The Raman spectra of the three varieties of hydrated calcium carbonate, monohydrate, hexahydrate and 'amorphous,' are reported for the first time and discussed according to the already known crystal structures. The transformation by dehydration of these hydrated phases into the usual anhydrous forms was followed by in situ micro-Raman spectroscopy. It is shown that the dehydration of the hexahydrate proceeds through the formation of the 'amorphous' phase. The latter is better described as microcrystalline monohydrate.

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