Mussels have great annual production in the world and their shells are discarded as a waste. Shells are calcium and carbon accumulators and, formed as a consequence of calcium carbonate biomineralisation. They can be used as bio‐based composite materials in different fields. Hence, assessment of the chemical compound content and microstructure of shells is important for the prospective utility. For these reasons, structure of Cerastoderma edule, Chamelea gallina, Donax trunculus, Ruditapes decussatus shells were analyzed with X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS). In addition, microhardness was measured in shells. Our results indicated that shells have a flake like structure with irregular grains appearance morphologically at micrometric scale. The fractions of aragonite in particular shells were assessed. EDS analysis showed that Ca, C and O were major elements of C. edule, C. gallina, D. trunculus, R. decussatus in accordance with XRD data. The lowest microhardness (2.68±0.16 GPa) was in D. trunculus while the highest microhardness (4.28±0.32 GPa) was in C. edule
)