Identification of anhydrous CaCl2 and KCaCl3 in natural inclusions by Raman spectroscopy

Anhydrous chlorides - CaCl₂, and KCaCl₃ (chlorocalcite) were identified as mineral inclusions in halite from the Siberian Large Igneous Province at the contact of magmatic intrusions and evaporates. Chlorocalcite was also found as daughter mineral in polyphase hypersaline inclusions. While Raman spectra of KCaCl₃ (chlorocalcite) in natural inclusions are similar to spectra of synthetic analogue, the Raman spectra of natural CaCl₂ do not correspond to the published Raman spectra of synthetic CaCl₂. Simulations of Raman spectra using ab initio density-functional theory (DFT) allowed us to calculate the spectra of individual polymorphs of CaCl₂ and to discriminate anhydrous CaCl₂ phases in natural inclusions and synthetic CaCl₂. In the spectrum of the Pbcn polymorph of CaCl₂ twelve different peaks could be identified at 74, 95, 99, 107, 124, 158, 164, 179, 212, 236, 244, 256 cm⁻¹ in contrast to five peaks in the spectrum of the Pnnm polymorph of CaCl₂ at 115, 157, 160, 211 and 252 cm⁻¹. Naturally occurring CaCl₂ in inclusions in halite consist of Pbcn polymorph only, which probably results from a mechanical stress on cooling from magmatic to ambient temperatures. However, the Raman spectra of the synthetic CaCl₂ corresponds to the Pnnm phase with small contributions of the Pbcn phase.

Raman spectra of synthetic KCaCl₃ with main peaks at 58, 67, 90, 97, 133, 139, 147, 193 cm⁻¹ agrees well with the spectra of chlorocalcite in the natural inclusions. Positions of each atom in the KCaCl₃ structure were refined using the density-functional theory. There are no imaginary phonon modes for the optimized structure, indicating that the structure of KCaCl₃ is stable. Calculated Raman spectrum is in a good agreement with the Raman spectrum of synthetic and natural KCaCl₃ samples.