Phase transformations in calcite from electrical impedance measurements

Abstract

The phase transformation in calcite I-IV-V and calcite ⇔ aragonite have been characterized by electrical impedance measurements at temperatures 600–1200°C and pressures 0.5–2.5 GPa in a piston cylinder apparatus. The bulk conductivity σ has been measured from Argand plots in the frequency range 10⁵–10⁻² Hz in an electric cell representing a coaxial cylindrical capacitor. The synthetic polycrystalline powder of CaCO₃ and natural crystals of calcite were used as starting materials. The transformation temperature T_c was identified from resistivity-temperature curves as a kink point of the activation energy. At pressure above 2 GPa in ordered phase calcite I, the activation energy E _σ is c. 1.05 eV, and in disordered phase calcite V E _σ is c. 0.75 eV. The pressure dependence of T_c for the rotational order–disorder transformation in calcite is positive for pressures <1 GPa and negative for pressures >1 GPa. The transformation boundary of calcite 1–IV is observed only during first heating in samples after a long annealing at low temperatures. The activation energy of calcite I ⇔ IV decreases gradually from 1.8 to 1.05 eV with the pressure increase from 0.5 to 2 GPa. The kinetics of calcite ⇔ aragonite transformation has been monitored by measuring a time-variation of the electrical resistance of a calcite sample at 10³ Hz in the stability P-T field of aragonite. The variation of the impedance correlates with the degree of phase transformation, estimated from X-ray powder diffraction studies on quenched products of experiments. The kinetics of calcite ⇒ aragonite transformation may be fitted to the Avrami kinetics with the exponent m ˜ 1–1.5.

Keywords:

• Order–disorder
• Calcite
• Aragonite
• Phase transition
• Electrical impedance
• Kinetics

Acknowledgements

This work was funded by Special Program of German Science Foundation SPP 1150. A. S. thanks Exchange Program between GSF and Russian Academy of Sciences for covering his travel and stay costs in IfMG. The authors are grateful to R. Petscheck (IfPG, Frankfurt) for the diffractometer analysis of samples.