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Calcite of different particle sizes ranging from about 3 mu m to 4 mm in cross sectional distance was decomposed isothermally at 610 exp 0 C and 670 exp 0 C in vacuum with variation of sample amount, sample bed depth and packing density, all of which may influence the decomposition rate and the product solid properties. The effective initial flux of CO sub 2 from powder samples (approx. 3 to 20 mu m) and the true initial flux of CO sub 2 from larger single crystals (0.5 to 4 mm) and the true initial flux of CO sub 2 from larger single crystals (0.5 to 4 mm) were constant at (8.0 +- 1.0) x 10 exp -5 g/cm exp 2 sec and (2.3 +- 0.4) x 10 exp -6 g/cm exp 2 sec at 670 exp 0 C, respectively. While the decomposition rate of a powder sample was limited by a metastable equilibrium CO sub 2 pressure which gave a reaction gradient in the powder bed, an irreversible chemical step of decomposition was rate limiting for larger single crystals. The conventional kinetic model of contracting surface area was not applicable for calcite powder decomposition and was doubtful for larger crystal decomposition because the assumptions of the kinetic model did not accord with the calcite decomposition. 26 figures, 5 tables. (ERA citation 08:015605)