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IMA 2012-025 = omsite

Posted by Marco E. Ciriotti  
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Marco E. Ciriotti October 27, 2012 01:55PM
Reference:
▪ Mills, S.J., Kampf, A.R., Housley, R.M., Favreau, G., Pasero, M., Biagioni, C., Merlino, S., Berbain, C., Orlandi, P. (2012): Omsite, (Ni,Cu)2Fe3+(OH)6, a new member of the cualstibite group from Oms, France. Mineralogical Magazine, 76, 1347-1354.

Abstract:
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 μm 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−3. Crystals are uniaxial (−), with indices of refraction of ω = 1.728(3) and ε = 1.66(1), measured in white light. Pleochroism is ω = orange-yellow, ε = pale orange-yellow; ω > ε. The empirical formula is (Mg0.107)Σ1.916(As0.072Na0.029)Σ1.048OH11.967Cl0.033. Omsite crystallizes in space group P3, with unit-cell parameters a = 5.3506(8), c = 19.5802(15) Å, V = 485.46(10) Å3 and Z = 2 determined by single crystal X-ray diffraction. The five strongest lines in the X-ray powder diffraction pattern are as follows: 4.901, (100), (004); 4.575, (83), (011); 2.3539, (81), (114İ); 1.8079, (48), (118İ); 3.781, (34), (103). The crystal structure was solved to R1 = 0.0896 for 356 observed reflections 4σFo> 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.
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