The enthalpy of formation of sedimentary Ca-rich dolomite has been det
ermined by oxide melt calorimetry. The results show that Ca-rich dolom
ite is energetically different from well-ordered stoichiometric dolomi
te. The enthalpy of formation from calcite and magnesite becomes stron
gly more endothermic with increasing excess Ca content. This supports
the idea that the substitution of Ca in the Mg layer of dolomite is en
ergetically unfavorable. Ca-rich dolomite is unstable relative to well
-ordered stoichiometric dolomite and calcite. The enthalpy behavior fo
r excess Ca substitution in dolomite is different from that of Mg subs
titution in calcite; the enthalpy change is much larger in magnitude i
n dolomite and is more strongly dependent on composition. Differences
in cation order as well as the presence of a modulated structure and l
ow-symmetry domains in Ca-rich dolomite cannot be discerned from the e
nthalpy data. The findings confirm that the growth and occurrence of C
a-rich dolomite in sedimentary environments must be attributed to kine
tic factors rather than to equilibrium.