Biological aragonites show various chemical compositions (Sr and Mg) related to the systematic position of the organisms. It is well known that Cnidaria tests have high Mg and Sr aragonites, whereas Mollusca shells have low Mg and Sr aragonites. Sponges have low Mg and high Sr aragonites. The linear correlation between Mg and Sr concentrations is positive. DRIFT spectra show that the wavenumber of a part of the doublet ν<sub>2</sub> significantly differs in the Mollusca shells and the Sponges and Cnidaria set: 863.7 cm<sup>-1</sup> in Mollusca, 858.8 cm<sup>-1</sup> in Sponges + Cnidaria. The other part of the doublet is similar in the two sets (844.1 cm<sup>-1</sup>). ν<sub>1</sub> and the doublet ν<sub>4</sub> do not seem to be altered. There is a strong linear correlation between the ν<sub>2</sub> wavenumber and the Mg and Sr concentrations in these biological aragonites. It is known that the calcite-aragonite transition apparently shifts the wavenumber of ν<sub>2</sub>, but such a shift has not been observed in one mineral.
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