The fundamental vibration spectra of aragonite and calcite in the 2- to 15-μ region of the infrared have been described in previous investigations. Recently run curves, however, cast doubt on the assignment of absorption bands in the 11- to 12-μ range. Investigation of the spectra of some 20 mineral specimens indicates that the band at 11.65 μ is specific for aragonite wheras the band at 11.41 μ is caused by calcite. Five of the specimens studied yield spectra with both bands, suggesting that natural aggregation of the two minerals is a rather common occurrence.
In artificial mixtures, the intensity ratio of the bands at 11.41 and 11.65 μ is found to be approximately proportional to the ratio of concentration of aragonite and calcite in a given sample. However, considering the range in absorption intensity inherent in different specimens, the method as now applied yields at best semiquantitative data accurate to within 10 percent (absolute) of the mineral content in the specimen. Spectrograms obtained on recent and fossil invertebrates suggest application of the method to the study of the composition of calcareous shells.