Polymorphic Transitions in Calcite and the Kinetics of the Aragonite-Calcite Transformation: Evidence from Electrical Impedance Measurements at Pressures up to 2.5 GPa

摘要

High-temperature phase transitions in calcite and the calcite-aragonite transformation were investigated on a piston-cylinder apparatus by means of an electrical impedance measurement within temperature and pressure intervals of 600-1200 deg C and 0.5-2.5 GPa. The measurement cell consisted of a coaxial capacitor with a geometric factor of 6 cm. The bulk component of electrical conductivity was obtained from Argand dia-grams in the fequency range 10~(-1)-10~5 Hz. The starting materials were represented by the polycrystalline CaCO_3 reagent and pressed powders of natural calcite crystals. t each pressure, the transition temperature was recorded by a stepwise change in activation energy, which was determined from the curves of electrical con-ductivity versus temperature. The pressure dependency of the temperature of the transition into the disordered calcite-V phase shows a maximum at a pressure of 10 kbar. The curve that has previously been regarded in the literature as a transition to the intermediate calcite-IV phase was interpreted as the pressure dependency of the onset of continuous orientation disorder in the initial calcite-I phase. The kinetics of the aragonite-calcite trans-formation was determined at pressures of 15 and 25 kbar by the continuous monitoring of changes with time in the electrical conductivity of a sample at conrrelated with the degree of phase transformation.