Vibrational spectroscopic characterization of growth bands in Porites coral from South China Sea

•We have studied the Raman spectra in different growth bands of Porites coral.•Raman spectra show vaterite could be the precursor of coral aragonitic skeleton.•The positional shift in the infrared spectra correlate significantly to minor elements.

A series of samples from different growth bands of Porites coral skeleton were studied using Raman, infrared reflectance methods. The Raman spectra proved that skeleton samples from different growth bands have the same mineral phase as aragonite, but a band at 133 cm−1 for the top layer shows a transition from ∼120 cm−1 for vaterite to ∼141 cm−1 for aragonite. It is inferred that the vaterite should be the precursor of aragonite of coral skeleton. The positional shift in the infrared spectra of the skeleton samples from growth bands correlate significantly to their minor elements (Li, Mg, Sr, Mn, Fe and U) contents. Mg, Sr and U especially have significant negative correlations with the positions of the antisymmetric stretching band ν3 at ∼1469 cm−1. And Li shows a high negative correlation with ν2 band (∼855 cm−1), while Sr and Mn show similar negative correlation with ν4 band (∼712 cm−1). And Mn also shows a negative correlation with ν1 band (∼1082 cm−1). A significantly negative correlation is observed for U with ν1 + ν4 band (∼1786 cm−1). However, Fe shows positive correlation with ν1, ν2, ν3, ν4 and ν1 + ν4 bands shifts, especially a significant correlation with ν1 band (∼1082 cm−1). New insights into the characteristics of coral at different growth bands of skeleton are given in present work.

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