Puzzling calcite-III dimorphism: crystallography, high-pressure behavior, and pathway of single-crystal transitions

Abstract

High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO₃ were investigated by analytical in situ high-pressure high-temperature experiments on oriented single-crystal samples. All experiments at non-ambient conditions were carried out by means of Raman scattering, X-ray, and synchrotron diffraction techniques using diamond-anvil cells in the pressure range up to 6.5 GPa. The composite-gasket resistive heating technique was applied for all high-pressure investigations at temperatures up to 550 K. High-pressure Raman spectra reveal distinguishable characteristic spectral differences located in the wave number range of external modes with the occurrence of band splitting and shoulders due to subtle symmetry changes. Constraints from in situ observations suggest a stability field of CaCO₃-IIIb at relatively low temperatures adjacent to the calcite-II field. Isothermal compression of calcite provides the sequence from I to II, IIIb, and finally, III, with all transformations showing volume discontinuities. Re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to determine the transition sequence. Clausius–Clapeyron slopes of the phase boundary lines were determined as: ΔP/ΔT = −2.79 ± 0.28 × 10⁻³ GPa K⁻¹ (I–II); +1.87 ± 0.31 × 10⁻³ GPa K⁻¹ (II/III); +4.01 ± 0.5 × 10⁻³ GPa K⁻¹ (II/IIIb); −33.9 ± 0.4 × 10⁻³ GPa K⁻¹ (IIIb/III). The triple point between phases II, IIIb, and III was determined by intersection and is located at 2.01(7) GPa/338(5) K. The pathway of transition from I over II to IIIb can be interpreted by displacement with small shear involved (by 2.9° on I/II and by 8.2° on II/IIIb). The former triad of calcite-I corresponds to the [20-1] direction in the P2₁/c unit cell of phase II and to [101] in the pseudomonoclinic C ${\bar{1}}$ setting of phase IIIb. Crystal structure investigations of triclinic CaCO₃-III at non-ambient pressure–temperature conditions confirm the reported structure, and the small changes associated with the variation in P and T explain the broad stability of this structure with respect to variations in P and T. PVT equation of state parameters was determined from experimental data points in the range of 2.20–6.50 GPa at 298–405 K providing $K_{{{\text{T}}_{0} }}$ = 87.5(5.1) GPa, (δK _T/δT) _P = −0.21(0.23) GPa K⁻¹, α ₀ = 0.8(21.4) × 10⁻⁵ K⁻¹, and α ₁ = 1.0(3.7) × 10⁻⁷ K⁻¹ using a second-order Birch–Murnaghan equation of state formalism.