Silica, the Three-dimensional Silicates, Borosilicates and Beryllium Silicates

Abstract

Strunz (1966) arranged 15 groups of minerals in a tectosilicate division of his system, describing about 72 of these three-dimensional structures in detail. The 10 minerals of the quartz group are listed in an oxide and hydroxide class, but in considering their vibrations and spectra these silica minerals are more naturally grouped with the tectosilicates. In a first comprehensive survey (Milkey, 1960), one can find IR spectra (1400–667 cm⁻¹) of 57 tectosilicates. Of the 473 spectra (1800–400 cm⁻¹) included in Moenke's Mineralspektren (1962, 1966), 40 are of tectosilicates. Lyon (1962a) refers to 93 tectosilicates in his bibliography, which includes 269 silicates in all. Thus the field, which includes the important sub-groups of felspars, zeolites and silica has been well surveyed, although the restricted range of older spectra (down to 670 cm⁻¹) limits their value. It has also been considered desirable to include the borosilicates and beryllium silicates in this chapter. Although some of these cannot be regarded as tectosilicates, they show some distinctive features which make it convenient to deal with them in one place. The strongest absorption bands of the tectosilicates lie in the 950-1200 cm region, and these can be characterized as antisymmetric Si-O-Si and Si-O-(AI) stretching vibrations. This range lies significantly higher than that found for nesosilicates (820-1000 crrr") but overlaps the range found for layer and chain structures. The second strongest region of absorption usually lies between 400-550 cm-I, and can be broadly characterized as O-Si-O bending