What do greenhouse gases, pearls, antacids, lasers, cement and sea shells have in common? They are all related to calcium carbonate phases. Calcium carbonate is one of the most commonly occurring natural minerals. It can be observed in three different forms, calcite, aragonite and vaterite. Figure 1 shows the calcite and aragonite forms. These forms share the exact same chemical formula, but have a slightly different crystal structure and are known as polymorphs.
Polymorphs have different energies of activation, dissolution rates and degrees of reactivity. Therefore it is critical to ensure the correct polymorph is present for effective functionality. These polymorphs can easily be detected through X-ray diffraction with the MiniFlex diffractometer, as seen in Figure 2.
Aragonite, the orthorhombic polymorph, is commonly found in mollusks and the pearly lining of sea shells and certain corals. Pearls are composed almost entirely of aragonite. So how does this relate to greenhouse gases? Coral and other sea creatures absorb carbon dioxide from the sea water to produce the aragonite component of their structures. This allows for more carbon dioxide gases from the atmosphere to be dissolved in the sea water. This cycle is perceived to be one natures way of controlling greenhouse gases. Aragonite, however is not the stable polymorph of calcium carbonate. If aragonite is heated to 400° Celsius, it will spontaneously convert to calcite. Calcite, the hexagonal polymorph, is found in cements, and antacids and functions as a mild pH buffer. Optically clear calcite, a natural mineral called Iceland spar, is used in lasers and polarizing devices. Since the natural supply is dwindling, synthetic production of optically clear calcite is in high demand. Therefore, the need to ensure polymorph purity is also of significant concern.