Crystallization of Manganese Silicates in the Na₂O—MnO—SiO₂—H₂O System with MnO/SiO₂ > 1

Abstract

Excess of manganese oxide over silica in a complex silicate system causes certain difficulties in research on the formation of the phases because manganese hydroxides and phases of indefinite composition of the braunite type are produced. This is characteristic of other silicate systems such as those form by In, the rare earths, and (especially) Ga. The hydroxides are produced in the hydrothermal reaction by hydration of the Me_XO_y component, which readily goes over to a sparingly soluble compound of the form Me_x(OH)_y under hydrothermal conditions, whereas the silica remains in the solution or is deposited as quartz. The rate and extent of the hydration can vary widely, so the system may produce phases of indefinite composition of the type of braunite (the Mn system) or tobermorite (the Ca system). These secondary effects can be suppressed to some extent by using ready-made crystalline silicates such as MnSiO₃ (chemically pure reagent grade), Mn₂SiO₄ (synthetic), and MnO/Si0₂ sinters of ratio 2–2.5, in contrast to the use of mixed oxides [1]. However, it has not proved possible to suppress the production of manganese hydroxides and braunite completely.