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Abstract. The temperature dependence of the infrared active modes of meteoritic and synthetic tridymite have been investigated between 23 K and 1073 K in IR absorption and IR emission experiments. At room temperature both tridymite samples consist of a mixture of low temperature forms, in different proportions, due to the grinding. The sequence of phase transitions in Steinbach tridymite deduced from the IR data agrees well with recent X-ray and calorimetry studies using identical samples (Cellai et al. 1994). The previously suspected structural phase transition P6322.**.P63/mmc is confirmed by the disappearance of the 470 cm 1 mode and a temperature anomaly of the spectral shift of the 790 cm-1 mode. Changes in the infrared spectra of synthetic tridymite give a different sequence of phase transitions from those of the meteoritic sample, consistent with the structural phase transitions observed in a 29Si MAS NMR investigation using the same sample (Xiao et al. 1993).

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