Mineral Spotlight: Aragonite (and Calcite)
Aragonite (CaCo3) is one polymorph (common chemical formula, but not structure) of calcium carbonate - which is more commonly known in the form of calcite, an important mineral to sea life. But how does aragonite differ? What is its importance?
The primary difference between calcite and aragonite is their crystal class, which is based on the symmetry elements of the lattice structure. Aragonite belongs to the orthorhombic class - think of a rhombic prism, with one “two-fold axis” of rotation (if you rotate along this axis, every 180 degrees, you will see the same image) and two mirror planes (planes across which the image can be reflected). Calcite, contrarily, is trigonal, with one three-fold rotational axis (repeat every 120 degrees of rotation).
These differences in structure make a difference between the materials. First, aragonite (Mohs hardness of 3.5-4) is harder than calcite (Mohs hardness of 3). The minerals also vary in cleavage, fracture, and density, although in general they are very close. Additionally, aragonite is more soluble than calcite, which is very important when it comes to ocean chemistry, which we will get to.
The occurrence of the minerals also differs. Calcite these days is far more common than aragonite. It occurs in marble (metamorphic) and limestones (sedimentary), as well as at hydrothermal vents. Meanwhile, aragonite appears in caves, as deposits at geysers and hot springs, and ore deposits.
Probably the most important difference between calcite and aragonite on the global scale, however, is their relationship with ocean chemistry.
Currently, our seas are dominated by aragonite - but that was not always the case! Whether aragonite or calcite precipitates from solution (ocean water, really) depends on a variety of factors (Figure 3), but largely on magnesium content, as well as temperature/pressure. Higher magnesium content leads to dominantly aragonitic seas, while the reverse conditions lead to calcitic seas, because magnesium decreases the stability of calcite in solution, thus leading to more aragonite precipitation than calcite (Berner 1975).
Calcite seas occurred, for example, during from the Cretaceous to the Early/Middle Cenozoic and other times, while aragonitic seas occurred during the later Cenozoic (now) and late Paleozoic (and other times as well).
The organisms that thrive better with one or the other mineral also differ. Soft corals, echinoderms (sea stars, crinoids, etc), sponges, barnacles, red algae, and some mollusks, like oysters, use calcite to build shells, skeletons, and spicules. Aragonite is used by other mollusks (and even other mollusks use both aragonite and calcite!), green algae, “hard” corals, and certain plankton.
But what about oceans today? What is the state of aragonite? According to the EPA, rising atmospheric levels of CO2 (measured since the 1880s) has led to lower saturation, or higher dissolution, of aragonite - which, remember, is more soluble than calcite (from Fabry et al., 2008). Aragonite is currently being depleted.
Models suggest that rising CO2 levels also lead to lower dissolved inorganic carbon, from which aragonite forms. Furthermore, rising temperatures from rising CO2 levels leads to more dissolution of aragonite. Projections for the dissolution of aragonite-based organisms like corals are quite frightening - when CO2 levels reach 450-560 ppm from current levels of 390 ppm, reefs are expected to start to dissolve and be unable to calcify (Silverman et al., 2009; Cao and Caldeira, 2008).
(Photos: 1 | 2 | 3; info sources: 1 | 2 | 3 | 4 | 5 | 6 - 1 and 2 have neat interactive applets so you can rotate both the crystal system and the lattice!)