Mixtures of powdered calcite and quartz were reacted for various lengths of time at a total pressure of 2000 bars, 650 °C, in an H2O–CO2 fluid. Extent of formation of wollastonite was determined by partial chemical analysis of run products. Experimental scatter precludes detailed analysis of the reaction mechanism.The data show that approximately 40% reaction takes place in the first 24 h, and that after this initial period the rate decreases. Examination of run products shows the calcite to be surrounded by radiating wollastonite crystals. This suggests that the wollastonite cover forms during the initial stages of reaction and, by shielding the calcite, suppresses the reaction.Single cleavage flakes of calcite were packed in −325 mesh quartz and allowed to react under the same conditions as the powders. A scanning electron microscope was used to examine and photograph the fragment surfaces at the conclusion of the experiments.Examination of the photographs shows the calcite to be completely covered with a thin layer of wollastonite crystals. Details of the morphology suggest that the wollastonite grew outward from nucleation centers and that the solution of calcite and quartz may have been accelerated near growing wollastonite. Although no details of the reaction mechanism can be deduced, the model favored here is that growth was most rapid near the calcite–wollastonite interface and the mantling effect slowed the reaction by preventing the transport of silica to the calcite surface.