Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analyses of mineral and organic matrix during heating of mother of pearl (nacre) from the shell of the mollusc Pinctada maxima

Abstract

Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction patterns were used to analyze the mineral structure and organic matrix composition and thermal behavior of the internal nacreous layer (mother of pearl or nacre) of the shell of the giant oyster Pinctada maxima. Nacre is a natural biomaterial with osteogenic properties. The mineral of nacre is calcium carbonate crystallized as aragonite and it is highly crystallized. The FT-IR spectra showed amide, amine, and carboxylic acid groups in the organic matrix of the whole (organic and mineral) nacreous layer, with the HCO(-)(3) groups possibly at the organic-mineral interface. The insoluble organic matrix remaining after decalcification contained amide, amine, and carboxylic groups. The heated aragonite mineral structure of nacre underwent two transformations (X-ray diffraction), aragonite to calcite at 300-400 degrees C, and calcite to calcium oxide (CaO) at 500-600 degrees C. The organic matrix of nacre was destroyed around 550-600 degrees C, the same temperature as the calcite to CaO transformation, revealing the great thermal stability of the organic matrix and the organic-mineral bonding. This could be an useful feature for the in vivo use of this natural biomaterial as an implant.