Atomic order of aragonite crystals formed by mollusks

Aragonite is the most common polymorph of calcium carbonate produced by mollusks. The crystals have different shapes and sizes, and are intimately associated with an organic matrix. Here we adapt an infrared spectroscopic method that was developed to characterize atomic disorder in calcite crystals, in order to characterize aragonite crystals produced by mollusks, as well as geogenic and synthetic aragonite crystals. The method involves systematically changing the particle size distribution by repeated grinding of the sample. We show that large single geogenic aragonite crystals are well ordered compared to aragonite formed from boiling water, which in turn is better ordered than synthetic aragonite crystals produced in vitro in the presence of additives. The most ordered biogenic aragonite is from the crossed lamellar structure. Nacre and myostracum are less ordered and the large ligament crystals are even less ordered than nacre. These trends do not correspond to those observed from X-ray diffraction peak broadening, indicating that the two techniques reflect different types of ordering. The differences in atomic order observed by infrared spectroscopy between biogenic aragonite crystals and also between geogenic aragonite crystals presumably reflect different modes of crystal formation.