated cell parameters of a = 19.68A, b = 9.89A, c = 9.82A α = γ = 90°, β-96.93° and V = 1897.83A3. Chemical analysis yields a formula of (Ca0.10 Mg0.09)Σ0.19Al0.53(UO2)2.04((PO4)1.99(AsO4)0.01)Σ2.00·11.22H2O. EMPA mapping shows that the mineral is compositionally uniform with no micron-scale layering. Charge of cations including Ca and Mg in the cation-H2O layer is 1.98 being identical to that of autunite group minerals. This suggests that the charge balance in the cation-H2O layer of the mineral could be made by the alkaline earth or alkaline elements rather than by hydrogen ions.