Catalysed phase transformations of silica were studied at 1400°C employing DTA method for determination of the crystalline forms of silica. The consecutive transformation of quartz to tridymite proceeded through metastable cristo- balite and intervening transition phases. The decomposition of quartz in presence of mineralisers that produced liquid phases proceeded with uniform radial velocity, while with those acting in the solid state, the decomposition velocity progressively decreased with time, and the conversion kinetics were in approximate agreement with a phase boundary-controlled and a diffusion-controlled process respectively. The dependence of cristobalite formation rate on the transition phase concentration suggested that development of cristobalite occurred at the nucleation centres distributed inside the whole volume of the transition phase. © 1976, Taylor & Francis Group, LLC.