(Vyacheslavite U4+(PO4)(OH).nH2O - a new uranium phosphate.

Vyacheslavite is found in the zone of secondary enrichment of a U deposit and occurs in veinlets as dense green aggregates on quartz crystals; SEM shows elongated lamellar crystals = or <8 mu m long. Under the microscope the crystals are weakly pleochroic in shades of green and have alpha 1.700, beta 1.726-1.729, gamma 1.729-1.731, 2Valpha small; parallel extinction; electron microdiffraction indicates orthorhombic symmetry. Indexed XRD powder data are tabulated; strongest lines 6.19(10), 2.69(7), 4.56(6), 4.13(6), 3.68(5), 2.71(5) A; a 6.96, b 9.10 c = or < 12.38 A; space group CmCm, Cmc21 or C2cm; Dcalc 5.09/cm3. Microprobe analysis gave UO2 69.75-67.63, CaO 0.5-0.55, P2O5 16.9-17.1, H2O (by diff.) 12.85-14.72, corresponding with the formula U4+(PO4)(OH).nH2O where n approx 2.5. The name is for Vyacheslava G. Melkova.-R.A.H.