Silicifications, Flint and Growth of Crystals

Abstract

It may seem surprising to study siliceous rocks and silicifications in a treatise on clays. In fact, I hope that the reading of this chapter will show how necessary it is. Here are the principal reasons. Since clay minerals are silicates, the architecture and development of their lattice are chiefly controlled by the assemblage of oxygen tetrahedra with a central silicon ion. Now this fundamental elemental structure is that of crystallized silica in its three forms: quartz, cristobalite and tridymite. Many rocks of pedogenetic or sedimentary origin are siliceous, that is, formed chiefly of silica tetrahedra. It is obvious that these siliceous formations formed at low temperature, under conditions of the hydrosphere which are the conditions of the formation of clays; moreover, they are frequently closely associated with the latter. Thus, looking at clays and siliceous formations on a structural scale, one sees the similarity in their natures. In clays, the tetrahedra are arranged in a two-dimensional planar layer; in siliceous formations, they are arranged spatially in a three-dimensional lattice. This is the fundamental difference for the specialist in crystalline structures, but for the common people, is it not only a difference between two types of occurrence of silica? This chapter is designed to show what this difference represents for the geologist.