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Abstract
The P21/a to A2/a transition in titanite has been studied at high pressure (and room temperature) by X-ray powder diffraction. On the basis of the disappearance of k + l= odd reflections from the diffraction pattern, the transition was located between 3·351(3) and 3·587(3) GPa. The variation with pressure of the spontaneous strain in the P21/a phase indicates that the transition has an effective critical exponent significantly less than 1/2. Within the uncertainties of the data, the transition could be weakly first-order in character or be continuous with an effective critical exponent in the range of ∼0·13-0·25. The A2/a to P21/a transition is accompanied by a significant expansion of the a-axis as a result of the bond-valence sum requirements of the Ti atoms: in the high-pressure phase they occupy the centres of the TiO6 octahedra, but they are displaced along the a-axis in the P21/a phase to form alternating short and long Ti-O bonds.