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An in situ study of the phase transitions among CaCO3 high-pressure polymorphs

2018; Cambridge University Press; Volume: 83; Issue: 02 Linguagem: Inglês

10.1180/mgm.2018.140

1471-8022

Xueyin Yuan, Chao Gao, Jing Gao,

Crystallography and molecular interactions

Abstract The phase transitions involving calcite (CaCO 3 -I), CaCO 3 -II, CaCO 3 -III and CaCO 3 -IIIb were investigated using a diamond anvil cell and micro-Raman spectroscopy. Based on the results obtained from in situ observations and Raman measurements made with six natural calcite crystals, the phase transition from calcite to CaCO 3 -II took place between 1.56 and 1.67 GPa under ambient temperature. Under a precise pressure of 1.97 ± 0.03 GPa, three CaCO 3 samples were observed to transform from CaCO 3 -II directly to CaCO 3 -III, while in the other three samples both CaCO 3 -III and CaCO 3 -IIIb crystal structures were detected. Transformation from CaCO 3 -IIIb to CaCO 3 -III was completed in a short period in one sample, whereas in the other two samples coexistence of CaCO 3 -III and CaCO 3 -IIIb was observed over a wide pressure range from 1.97 to 3.38 GPa, with sluggish transformation from CaCO 3 -IIIb to CaCO 3 -III being observed after the samples were preserved under 3.38 GPa for 72 h. Hence, it can be concluded that CaCO 3 -IIIb is a metastable intermediate phase occurring during the reconstructive transformation from CaCO 3 -II to CaCO 3 -III. Splitting of the C–O in-plane bending ( ν 4 ) and symmetric stretching ( ν 1 ) vibrations and appearance of new lattice vibrations in the Raman spectra of CaCO 3 -III and CaCO 3 -IIIb suggest a lowering in crystal symmetry during the transformation from CaCO 3 -II through CaCO 3 -IIIb to CaCO 3 -III, which is in good agreement with the observed sequence of phase symmetries.