High-temperature Raman spectroscopy of K2Ca(CO3)2 bütschliite and Na2Ca2(CO3)3 shortite

Abstract Raman spectra of Na2Ca2(CO3)3 shortite and K2Ca(CO3)2 bütschliite were measured to 715 and 740 °C, respectively, under vacuum. The vibrational spectra demonstrate that shortite converts to nyerereite [Na2Ca(CO3)2] and calcite at 535 °C. This assemblage remains stable up to ∼700 °C, at which point nyerereite begins to decompose. Bütschliite converts to the isochemical phase fairchildite at 570 °C, which is stable to 665 °C, where it decomposes to an assemblage of K2CO3 and CaO. The variation of anharmonicity between different vibrational modes of each of the low-temperature phases is assessed, and these yield insights into inter-carbonate group couplings. Both fairchildite and nyerereite exhibit spectral features consistent with extensive disordering.