Structural effects in the chemical reactivity of silica and silicates∗

The attack of quartz by hydrofluoric acid proceeds at a rate approximately a hundredfold greater on the optic axial plane than on the prism faces. The rate of attack on coesite is nil. Mica and certain other silicates are resistant to hydrofluoric acid attack on certain crystal faces. A reaction mechanism is proposed to account for these facts. This mechanism predicts that in a perfect crystal, only those crystal faces which expose simultaneously two of the four oxygen atoms of a given silica tetrahedron are attacked by hydrofluoric acid. Coesite is a special case, in that the close-packing which is assumed to exist provides double the normal number of tetrahedral sites. Surface silicon atoms may occupy some of these vacant sites, with the result that no unsaturated oxygen atoms are exposed at the surface.