It is pointed out that many of the thermal analysis techniques commonl
y utilized to measure the rates of thermal decomposition reactions are
plagued with problems of mass and energy transport limitations. Becau
se the technique employed here, dynamic X-ray diffraction (DXRD), allo
ws for in situ observations of the solid reactants, intermediates, and
products, and for better control of the heat and mass transfer resist
ance, most of these problems are obviated. Results are presented which
not only compare the dramatic differences in global reaction rates be
tween TGA and DXRD experiments, but also demonstrate the effects of sa
mple preparation on calcite decomposition. Specifically, it is shown t
hat sample cleaning, which tends to remove nucleation sites, as well a
s sample de-gassing, which probably removes water vapor, can lead to v
ery different calcite decomposition rates.