Other Group IIA carbonates

The decomposition of MgC03 (magnesite) is an interface process [734] between 813—873 K and E = 150 kJ mole-1. In the presence of C02, E was increased to 234 kJ mole-1 but was reduced slightly on the addition of ZnO or NiO. Admixture with CaO reduced the value of E to 54 kJ mole-1. This is a surprising result since the value of E for decomposition [734,753] of the mixed carbonate (Ca, Mg)C03, dolomite, is 220 kJ mole-1, larger than the value for each constituent. The influence of PCOz and of alkali metals on MgC03 decomposition has been the subject of a DTA study [404]. 

Basu and Searcy [736] have applied the torsion—effusion and torsion— Langmuir techniques, referred to above for calcite decomposition [121], to the comparable reaction of BaC03, which had not been studied previously. The reaction rate at the (001) faces of single crystals was constant up to a product layer thickness of 1 mm. The magnitude of E (225.9 kJ mole-1) was appreciably less than the enthalpy of the reaction (252.1 kJ mole-1). This observation, unique for carbonates, led to the conclusion that the slowest step in BaC03 vacuum decomposition at 1160—1210 K is diffusion of one of the reaction components in a condensed phase or a surface reaction of C02 prior to desorption. 

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