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Stepite, tetragonal U(AsO3OH)(2)(H2O)(4) (IMA 2012-006), is the first natural arsenate of tetravalent uranium. It occurs in the Geschieber vein, Jachymov ore district, Western Bohemia, Czech Republic, as emerald-green crystalline crusts on altered arsenic. Associated minerals include arsenolite, behounekite, claudetite, gypsum, kaatialaite, the new mineral vysolcrite (IMA 2012-067) and a partially characterized phase with the formula (H3O)(2+)(UO2)(2)(AsO4)(2)center dot 6H(2)O. Stepite typically forms tabular crystals with prominent {001} and {010} faces, up to 0.6 mm in size. The crystals have a vitreous lustre and a grey to greenish grey streak. They are brittle with an uneven fracture and a very good cleavage on (001). Their Mobs hardness is about 2. Stepite is not fluorescent in either short-wave or long-wave ultraviolet light. It is biaxial (-) with refractive indices (at 590 nm) of alpha = 1.636(2), beta = 1.667(3), gamma = 1.672(2) and 2V(obs) < similar to 5 degrees, anomalous greyish to pale yellow interference colours, and no pleochroism. The composition is as follows: 0.12 Na2O, 50.19 UO2, 0.04 SiO4, 0.09 P2O5, 0.93 As2O5, 1.95 SO3, 16.41 H2O; total 107.90 wt.%, yielding an empirical formula (based on 12 0 a.p.f.u.) of (U1.01Na0.02)(Sigma 1.03)[(ASO(3)OH)(1.82)(PO3OH)(0.04)(SO4)(0.13)(SiO4)(0.01)](Sigma E2.00)center dot 4H(2)O. Stepite is tetragonal, crystallizing in space group I4(1)/acd, with a = 10.9894(1), c = 32.9109(6) angstrom, V = 3974.5(1) angstrom(3), Z = 16 and D-calc = 3.90 g cm(-3). The six strongest peaks in the X-ray powder-diffraction pattern [d(obs) in angstrom (I) (hkl)] are as follows: 8.190(100)(004), 7.008(43)(112), 5.475(18)(200), 4.111(16)(008), 3.395(20)(312,217), 2.1543(25)(419). The crystal structure of gtepite has been solved from single-crystal X-ray diffraction data by the charge-flipping method and refined to R-1 = 0.0353 based on 1434 unique observed reflections, and to wR(2) = 0.1488 for all 1523 unique reflections. The crystal structure of gtepite consists of sheets perpendicular to [001], made up of eight-coordinate uranium atoms and hydroxyarsenate 'tetrahedra'. The ligands surrounding the uranium atom consist of six oxygen atoms which belong to the hydroxyarsenate groups and two oxygen atoms which belong to interlayer H2O molecules. Each UO8 polyhedron is connected to five other U polyhedra via six AsO3OH groups. Adjacent electroneutral sheets, of composition [U4+(AsO3OH)(22)(-)](0), are linked by hydrogen bonds involving H2O molecules in the interlayers and OH groups in the sheets. The new mineral is named in honour of Josef Step (1863-1926), a Czech mining engineer and 'father' of the world's first radioactive spa at Jachymov.
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