Hydrous CaCO ₃ gets a new structure: Calcium carbonate (CaCO ₃ ) forms important minerals on Earth and is a model system for understanding crystal nucleation. Three different structures of CaCO ₃ are known, along with two structures that are hydrated. Zou et al. found a third hydrated CaCO ₃ structure formed from amorphous CaCO ₃ in the presence of magnesium ions. The discovery illustrates the importance of amorphous precursors for producing new materials. Science , this issue p. 396

Double Vision: Vaterite is the least stable form of anhydrous crystalline calcium carbonate. While rarely found in geological contexts, it is an important biological precursor and occurs as a minor component in the shells of some organisms. The crystal structure of vaterite has long been debated with no model able to explain all the experimentally observed diffraction spots. Kabalah-Amitai et al. (p. 454 ) show that vaterite contains two coexisting crystallographic structures that form a pseudo-single crystal.

Many roads to being tough: A number of routes exist to increase toughness in both natural and human-made materials—for example, using secondary phases and precipitates or exploiting tailored architectures and shaped crystals. Polishchuk et al. detail the nanoscale internal structure of calcitic microlenses formed by a brittlestar (see the Perspective by Duffy). The segregation of magnesium-rich particles forms a secondary phase that places compressive stresses on the host matrix. This toughening mechanism resembles Guinier-Preston zones known in classical metallurgy. Science , this issue p. 1294 see also p. 1254