Thermal transformations of natural calcium oxalate dihydrate known in mineralogy as weddellite have been undertaken using a combination of Raman microscopy and infrared emission spectroscopy. The vibrational spectroscopic data was complimented with high resolution thermogravimetric analysis combined with evolved gas mass spectrometry. TG-MS identified three mass loss steps at 114, 422 and 592 degreesC. In the first mass loss step water is evolved only, in the second and third steps carbon dioxide is evolved. The combination of Raman microscopy and a thermal stage clearly identifies the changes in the molecular structure with thermal treatment. Weddellite is the phase in the temperature range up to the pre-dehydration temperature of 97 degreesC. At this temperature, the phase formed is whewellite (calcium oxalate monohydrate) and above 114 degreesC the phase is the anhydrous calcium oxalate. Above 422 degreesC, calcium carbonate is formed. Infrared emission spectroscopy shows that this mineral decomposes at around 650 degreesC. Changes in the position and intensity of the C=O and C-C stretching vibrations in the Raman spectra indicate the temperature range at which these phase changes occur. (C) 2003 Elsevier Science B.V. All rights reserved.