Cloudite, BaFe3+3(PO4)(SO4)(OH)6, a New Alunite-Supergroup Mineral from Iron Monarch, South Australia, Australia

Abstract Cloudite, BaFe3+3(PO4)(SO4)(OH)6, is a new member of the beudantite group of the alunite supergroup from the Iron Monarch quarry, Iron Knob, Middleback Range, Eyre Peninsula, South Australia, Australia. It was found in seams in a matrix of quartz and hematite in association with gorceixite, fluorapatite, and baryte. Cloudite forms orange tabular to equant crystals up to 40 mm across with forms {001}, {101}, {011}, and {012}. The streak is yellow and the luster is vitreous. The cleavage is excellent on {001}. Optically, cloudite crystals are uniaxial (+), with ω = 1.805(5) and ε = 1.845(5) (measured in white light). The empirical formula from electron microprobe analyses is (Ba0.94K0.05Ca0.03)Σ1.02(Fe2.81Al0.20)Σ3.01[(PO4)1.12(SO4)0.89](O5.96H6.04). Cloudite is trigonal with space group R3¯m and unit-cell parameters a = 7.2709(5), c = 17.2799(10) Å, V = 791.13(12) Å3, and Z = 3. The crystal structure was solved using single-crystal X-ray diffraction data and refined to R1 = 5.04% for 257 unique reflections with I > 2σI. The crystal structure has the same topology as that for alunite-supergroup minerals. It is characterized by (001) sheets of corner-sharing octahedra and tetrahedra that link along [001] via a [12]-coordinated large cation site.