Redgillite, Cu_6(OH)_10(SO_4) center dot H_2O, a new mineral from Caldbeck Fells, Cumbria, England: description and crystal structure

摘要

Redgillite, Cu_6(OH)_10(SO_4) centre dot H_2O, space group P2_1/c, a 3.155(3) A, b 10.441(8) A, c 19.436(16) A, P 90.089(13)°, V = 640.2(9) A3, Z = 2, is a new mineral from Silver Gill, Caldbeck Fells, Cumbria, England. The strongest six lines of the X-ray powder-diffraction pattern [d in A, (I) (hkl)] are: 9.72 (90) (002), 7.11 (100) (012), 4.60 (30) (022), 4.068 (20) (023), 2.880 (30) (112,112), 2.318 (50) (131,131). It occurs as translucent to transparent grass-green bladed crystals up to 0.15 mm long with squared-off or tapering terminations; usually in radiating groups. Forms observed are {001} prominent, {010} as composite stepped faces, and {100} irregular. Redgillite has white streak, vitreous lustre and Mohs hardness of ~2. Blades are slightly flexible with irregular fracture and exhibit a perfect {001} cleavage and good {100} and {010} cleavages. The measured density (by sink-float) is 3.45(5) g/cm3; the calculated density is 3.450 g/cm~3. The mineral dissolves slowly in dilute HCl. Redgillite is biaxial-negative with a = 1.693(2), p = 1.721(2), y = 1.723(2), 2V = 30(2) deg (meas.) and 30 deg (calc); dispersion is r > v, medium; pleochroism: Y blue-green > X blue-green > Z yellow-green; orientation: X ≈ c, Y = b, Z ≈ a. Electron microprobe analyses yielded CuO 68.9, SO_3 11.6, total 80.5. With water inferred from the structure analysis, the empirical formula is: Cu_5. 995 (OH)_9.991(SO_4)_1. 003 centre dot H_2O. Redgillite is typically found in thin fractures in partly oxidized sulphides where it is commonly associated with langite and more rarely with malachite, cuprite, connellite and brochantite. The name is for the Red Gill mine, from which the mineral is best known. The crystal structure of redgillite was determined and refined to R = 0.090 for 1529 observed reflections [I > 2sigma(I)]. The redgillite structure consists of Jahn-Teller distorted CuO_6 octahedra and SO_4 tetrahedra. The octahedra share edges to form sheets that are zig-zag in cross section. The SO_4 tetrahedra share an oxygen with the Cu layer and link the layers by hydrogen bonds to OH groups. The crystal structures of wroewolfeite, langite, posnjakite, spangolite and schulenbergite are similar to redgillite. They all contain edge sharing CuO_6 layers connected to SO_4 groups with the layers bridged via hydrogen bonds.