A RICH variety of elaborate microscopic skeletal structures composed of inorganic materials are produced in nature(1). Such complex, three-dimensional structures, if produced synthetically, could find important applications as light-weight ceramics, catalyst supports, biomedical implants and robust membranes for high-temperature separation technology. Here we describe a method for synthesizing hollow porous shells of crystalline calcium carbonate (aragonite) that resemble the coccospheres of certain marine algae. We show that thin cellular frameworks of either mesoporous or macroporous aragonite can be formed from oil-water-surfactant microemulsions supersaturated with calcium bicarbonate, with the pore size determined by the relative concentrations of water and oil. Using micrometre-sized polystyrene beads as the substrate for the microemulsion, hollow spherical shells of the honeycomb architecture can be produced. We propose that these cellular frameworks originate from rapid mineralization of aragonite, with a self-organized foam of oil droplets acting as a structural template, and suggest that similar processes could be of general importance in materials chemistry.