- journal_title：Mineralogical Magazine
- Contributor：S. J. Mills ; A. R. Kampf ; R. M. Housley ; G. Favreau ; M. Pasero ; C. Biagioni ; S. Merlino ; C. Berbain ; P. Orlandi
- Publisher：Mineralogical Society of Great Britain and Ireland
- Date：2012-10-01
- Format：text/html
- Language：en
- Identifier：10.1180/minmag.2012.076.5.16
- journal_abbrev：Mineralogical Magazine
- issn：0026-461X
- volume：76
- issue：5
- firstpage：1347
- section：CNMNC Newsletter 14
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 [based on 12 (OH + Cl) p.f.u.] is (class="inline-formula" id="inline-formula-1">class="math mml" alt="Formula" src="1347/embed/mml-math-1.gif" /> Mg0.107class="inline-formula" id="inline-formula-2">
class="math mml" alt="Formula" src="1347/embed/mml-math-2.gif" /> )Σ1.916class="inline-formula" id="inline-formula-3">
class="math mml" alt="Formula" src="1347/embed/mml-math-3.gif" /> (class="inline-formula" id="inline-formula-4">
class="math mml" alt="Formula" src="1347/embed/mml-math-4.gif" /> As0.072Na0.029)Σ1.048OH11.967Cl0.033. Omsite crystallizes in space group P3İ, 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 [d in Å, (Irel), (hkl)] 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 [Fo>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.