Putnisite, SrCa (sub 4) Cr (sub 8) (CO (sub 3) ) (sub 8) SO (sub 4) (OH) (sub 16) .25H (sub 2) O, a new mineral from Western Australia; description and crystal structure

Putnisite, SrCa (sub 4) Cr (super 3+) (sub 8) (CO (sub 3) ) (sub 8) SO (sub 4) (OH) (sub 16) .25H (sub 2) O, 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 1 1/2-2. The measured density is 2.20(3) g/cm (super 3) and the calculated density based on the empirical formula is 2.23 g/cm (super 3) . Optically, putnisite is biaxial negative, with alpha = 1.552(3), beta = 1.583(3) and gamma = 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) Aa, c = 18.270(4) Aa, V = 5727(2) Aa (super 3) (single-crystal data), and Z = 4. The strongest five lines in the X-ray powder diffraction pattern are [d(Aa)(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.%): Na (sub 2) O 0.17, MgO 0.08, CaO 10.81, SrO 5.72, BaO 0.12, CuO 0.29, Cr (sub 2) O (sub 3) 31.13, SO (sub 3) 3.95, SiO (sub 2) 0.08, Cl (super -) 0.28, CO (sub 2calc) 17.94, H (sub 2) O (sub calc) 30.30, O=Cl -0.06, total 100.81. The empirical formula, based on O + Cl = 69, is: Cr (super 3+) (sub 8.02) Ca (sub 3.78) Sr (sub 1.08) Na (sub 0.11) CU (super 2+) (sub 0.07) Mg (sub 0.04) Ba (sub 0.02) [(SO (sub 4) ) (sub 0.96) (SiO (sub 4) ) (sub 0.03) ] (sub 0.99) (CO (sub 3 ) ) (sub 7.98) (OH) (sub 16.19) Cl (sub 0.15) .24.84H (sub 2) O. The crystal structure was determined from single-crystal X-ray diffraction data (MoKalpha , CCD area detector and refined to R1 = 5.84% for 3181 reflections with F (sub 0) > 4sigma F. Cr(OH) (sub 4) O (sub 2) octahedra link by edge-sharing to form an eight-membered ring. A 10-coordinated Sr (super 2+) cation lies at the centre of each ring. The rings are decorated by CO (sub 3) triangles, each of which links by corner-sharing to two Cr(OH) (sub 4) O (sub 2) octahedra. Rings are linked by Ca(H (sub 2) O) (sub 4) O (sub 4) polyhedra to form a sheet parallel to (100). Adjacent sheets are joined along [100] by corner-sharing SO (sub 4) tetrahedra. H (sub 2) O molecules occupy channels that run along [100] and interstices between slabs. Moderate to weak hydrogen bonding provides additional linkage between slabs.