Putnisite, SrCa₄$Cr83+$(CO₃)₈SO₄(OH)₁₆·25H₂O, a new mineral from Western Australia: description and crystal structure

Putnisite, SrCa₄$Cr83+$(CO₃)₈SO₄(OH)₁₆·25H₂O, is a new mineral from the Polar Bear peninsula, Southern Lake Cowan, Western Australia, Australia. The mineral forms isolated pseudocubic crystals up to 0.5 mm in size in a matrix composed of quartz and a near amorphous Cr silicate. Putnisite is translucent, with a pink streak and vitreous lustre. It is brittle and shows one excellent and two good cleavages parallel to {100}, {010} and {001}. The fracture is uneven and the Mohs hardness 1½–2. The measured density is 2.20(3) g/cm³ and the calculated density based on the empirical formula is 2.23 g/cm³. Optically, putnisite is biaxial negative, with α = 1.552(3), β = 1.583(3) and γ = 1.599(3) (measured in white light). The optical orientation is uncertain and pleochroism is distinct: X pale bluish grey, Y pale purple, Z pale purple. Putnisite is orthorhombic, space group Pnma, with a = 15.351(3), b = 20.421(4) Å, c = 18.270(4) Å, V = 5727(2) ų (single-crystal data), and Z = 4. The strongest five lines in the X-ray powder diffraction pattern are [d(Å)(I)(hkl)]: 13.577 (100) (011), 7.659 (80) (200), 6.667 (43) (211), 5.084 (19) (222, 230), 3.689 (16) (411). Electron microprobe analysis (EMPA) gave (wt.%): Na₂O 0.17, MgO 0.08, CaO 10.81, SrO 5.72, BaO 0.12, CuO 0.29, Cr₂O₃ 31.13, SO₃ 3.95, SiO₂ 0.08, Cl⁻ 0.28, CO_2calc 17.94, H₂O_calc 30.30, O=Cl −0.06, total 100.81. The empirical formula, based on O + Cl = 69, is: $Cr8.023+$Ca_3.78Sr_1.08Na_0.11$Cu0.072+$Mg_0.04Ba_0.02[(SO₄)_0.96(SiO₄)_0.03]_0.99(CO₃)_7.98(OH)_16.19Cl_0.15·24.84H₂O. The crystal structure was determined from single-crystal X-ray diffraction data (MoKα, CCD area detector and refined to R1 = 5.84% for 3181 reflections with F₀ > 4σF. Cr(OH)₄O₂ octahedra link by edge-sharing to form an eight-membered ring. A 10-coordinated Sr²⁺ cation lies at the centre of each ring. The rings are decorated by CO₃ triangles, each of which links by corner-sharing to two Cr(OH)₄O₂ octahedra. Rings are linked by Ca(H₂O)₄O₄ polyhedra to form a sheet parallel to (100). Adjacent sheets are joined along [100] by corner-sharing SO₄ tetrahedra. H₂O molecules occupy channels that run along [100] and interstices between slabs. Moderate to weak hydrogen bonding provides additional linkage between slabs.