Phase transformation of calcium oxalate dihydrate-monohydrate: effects of relative humidity and new spectroscopic data.

•We study the vibrational properties of monohydrate and dihydrate phases of calcium oxalate.•We investigate the phase transformation of calcium oxalate dihydrate.•We report a re-investigation of the vibrational spectra (Raman and IR) of whewellite.•X-ray diffraction show the influence of relative humidity on the stability of weddellite.•The new data achieved will provide essential information in medical and Cultural Heritage fields.