Kinetics of the calcite ⇌ aragonite transformation

Abstract

A beryllium pressure vessel mounted in an X-ray diffractometer has been used to study the rates of the transformation calcite ⇌ aragonite at temperatures to 500°C and pressures to 15 kb. The rates of transformation are shown to be both temperature and pressure dependent. It is proposed that the tail-off shown by all rate curves presented here results from a decrease in nucleation and loss of stored strain energy in the sample as the transformation progresses. In the vicinity of 400°C the rate of the calcite ⇀ aragonite transformation at 15±1 kb is similar to the reverse transformation at 1 bar. A decreasing-temperature extrapolation of the rate constant for the aragonite ⇀ calcite transformation shows that this transformation at room temperature progresses at a negligible rate. The activation energy for this transformation at 1 bar is 106 kcal/mole. The use of the experimental rate data as well as calculated transformation times obtained from an empirical relationship describing the transformation has permitted the construction of curves of time for 99% transformation of aragonite ⇀ calcite in the portion of the calcite stability field lying below 500°C.