Puzzling calcite-III dimorphism
- Autor(en)
- Thomas Pippinger, Ronald Miletich-Pawliczek, Marco Merlini, Paolo Lotti, Pascal Schouwink, Takehiko Yagi, Wilson A. Crichton, Michael Hanfland
- Abstrakt
High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO3 were investigated by anal. in situ high-pressure high-temp. expts. on oriented single-crystal samples. All expts. 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 temps. up to 550 K. High-pressure Raman spectra reveal distinguishable characteristic spectral differences located in the wave no. 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 temps. 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 vol. discontinuities. Re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to det. the transition sequence. Clausius-Clapeyron slopes of the phase boundary lines were detd. as: P/T = -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 detd. 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 C1[n.773] setting of phase IIIb. Crystal structure investigations of triclinic CaCO3-III at non-ambient pressure-temp. conditions confirm the reported structure, and the small changes assocd. 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 detd. from exptl. data points in the range of 2.20-6.50 GPa at 298-405 K providing KT0 = 87.5(5.1) GPa, (dKT/dT)P = -0.21(0.23) GPa K-1, a0 = 0.8(21.4) × 10-5 K-1, and a1 = 1.0(3.7) × 10-7 K-1 using a second-order Birch-Murnaghan equation of state formalism.
- Organisation(en)
- Institut für Mineralogie und Kristallographie
- Externe Organisation(en)
- Università degli Studi di Milano-Bicocca, Universität Heidelberg, Ehime University, European Synchrotron Radiation Facility ESRF
- Journal
- Physics and Chemistry of Minerals
- Band
- 42
- Seiten
- 29-43
- Anzahl der Seiten
- 15
- ISSN
- 0342-1791
- DOI
- https://doi.org/10.1007/s00269-014-0696-7
- Publikationsdatum
- 08-2014
- Peer-reviewed
- Ja
- ÖFOS 2012
- 105116 Mineralogie, 105113 Kristallographie
- Schlagwörter
- ASJC Scopus Sachgebiete
- Geochemistry and Petrology, Allgemeine Materialwissenschaften
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/312d7fd2-1b78-4782-9945-2211d7b1fda6