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

Neotocite, an X-ray amorphous hydrous manganese silicate, is the mineral typical of low-grade metamorphosed sedimentary manganese ores of the Ushkatyn-III deposit in Central Kazakhstan. It occurs both in the ore groundmass, where it associates with hausmannite, tephroite, caryopilite, friedelite, pennantite, rhodochrosite, kutnohorite, and some other minerals, as well as in veinlets crossing these ores. In the veinlets, segregations of homogeneous neotocite reach several cubic centimeters in size. The study of such large segregations allows us to characterize optical, mechanical and thermal properties, IR spectra, and chemical composition of neotocite using modern analytical methods. It is assumed that manganese in the neotocite is mainly (or even completely) divalent. The stoichiometry of the mineral with allowance for the chemical and thermal analyses and IR spectroscopic data corresponds to the ideal formula Mn7(Si8O20)(OH)6⋅nH2O. This formula reliably reflects the silicon to manganese ratio in the mineral, its layered crystal structure, and at least two different hydrogen speciations: (OH)-groups and H2O molecules. The above-given formula of neotocite could be accepted as the idealized one. Neotocite was formed at low temperature reducing conditions during the burial of metal-bearing deposits containing a Mn–Si–H2O substance (gel?), or later through the hydrothermal alteration of already formed manganese ores.