Carbonates and the Ocean

Abstract

Carbonate minerals include those minerals composed of CO ⁼₃ and one or more cations. The various cations that are usually incorporated in carbonates are listed in Table 1. Most carbonate minerals are either rhombohedral or orthorhombic in

crystal habit. Rhombohedral carbonate minerals have six-fold coordination (2 cations for every oxygen), while orthorhombic minerals have nine-fold coordination (3 cations for every oxygen). Smaller cations, such as Mg, Fe, Mn, Zn and Cu, are energetically favored in six-fold coordination, while larger cations, such as Sr, Pb and Ba, are favored in the larger orthorhombic structure. Calcium occupies a unique position in that its ionic radius (0.99 Å) is intermediate between the small and large cations, and as such can form either rhombohedral (calcite) or orthorhombic (aragonite) carbonates. Aragonite is the high pressure polymorph of calcium carbonate (Jamieson 1953; MacDonald 1956; Crawford and Fyfe (1964) (Fig. 1) and at present earth-surface conditions is metastable. Calcium also can form a highly unstable spherulitic hexagonal polymorph, vaterite, sometimes referred to as μ-CaC0₃ (Johnston and others, 1916); this polymorph rarely occurs in nature. Other common calcite-type rhombohedral carbonates include magnesite (MgCO₃), rhodochrosite (MnC0₃) and siderite (FeCO₃). Aragonite-type orthorhombic carbonates include strontianite (SrC0₃), witherite (BaCO₃) and cerrusite (PbCO₃).