Aragonite

Aragonite is calcium carbonate (CaCO₃), with the same chemical formula as calcite, but its carbonate ions are packed differently. That is, aragonite and calcite are polymorphs of calcium carbonate. It is harder than calcite (3.5 to 4, rather than 3, on the Mohs scale) and somewhat denser, but like calcite it responds to weak acid by vigorous bubbling. You may pronounce it a-RAG-onite or AR-agonite, though the majority of American geologists use the first pronunciation. It is named for Aragon, in Spain, where notable crystals occur.

Aragonite occurs in two distinct places. This crystal cluster is from a pocket in a Moroccan lava bed, where it formed at high pressure and relatively low temperature. Likewise, aragonite occurs in greenstone during the metamorphism of deep-sea basaltic rocks. At surface conditions, aragonite is actually metastable, and heating it to 400°C will make it revert to calcite. The other point of interest about these crystals is that they are multiple twins that make these pseudo-hexagons. Single aragonite crystals are shaped more like tablets or prisms.

The second major occurrence of aragonite is in the carbonate shells of sea life. Chemical conditions in seawater, notably the concentration of magnesium, favor aragonite over calcite in seashells, but that changes over geologic time. Whereas today we have "aragonite seas," the Cretaceous Period was an extreme "calcite sea" in which the calcite shells of plankton formed thick deposits of chalk. This subject is of great interest to many specialists.

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