Figure 4: Raman spectroscopy of the outer regions of the spike cuticle. (A) BSE-SEM image highlighting the locations of the Raman spectra and mapping (scale bar: 10µm). (B) Raman spectra obtained in different locations across a straight line from the heavily mineralized outer surface to the striated region. Three different peaks corresponding to amorphous calcium phosphate (ACP, 955cm-1), fluorapatite (FAP, 965 cm-1) and amorphous calcium carbonate (ACC, 1080 cm-1) are emphasized. (C-G) Raman mapping of (C) phosphate intensity, (D) ACC/ACP ratio, (E) phosphate peak position and (F) full width at half maximum (FWHM), (G) polarization ratio on the phosphate peak (I-0° / I-90°). (H) Schematic representation of the main compositional features of the spike (the underlying predominant fibers arrangement is also reported).