The polymorphism of silicon dioxide and the structure of tridymite

While the importance of some minerals arises from their scarcity, silica establishes its position by its abundance, accounting for about 12 per cent, of the entire lithosphere in the form of quartz alone (1), whilst up to 60 per cent, is attributed to SiO2 as a result of geochemical analysis (2). This fact alone should be of sufficient importance to warrant an extensive survey of the internal structures of the materials, but when one takes into account the industries in which the material is of primary importance, e. g., glass, clay, etc., and the various purposes to which silica in one form or another is applied, interest in its structures is heightened. A consideration in turn of each of the physical properties of quartz and fused silica especially recalls at once some particular application, sometimes limited, sometimes proving of wide scope. There are, for example, the extremely small coefficient of expansion of fused silica (3) (fig. 1), resulting in its ability to withstand sudden and large changes of temperature; its viscous properties, enabling fibres of great strength to be drawn (4, 5); its transparency to ultra-violet light (6), etc. In the case of quartz there are its comparatively high thermal conductivity, its high electrical resistivity (7), and its optical rotatory power, whilst lately its piezoelectric properties have received several new applications, notably in electromechanically coupled oscillatory systems devised as frequency standards and controls in the radio frequency range (8). Even its crystalline form has led to its use by Larsen and Wright (9) as a geological thermometer.