The mineralogical characterization of argentian cryptomelane from Xiangguang Mn–Ag deposit, North China

Argentian cryptomelane as a quite rare variety is determined during the investigation of Mn–Ag ore samples from Xiangguang deposit along the northern margin of North China craton. The mineral observed by a polarizing petrographic microscope involves concentric ring–band, pisolitic and veinlet structures and greyish white color. The scanning electron microscopy reveals a large number of elongated nanocrystals in the forms of nanofibers and nanorods in this densely natural argentian cryptomelane. The specifically chemical features in two samples of XG–C–1 and XG–C–2 of cryptomelane are: (1) (K0.55Na0.08Ca0.06Zn0.04Ag0.03Pb0.02Mg0.01)0.79(Mn7.21Fe0.52Al0.09Si0.09)7.91O16•nH2O; (2) (K0.37Ca0.28Ag0.13Na0.07Mg0.07Zn0.06Cu0.02)1.00(Mn7.01Fe0.40Al0.39Si0.03Ti0.01Cr0.01)7.85O16• nH2O. The silver content ranges from about 0.22–3.15 wt%, which is much higher than that of other manganese oxides including ranciéite, chalcophanite and coronodite found in this deposit as well. Both of two argentian cryptomelane samples feature two main Raman scattering contributions at about 580 cm−1 and 630 cm−1, belonging to the Mn–O lattice vibrations within the MnO6 octahedral double chains, which can distinguish from other three manganese oxides. The Ag+ prefers to locate in the tunnel sites substituting K+ of cryptomelane due to its large radius and the same monovalent state with K+. Some chain–width disorders characterized by transmission electron microscopy are probably caused by these cation substitutions.