Neotocite from Manganese Ores of the Ushkatyn-III Deposit, Central Kazakhstan

Neotocite, an X-ray amorphous hydrous manganese silicate is the mineral characteristic for low-grade metamorphosed sedimentary manganese ores of Ushkatyn-III deposit in Central Kazakhstan. It occurs both in the groundmass of ores, with hausmannite, tephroite, caryopilite, friedelite, pennantite, rhodochrosite, kutnogorite, and in veinlets crossing these ores. In veinlets, segregations of homogeneous neotocite reach several cubic centimeters. The study of such large segregations allows to characterize optical, mechanical and thermal properties, IR spectra, and chemical composition of neotocite using modern analytical methods. It is assumed that manganese in neotocite is mainly (or even completely) divalent. The stoichiometry of the mineral, taking into account chemical, thermal analyzes and IR spectroscopy data, corresponds to the ideal formula Mn₇(Si₈O₂₀)(OH)₆∙nH₂O. This formula reflects correctly the ratio of silicon and manganese in the mineral, its layered crystal structure and at least two different forms of H-bearing: in (OH)-groups and H₂O molecules. The above given formula of neotocite could be accepted as the idealized one. Neotocite was formed at low temperatures and in reducing conditions during the sedimentary burial of metal-bearing deposits containing a Mn–Si–H₂O substance (gel?), or later – due to hydrothermal alteration of already formed manganese ores.