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 3̄ , 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 (Ca0.85Mn2+0.06K0.05Mg0.02Fe3+0.01Ba0.01) (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 Ca0.19Mn2+0.01K0.01(Mn4+0.80Mn3+0.09□0.11)O2 ·∼ 0.6 H2O (calculated on the basis of O = 2).