Organisms that live on and near the surface of the Earth affect the cycling of sulfur and metals and thus the formation and decomposition of sulfide minerals. Biological mediation of mineral formation can take many forms. Some organisms have evolved to synthesize minerals that are used for a particular function, such as structural support, protection against predators, hardening, or magnetic sensing. In these cases, the organism exerts strict control over the properties and the location of the mineral. The process by which such minerals form is termed biologically controlled mineralization (BCM) (Lowenstam and Weiner 1989).