Rancieite crystals from Friesach, Carinthia, Austria

Rancieite 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. Rancieite 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 rancieite are described. Structure modeling, by the Rietveld method and with difference-Fourier summations shows, that rancieite is most probably trigonal in structure, in contrast to the monoclinic structure of birnessite. The layers of the (Mn 4+ ,Mn 3+ )O 6 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 rancieite was indexed on a trigonal cell, space group P 3 [147], with a = 2.845(1) and c = 7.485(1) A, a:c = 1: 2.632, Z = 1. The unit formula can be written as (Ca 0.85 Mn 2+ 0.06 K 0.05 Mg 0.02 Fe 3+ 0.01 Ba 0.01 ) (Mn 4+ 3.60 Mn 3+ 0.40 )O 9 ·∼ 2.8 H 2 O (calculated on the basis of O = 9; water and the valence states of Mn were calculated), or as Ca 0.19 Mn 2+ 0.01 K 0.01 (Mn 4+ 0.80 Mn 3+ 0.09□0.11 )O 2 ·∼ 0.6 H 2 O (calculated on the basis of O = 2).