Needle-like or branch-like aragonite was prepared from sparingly soluble magnesium carbonate and calcium chloride aqueous solution. The morphology, particle size, and aspect ratio (ratio of length to diameter of the particle) of the aragonite particles were characterized with SEM. The polymorphs and crystal size were characterized with XRD. Comparing with the reactants calcium sulfate and sodium carbonate, which were used to synthesize needle-like aragonite in our previous study, a high-reactant concentration region can be used when magnesium carbonate and calcium chloride aqueous solution are used as reactants. It was also found that low calcium chloride concentration is not favorable to the high aspect ratio of needle-like aragonite. Magnesium ions can increase aragonite fraction but slightly decrease aspect ratio. High synthesis temperature is favorable to both aragonite fraction and higher aspect ratio. High pH is beneficial to the formation of aragonite. Aging will result in a transformation of aragonite into calcite if calcite exists in the suspension. For an aragonite suspension without calcite and with Mg2+, aging will decrease the needle length and thus decrease the aspect ratio of aragonite particles. (C) 2004 Elsevier B.V. All rights reserved.