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

Abstract

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 densityfunctional 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.