Polarization dependence of aragonite calcium L-edge XANES spectrum indicates c and b axes orientation

Calcium carbonate minerals are frequently found in biomineral structures and are the predominant mineral in invertebrates. While there are several calcium carbonate polymorphs, aragonite and calcite are the two most commonly found in biogenic systems. Currently, calcium L-edge X-ray absorption near-edge structure (XANES) spectra are used to distinguish between different calcium carbonate polymorphs, including calcite and aragonite, while oxygen and carbon K-edge XANES spectra are often used to determine the c axis orientation of a given calcium carbonate crystal. By doing a full analysis of the calcite and aragonite calcium L-edge XANES spectrum for both geologic and biogenic systems, we were able to show that aragonite has a polarization-dependent peak while calcite does not. Analysis based on both multiplet models and density functional calculations show how the polarization dependence arises from directional bonds between the calcium and oxygen atoms within aragonite. These data not only enable an interpretation of the aragonite calcium L-edge XANES spectrum but also the ability to determine the orientation of the c and b axes of aragonite crystals within a biomineral sample.