Extending the mineralogy of U ⁶⁺ (VII.): Kristekite, [Cu ₂ (H ₂ O) ₄ (UO ₂ )(SeO ₃ ) ₃ ]·4H ₂ O, a new mineral from Příbram, Czech Republic.

The new mineral kristekite (IMA2025-101), [Cu 2 (H 2 O) 4 (UO 2 )(SeO 3 ) 3 ]•4H 2 O, was found in the dump material of the former uranium mine at Bytíz, near Příbram, Central Bohemia, Czech Republic, where it occurs as hedgehog-like aggregates, consisting of yellowish-green tabular crystals (up to about 0.2 mm in diameter) on the surface of strongly weathered dump material with abundant Cu-selenides and pitchblende.Among other supergene phases, clinochalcomenite, devilline, brochantite, and a rare piretite were found.The streak of ktistekite is pale green.Crystals are transparent with strong vitreous lustre.The tenacity is brittle; the Mohs hardness is about 2-3; the fracture is curved.Kristekite is optically biaxial (-) with α = 1.717(4), β = 1.755(5) and γ = 1.779(5) (589 nm); 2V = 76.5(2)°,2V calc.= 75.5°.Dispersion r < v is strong; orientation: X ^ c ~ -6°, Y ^ a ~ +8°, Z = b; pleochroism: X, Y = pale yellowish green, Z = yellow; X < Y < Z. Electron microprobe analyses yielded Cu 1.99 (UO 2 ) 1.01 (SeO 3 ) 3.01 •8H 2 O (D calc .empirical formula at 100K = 3.729 g•cm -3 ).Kristekite is monoclinic, P2 1 /m, a = 6.3665(2), b = 15.8932(4),c = 8.1248(3) Å, β = 91.437(2)°and V = 821.84(4)Å 3 and Z = 2 (at 100K).The crystal structure (R 1 = 0.0227 for 1504 I > 3σ(I) reflections) contains topologically novel uranyl-selenite sheets with incorporated Cu 2+ within these sheets and the interlayer occupied by molecular H 2 O only.The structural unit of kristekite is characteristic of the low value of the charge deficiency per anion, 0.11 vu, which is of similar magnitude as, e.g., uranyl-oxide hydroxy-hydrate mineral schoepite.The observed stereochemical properties are discussed in relation to the origin of the mineral and the observed mineral association.