Putnisite, SrCa₄C^r3+₈ (CO₃)₈SO₄(OH)₁₆•25H₂O, a new mineral from Western Australia: description and crystal structure

Author(s)

Peter Elliott, Gerald Giester, Ralph Rowe, Allan Pring

Abstract

Putnisite, SrCa4Cr3+,8 (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 12.,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 a = 1.552(3), b = 1.583(3) and g = 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) A ° , c = 18.270(4) A ° , V = 5727(2) A ° 3 (single-crystal data), and Z = 4. The strongest five,lines in the X-ray powder diffraction pattern are [d(A° )(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, CO2calc 17.94, H2Ocalc 30.30, O=Cl 0.06, total 100.81. The empirical formula,,based on O + Cl = 69, is: Cr3+,8.02Ca3.78Sr1.08Na0.11Cu2+,0.07Mg0.04Ba0.02[(SO4)0.96(SiO4)0.03]0.99,(CO3)7.98(OH)16.19Cl0.15·24.84H2O. The crystal structure was determined from single-crystal X-ray,diffraction data (MoKa, CCD area detector and refined to R1 = 5.84% for 3181 reflections with F0 >,4sF. 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.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

University of Adelaide, Canadian Museum of Nature, Flinders University

Journal

Mineralogical Magazine

Volume

78

Pages

131-144

No. of pages

14

ISSN

0026-461X

DOI

https://doi.org/10.1180/minmag.2014.078.1.10

Publication date

02-2014

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 105113 Crystallography

Keywords

ASJC Scopus subject areas

Geochemistry and Petrology

Portal url

https://ucrisportal.univie.ac.at/en/publications/3bad93b2-4157-4200-ad52-d21261f2f8a0