A pseudomorphous aggregate after titanite composed of smectite, anatase and residual titanite of composition (Ca0.98, Mn0.02)(Ti0.65, Al0.35)[SiO4](O0.65, OH0.35), from a depth of 450 m in the Broken Hill South Mine, New South Wales, Australia, was investigated by electron microscopy and microanalysis to characterize the alteration products and the mobility of Ti. Examination of the pseudomorph showed randomly oriented anatase crystals dispersed throughout a matrix of beidellite, with 9% porosity. Around the periphery and along the (110) cleavage plane of titanite, alteration was most developed. The range of Ti mobility was found to be limited to ~500 nm, and the ratio between the average diameter of anatase crystals and the average distance between them is ~1.3. This ratio is consistent with an alteration process in which Ti is conserved and the anatase crystals grow from the Ti available immediately around them. It is unlikely that Ti migrated beyond the titanite pseudomorph.