High-temperature phase transitions in Steinbach tridymite

Tridymite from the Steinbach meteorite has been studied by high- resolution XRD, DSC and hard-mode IR spectroscopy. Phase transitions at 350 and 465 degrees C have been analysed in detail; the 465 degrees C transition is second-order in character and the coefficients in a Landau potential are: G = 1.2(T-738)Q (super 2) + 443Q (super 4) J/mol. This phase change appears to be driven by a volume decrease and the excess free energy is relatively small. The 350 degrees C transition also appears to be continuous and could be close to second-order in character; the excess free energy is again small. Two mechanisms are presented to account for the thermal behaviour; a dynamic model involving the 'flipping' of apical oxygen atoms and a purely displacive model involving shifts in atomic positions and the appearance of an intermediate phase. The first is consistent with the XRD results but does not accord with the IR measurements, while the second satisfies the IR data but not the XRD observations.