Omsite , ( Ni , Cu ) 2 Fe 3 + ( OH ) 6 [ Sb ( OH ) 6 ] , a new member of the cualstibite group from Oms , France

Omsite (IMA 2012-025) is a new mineral from the Correc d’en Llinassos, Oms, Pyrénées-Orientales Department, France. It occurs as bright yellow to amber yellow discoidal tablets, flattened on {001}, which form rosettes typically 50 100 mm in diameter. Omsite generally crystallizes on siderite without associated supergene minerals; it occurs less commonly with glaukosphaerite. Crystals have a vitreous to resinous lustre, and are transparent to translucent. Omsite is not fluorescent in either short-wave or long-wave ultraviolet light. It has an estimated hardness of 3 on the Mohs’ scale, is brittle with an irregular fracture, and has one poor cleavage on {001}. The calculated density is 3.378 g cm . Crystals are uniaxial ( ), with indices of refraction of o = 1.728(3) and e = 1.66(1), measured in white light. Pleochroism is o = orange-yellow, e = pale orange-yellow; o > e. The empirical formula [based on 12 (OH + Cl) p.f.u.] is (Ni 1.099Cu 2+ 0.665Mg0.107Fe 3+ 0.045)S1.916Fe 3+ 1.000(Sb 5+ 0.947As0.072Na0.029)S1.048 OH11.967Cl0.033. Omsite crystallizes in space group P3̄, with unit-cell parameters a = 5.3506(8), c = 19.5802(15) Å, V = 485.46(10) Å and Z = 2 determined by single crystal X-ray diffraction. The five strongest lines in the X-ray powder diffraction pattern [d in Å, (Irel), (hkl)] are as follows: 4.901, (100), (004); 4.575, (83), (011); 2.3539, (81), (114̄); 1.8079, (48), (118̄); 3.781, (34), (103). The crystal structure was solved to R1 = 0.0896 for 356 observed reflections [Fo>4sFo] and 0.1018 for all the 469 unique reflections. Omsite is a layered double hydroxide (LDH) mineral, with a topology consistent with members of the hydrotalcite supergroup and cualstibite group.