Piretite, ideally Ca(UO 2 ) 3 (SeO 3 ) 2 (OH) 4 .4H 2 O, is a new mineral from the Shinkolobwe uranium deposit in Shaba, Zaire, that occurs as crusts in association with an orange masuyite-like U-Pb oxide on the surface of uraninite samples. The crystals are lemon-yellow in color with a pearly luster; they do not fluoresce under ultraviolet light. Cleavage: {001} good. D meas = 4.00 g/cm 3 and D calc = 3.87 g/cm 3 (empirical formula), 3.93 g/cm 3 (idealized formula); H Mohs = 2.5. Optically biaxial negative, 2V = 33(5) degrees , alpha 1.54(calc.), beta 1.73(1) and gamma 1.75 (1), with optical orientation xc, Ya, Zb. The dispersion r>v is weak, and the crystals are nonpleochroic. Piretite is orthorhombic, space group Pmn2 1 or Pmnm, with the following unit-cell parameters refined from powder data: a 7.010(3), b 17.135(7), c 17.606(4) Aa, V 2114.8(1) Aa 3 , a:b:c 0.409:1:1.027, Z = 4. The forms recognized are {100} {010} {001}, tenacity is weak, and the fracture is uneven. The strongest ten reflections of the X-ray powder pattern [d(in Aa)(1)hkl] are: 8.79(80)002, 8.56(40)020, 5.57(20)013, 4.43(20)130, 4.30(30)131, 3.51(100)200, 3.24(40)220, 3.093(50)115, 3.032(100)151 and 1.924(40)237. Electron-microprobe and thermogravimetric analyses gave: CaO 3.57, UO 3 72.00, SeO 2 19.29, H 2 O 8.00, total 102.86 wt.%. The empirical formula is 0.76 CaO.3.02 UO 3 .2.09 SeO 2 .5.33 H 2 O. Piretite results from the alteration of uraninite and primary selenium-bearing sulfides such as penroseite, in the presence of Ca derived from the host rocks. The name is chosen to honor the Belgian crystallographer Paul Piret. Holotype material is deposited in the mineralogical collection of the Royal Belgian Institute of Natural Sciences, Brussels, Belgium.

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