Structure versus composition: a comparative study across scales

Mollusk shells are composed of biominerals. While their mineral polymorphs are limited, their organic components number in the hundreds, if not thousands. Identifying these individual components is only the first step; understanding how they interact to form a shell remains an ongoing challenge. Infrared (IR) spectroscopy is a powerful technique for analyzing the structure and composition of these components while preserving their topographic relationships. This study employed three scales of observation and three samples: Concholepas, Pinctada, and cultivated pearls. Previously available data on their microstructure and compositions were utilized to explore potential correlations with results obtained from various techniques. IR analysis, being non-destructive, facilitates subsequent comparisons with other analytical methods such as Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Absorption Near Edge Structure (XANES), thanks to the precise localization of IR data. The findings reveal that data from earlier non-IR analyses align with results from new IR techniques, including Diffuse Reflectance Infrared Fourier Transform (DRIFT), Optical Photothermal Infrared Spectroscopy (O-PTIR), and Scattering-Type Scanning Near-Field Optical Microscopy (sSNOM). This concordance validates the application of these new IR methods for studying biogenic calcium carbonate. Furthermore, the high spatial resolution of O-PTIR and sSNOM enables detailed visualization of structural and compositional features. For instance, the techniques reveal the intricate inner structure of three-month-old pearls, the distribution of proteins, lipids, and sulphated sugars in Concholepas, and the nanoscale differences in the arrangement of nacre and prisms in Pinctada.