Montmorillonite Pseudomorphs after Amphibole from Melbourne, Australia

Well developed smectite “crystals” up to 10 mm long, which probably are pseudomorphic after titaniferous hornblende phenocrysts, are present in a weathered dike in a Melbourne clay pit. The pseudomorphs contain 77 per cent of montmorillonite as determined by X-ray powder diffraction, differential thermal analysis, thermo-gravimetric and chemical analysis. The pseudo-hexagonal shape of the pseuodomorphs probably reflects the original crystal habit of the hornblende. The mechanism of alteration of an amphibole to a smectite is discussed.