The Common Chain, Ribbon, and Ring Silicates

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Abstract
The minerals which form the subject of this work are the pyroxenes and pyroxenoids, the amphiboles, and beryl, cordierite and tourmaline. The less common chain, ribbon, and ring silicates, including all ring silicates with fewer than six tetrahedra in the ring, are omitted. Considerable emphasis has been placed on the interpretation of the fine structure of hydroxyl stretching bands in amphiboles, for these have yielded much useful information on the composition, structure and reactions of these minerals. By contrast, relatively little attention has been devoted to the vibrations of the silicate anions, or of the cation-oxygen polyhedra which link these anions. This selection of subjects reflects in part the existence of Lazarev's (1972) comprehensive work on silicate spectra and the absence of a comparable review of the hydroxyl spectra, and partly the relative paucity of mineralogical applications of the spectra of the complex anions. Sources of published spectra are listed in a supplementary bibliography to this chapter. Detailed descriptions of the crystal structures of the minerals considered here will be found in Deer, Howie and Zussman (1962). Taking bond strengths to be proportional to zlnr, where z is the formal charge on the cation, n its coordination number, and r the cation-oxygen distance, bond strengths in [Si04], [AI04], [AI06], [Mg06], and [NaOs] polyhedra in mineral structures are in the approximate ratios 12:9:5:3: 1. Thus the [Si04] groups which polymerize to form the complex chain, ring and ribbon anions present in minerals are joined to each other by bonds