A mollusc shell, the xiphoid from cuttlefish was found to be a substrate favouring the deposition of aragonite crystals from stable supersaturated solutions at pH 8.50 and 25 degrees C. The crystallization was studied at constant solution composition, thus making it possible for a relatively large amount of the overgrowths to be formed and to be identified exclusively as aragonite crystals. The apparent order found from kinetics data was n = 4.1 +/- 0.4, thus suggesting a polynuclear mechanism. A surface energy of 24 +/- 3 mJ m(-2) was calculated for the growing phase and a four-ion cluster forming the critical nucleus, according to the classical nucleation theory.