A B S T R A C T. Protasite, a new barium-cont aining member of the uranyl oxide hydrate group, occurs as bright orange pseudo-hexagonal platelets associated with uraninite and uranophane on unidentified rock matrix from Shinkolobwe mine, Zaire. Protasite is monoclinic, Pn with a 12.295(2), b 7.221(1), c 6.9558(8) A, fl 90.40(2) ~ and V 617.50(11)/~3. The tabular pseudo-hexagonal crystals are flattened on {010}, 0.1 mm to 0.5 mm wide and up to 0.1 mm thick. They are biaxial negative, 2 V = 60 65 ~ fl and 7 1.79 1.83, and X = b. Sector twinning is common. Microprobe analysis shows BaO 15.0, UO3 78.0, H20(diff.) 7.0%. The structural formula is Ba[(UO2)aO3(OH)2 ]. 3H20, Z = 2, and density(calc.) = 5.827(3) g cm-3. Complete crystal-structure analysis shows protasite to be the simplest model structure of the uranyl oxide hydrate group.

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21. Bits and pieces, 7.