Confinement generates single-crystal aragonite rods at room temperature

Significance Calcium carbonate is a widespread compound whose two common crystalline forms, calcite and aragonite, are important biominerals. Although aragonite is only marginally less stable than calcite under ambient conditions, it usually only crystallizes from solution at high temperatures or in the presence of magnesium ions. However, organisms readily form both calcite and aragonite biominerals, a capacity usually attributed to the action of specific organic macromolecules. By investigating calcium carbonate precipitation in submicron pores we here show that aragonite is promoted in confinement and that pure aragonite crystallizes in nanoscale pores in the absence of any additives. This is of great significance to biomineralization processes, which invariably occur in small volumes, and suggests that organisms may exploit confinement effects to control polymorph.