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Ranciéite crystals from Friesach, Carinthia, Austria

¹ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, A-1090 Wien, Austria
² Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560-0119, U.S.A.
³ School of Earth and Environmental Science, Seoul National University, Seoul 151-742, Korea
⁴ Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, A-1014 Wien, Austria
⁵ Geologische Bundesanstalt, Rasumovskigasse 23, A-1030 Wien, Austria

^* Corresponding author, e-mail: andreas.ertl{at}al.net

Ranciéite from Friesach, Carinthia, Austria, is described from an oxidation zone of a small (Mn-bearing) siderite deposit. This mineral is associated with "buserite-(Ca)", hollandite, braunite, romanechite, todorokite, cryptomelane, pyrolusite, manganite, nsutite, goethite, lepidocrocite, and calcite. Ranciéite rarely forms small purple-silver grey crystals with a bright metallic luster. Single crystals are up to 200 µm in length and 0.1 µm in thickness and exhibit well-formed crystal faces in some cases. The foliated habit of these crystals may be due to rapid growth on the plane of fissure. For the first time a relatively large number of X-ray powder diffraction lines (19 lines) of ranciéite are described. Structure modeling, by the Rietveld method and with difference-Fourier summations shows, that ranciéite is most probably trigonal in structure, in contrast to the monoclinic structure of birnessite. The layers of the (Mn⁴⁺ ,Mn³⁺)O₆ octahedra, arranged parallel to (00.1), are intercalated predominantly by Ca atoms and water molecules, which are in a statistic and disordered arrangement. The powder pattern of this ranciéite was indexed on a trigonal cell, space group P [147], with a = 2.845(1) and c = 7.485(1) Å, a:c = 1: 2.632, Z = 1. The unit formula can be written as (Ca_0.85Mn²⁺_0.06K_0.05Mg_0.02Fe³⁺_0.01Ba_0.01) (Mn⁴⁺_3.60Mn³⁺_0.40)O₉ · 2.8 H₂O (calculated on the basis of O = 9; water and the valence states of Mn were calculated), or as Ca_0.19Mn²⁺_0.01K_0.01(Mn⁴⁺_0.80Mn³⁺_0.090.11)O₂ · 0.6 H₂O (calculated on the basis of O = 2).

Key-words: ranciéite, birnessite-group, phyllomanganate, X-ray powder data, structure proposal.