Rancíeite crystals from Friesach, Carinthia, Austria

Authors: Ertl, Andreas; Pertlik, Franz; Prem, Markus; Post, Jeffrey E.; Kim, Soo Jin; Brandstätter, Franz; Alfschuster, R.

Source: European Journal of Mineralogy, Volume 17, Number 1, January 2005 , pp. 163-172(10)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

Rancíeite 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. Rancíeite 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 rancíeite are described. Structure modeling, by the Rietveld method and with difference-Fourier summations shows, that rancíeite 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 rancíeite was indexed on a trigonal cell, space group P3 [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·∼2.8H₂O (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⁺_0.09□_0.11)O₂·∼0.6H₂O (calculated on the basis of O = 2).

Keywords: RANCIEITE; BIRNESSITE-GROUP; PHYLLOMANGANATEX-RAY POWDER DATA; STRUCTURE PROPOSAL

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2005/0017-0163

Publication date: 2005-01-01

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