19. Thermophysical Properties of Natural Glasses at the Extremes of the Thermal History Profile

Natural amorphous glassy silicates are widely distributed and are found in quantities that range from micrograms to kilo tonnes and, hence, their occurrence is from microscopic glassy inclusions to “glassy mountains” [1]. These natural glasses have two generic origins which may be generalised as vitreous glasses, formed from the melt state by relatively rapid cooling at cooling rates that inhibit crystal formation, or diagenetic glasses, formed by a dissolution-precipitation mechanism where crystallisation is inhibited by the Ostwald's rule of stepwise petrogenesis [2]. The thermal histories of a range of natural glasses are depicted in the schematic of Fig. 19.1 and vary significantly from the typical conditions used in the glass industry which are optimised between processing speed and energy conservation. In the extremes, tektites like moldavites are formed by extremely fast heating and melting at very high temperatures (> 3,000 K) followed by quenching at extreme cooling rates (≥10 K/s). By contrast the formation of amorphous glasses from mineral diagenesis or biotic processes occurs at much lower temperatures and over longer time periods; the formation of sedimentary opal, for example, occurs at ambient temperatures, it is essentially isothermal, and takes place over long periods of time of the order of months to years.