We studied pathological bioapatite from patients undergoing valvular replacement due to severe aortic and mitral stenosis. Three different types of mineralized human cardiac valves were analyzed. We used infrared and Raman spectroscopy to infer the presence of the carbonate group and evaluate the carbonate substitution in bioapatite structure. The Raman spectra showed that the pathological bioapatite is a B-type “carbonate-apatite” (CO<sub>3</sub><sup>2?</sup> for PO<sub>4</sub><sup>3?</sup>) similar to the major mineralized products derived from normal biomineralization processes occurring in the human body. Fourier transform infrared spectra (FT-IR) confirmed the B-type carbonate substitution (CO<sub>3</sub><sup>2?</sup> for PO<sub>4</sub><sup>3?</sup>) and showed evidence for the partial replacement of [OH] by [CO<sub>3</sub>] (A-type substitution). The carbonate content of the samples inferred by the spectroscopic measurements is in good agreement with the range of values estimated for biological apatite. On the contrary, the crystal size of the pathological apatite estimated using the percentage area of the component at 1059 cm<sup>?1</sup> of the infrared spectrum is in the nanometer range and it is significantly smaller than the crystal size of normal mineralized tissues.

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