Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Pavel Povondra

Minerals of the hisingerite-neotocite series from Chvaletice, Železné hory Mts., eastern Bohemia, Czech Republic

Journal of the Czech Geological Society, volume 41 (1996), issue 1-2, 7 - 14

The hydrous ferri-manganese silicates from the Fe-Mn Chvaletice deposit correspond compositionally to [R₃Si₃O₁₀] . n H₂O, i.e. to the hisingerite-neotocite series. Both Fe and at least part of Mn in these mineral phases are trivalent, R : Si typically equals to unity, water content is highly variable and contents of magnesium, aluminium and alkali metals are low. Trace elements (e.g. Cu, Ni, Co, V) do not show any geochemical affinities to the major components. All physical parameters (optical properties, specific gravity, thermal properties, X-ray and IR absorption spectra) correspond well to those previously described for this mineral series. The minerals of the hisingerite - neotocite series have a gel-like structure (i.e. they are physically isotropic) but often contain anisotropic domains with incipient crystal structure. Most of the water present (up to 70 %) is free, probably occupying large open pores and it is released at temperatures less than 200 °C. The rest (30 % of water) is present in fine pores, probably bound via hydrogen bonds to the oxygen atoms of the co-ordination polyhedra and it is released at temperatures 200-400 °C. Minerals of the hisingerite-neotocite series in Chvaletice are likely to have originated through two different processes: (1) hypogeneous dissolution of primary Mn-silicates, e.g. knebelite and (2) reaction of cool silica-saturated solutions with solutions containing Fe and Mn. Product of such reaction - SiO₂ . FeO[OH] - has formed under slightly alkaline and oxidizing conditions.