The process and the formation of new minerals upon heating the carbonate rocks containing clay minerals, together with calcite are determined with thermal analysis, X-ray diffraction, infrared and Raman spectroscopy. The calcite-calcium oxide phase transition sequence was followed up to 947 degrees C in naturally occurring limestone samples. The spectral variations of the internal modes of the carbonate trigonal (nu(1), nu(2), nu(3) and nu(4)) were used to probe the structural phase transitions. The calcium oxide phase (which on reaction with atmospheric water forms portlandite) with an onset temperature of around 950 degrees C was also characterized by the appearance of the infrared mode around 450 cm(-1). The minerals, which were formed upon heating the calcite, were calcium oxide and wollastonite.