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Circular 82—Surface properties of silicate minerals

By R. A. Deju and R. B. Bhappu, 1965, 6 pp., 2 tables, 4 figs.

The basic structural unit of all silicate minerals is a tetrahedron with a silicon atom at the center and four oxygen atoms at the corners. The oxygen-silicon distance is about 1.6 Å and the oxygen-oxygen distance about 2.6 Å. The different types of oxygen-silicon frameworks in the various silicates are based entirely on the combinations of the tetrahedral oxygen-silicon groups through sharing of the oxygen atoms.

In recent years, infrared spectroscopy has permitted estimation of the ratio of the ionic character of the oxygen-silicon bond to be 2.3 times that of the oxygen-carbon bond. The ratio of electronegativity of the two bonds has substantiated this finding, since it amounts to 51% for the oxygen-silicon bond and only 22% for the oxygen-carbon bond; this means a ratio of 51:22, or 2.3:1. On this basis, it seems safe to assume that the oxygen-silicon bond is the strongest one occurring in silicate materials.

The composition and structures of the most important naturally occurring silicate minerals have been determined. A table in this report lists the accepted chemical compositions and some of the more important physical and optical properties of the silicates studied. As the oxygen-silicon ratio increases from quartz to the olivines, a greater percentage of the oxygen bonding power is available for bonding to cations other than silicon. Hence, with an increasing oxygen-silicon ratio, there is increasing oxygen-to-metal bonding. Upon the fracturing of a silicate mineral crystal, the oxygen-metal bond, which is almost entirely ionic in character, should break more easily that the stronger oxygen-silicon bond, resulting in a greater number of unsatisfied negative forces on the surface. If, then, the mineral is immersed in a liquid containing hydrogen ions, these negative forces should tend to be neutralized by hydrogen ions from solution, resulting in a change in pH of the solution. An increase in the degree of adsorption of hydrogen ions is to be expected as the oxygen-silicon ratio in the crystal structure increases.

This study attempted to correlate the oxygen-silicon ratio for various representative silicate minerals to the adsorption of hydrogen ions. Experiments to determine the effect of surface area on this adsorption and to investigate the effect of surface iron were also conducted. It is believed that such studies of the surface properties of silicate minerals will supply pertinent information about their behavior in froth flotation systems.

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