Understanding the 'Heterophyllosilicates'

摘要

Silicate minerals comprise about 25 percent of the known minerals and 40 percent of the common ones. As constituents of granites, basalts, and other igneous rocks, they make up about 90 percent of the Earth's crust (Klein and Hurlbut 1985). To understand their structures and classify them into workable groups, it is first necessary to recognize the SiO_4 tetrahedron as a basic building block. Through a combination of extremely strong Si-0 bond energy and the relative sizes of the Si~(4+) and O~(2-) ions, the (SiO_4)~(4-) group is extremely stable and is present as a unit even in molten magma. In silicate crystals, the SiO_4 tetrahedra may be isolated from other SiO_4 groups, or they may be linked to one another by sharing one oxygen ion so the tetrahedra are joined at a corner (or very rarely, sharing two oxygens so the tetrahedra are joined along one edge). Linkages can be extended in different ways leading to infinite chains, sheets, or frameworks, and the relationship of these tetrahedra to the coordination polyhedra representing the oxygens surrounding the other cations in turn determines the great diversity of silicate mineral structures.