Comparative nanoindentation study of biogenic and geological calcite

Biogenic minerals are often reported to be harder and tougher than their geological counterparts. However, quantitative comparison of their mechanical properties, particularly fracture toughness, is still limited. Here we provide a systematic comparison of geological and biogenic calcite (mollusk shell Atrina rigida prisms and Placuna placenta laths) through nanoindentation under both dry and 90% relative humidity conditions. Berkovich nanoindentation is used to reveal the mechanical anisotropy of geological calcite when loaded on different crystallographic planes, i.e., reduced modulus E_r{104} ≥ E_r{108} > E_r{001} and hardness H_{001} ≥ H_{104} ≥ H_{108}, and biogenic calcite has comparable modulus but increased hardness than geological calcite. Based on conical nanoindentation, we elucidate that plastic deformation is activated in geological calcite at the low-load regime (<20 mN), involving r{104} and f{012} dislocation slips as well as e{018} twinning, while cleavage fracture dominates under higher loads by cracking along {104} planes. In comparison, biogenic calcite tends to undergo fracture, while the intercrystalline organic interfaces contribute to damage confinement. In addition, increased humidity does not show a significant influence on the properties of geological calcite and the single-crystal A. rigida prisms, however, the laminate composite of P. placenta laths (layer thickness, ∼250-300 nm) exhibits increased toughness and decreased hardness and modulus. We believe the results of this study can provide a benchmark for future investigations on biominerals and bio-inspired materials.