Thermal properties of tridymite: 25°C–300°C

Abstract

Tridymite has been produced trom a variety of fluxes. Most samples gave two first-order inversions at ,-~ 117 and ~ 163 ~ Calorimetric study shows that the heats of inversion exhibit considerable variation, and are thus not constants of tridymite. These differences are related to the polytypism and disorder. The 117 ~ inversion occurs with considerable hysteresis: a method for determining equilibrium transition temperatures from dynamic data is presented. Some of the tridymite samples also exhibited a second-order inversion in the range 230--250 ~ Preparation of tridymiteQuartz and silicic acid were used as starting materials. The quartz was supplied by Thermal Syndicate Ltd., and is crushed natural crystal selected for the manufacture of 'Specpure' silica glass. The silicic acid was supplied by Johns-Manville Co.: our analysis gave 84.70% SiO2 the balance being almost entirely water, with only 0.02% material non-volatile with HF. The tridymite phase of silica cannot be formed directly from either of these starting materials. Fenner [1 ] found that sodium tungstate was an efficient flux for the interconversion of the low density silica modifications; tridymite formed in the range 870 ~ 1470 ~ Sodium tungstate was used in the present study: this and other fluxes were obtained as reagent-grade chemicals. The general procedure used in producing tridymite was to make a mechanical mixture of silica and an appropriate quantity of flux and to heat the mixture, contained in a platinum crucible, for the desired length of time in an electrically heated muffle. At the conclusion of a run the outside of the crucible was water-quenched to facilitate removing the sample. Conversion of quartz or cristobalite (the latter is formed rapidly from silicic acid) to tridymite was judged by petrographic examination and X-ray powder diffraction. The tridymitization is slow; runs of several hundred hours' duration may be required, even at 1300-1400 ~ to ensure complete conversion. The tungstate-fluxed samples were extracted by refluxing with 5 % ammonium chloride solution, followed by two washings with 2 % NaOH solution and finally with a double wash of 1% HCI solution and distilled water. This sequence of washings prevents precipitation of tungstosilicic acids and yields a very pure preparation of the appropriate silica Y. Thermal Anal. 2, 1970

A study of the thermal behavior of terrestrial tridymite by continuous X-ray diffraction

“…These results lend further support to discussions by other authors of the dependence of tridymite phase transitions on the degree of stacking disorder (e.g. Floerke 1955Floerke , 1967Eitel 1957;Sosman 1965;Shahid and Glasser 1970;Floerke and Mueller-Vonmoos 1971). In addition to stacking disorder, the type and complexity of twinning must also play an important role in transition behavior.…”

“…The major transition, between 159-165 ~ C, clearly represents one of the most well-documented phase transitions for tridymite, first reported by Fenner (1913). The transition hysteresis observed on cooling has been noted previously (Shahid and Glasser 1970;Nukui etal. 1978).…”

“…1978). Figure 13 shows additional changes occurring between 217-245 ~ C. Shahid and Glasser (1970) reported a transition for a synthetic crystal between 230-250 ~ C. Sato (1964) and Mosesman and Pitzer (1941) reported transitions at 250 and 275 ~ C, respectively. There are additional minor changes occurring between 302-309, 355-365, 372-390, and 440M50 ~ C respectively (Fig.…”

“…A number of minor effects were noted between 100-144 ~ C. Floerke (1955), Kihara (1978), Dollase (1967), Nukui et al (1978) reported transitions at 100, 105, 107, and 110 ~ C, respectively. Hill and Roy (1958), Shahid and Glasser (1970), and Wennemer and Thompson (1984b) all reported a transition at 113 ~ C. Many workers have reported a transition at 117~ (e.g. Fenner 1913; Mosesman and Pitzer 1941; Thompson and Wennemer 1979).…”

“…Dollase 1967;meteoritic 10. Shahid and Glasser 1970;synthetic 11. Shahid and Glasser 1970;terrestrial 12.…”