High-pressure phase transitions in Ca-Mn carbonates (Ca,Mn)CO3 studied by Raman spectroscopy

The influence of Mn content on the stability of the high-pressure CaCO 3 phases CaCO 3 -I, CaCO 3 - II, and CaCO 3 -III at 300 K has been investigated up to 40 mol% MnCO 3 using Raman spectroscopy recorded in situ with a diamond-anvil cell at pressures up to 14 GPa. Beyond about 5 mol% MnCO 3 , there is a progressive linear upward shift in the pressure of the CaCO 3 -I → CaCO 3 -II and CaCO 3 -II → CaCO 3 -III transitions, and expansion of the field of the CaCO 3 -II phase, with increase in MnCO 3 content. The shifts in transition pressure are 0.19 GPa/mol% for I → II and 0.26 GPa/mol% for II → III over the 5 to 40 mol% MnCO 3 composition interval, results fully consistent with elevation of transition pressure by the introduction of a smaller cation. However, minor and trace amounts of Mn appear to have a relatively insignificant influence on the pressure of these transitions.