Structural complexity in carbonatite liquid at high pressures

[1] The structure of a carbonatite liquid is examined using Raman spectroscopy at simultaneous high pressures and temperatures. At a pressure of 1.6 GPa and ∼700°C, the carbonatite shows marked structural differences from crystalline carbonates. The symmetric stretching vibration of the carbonate group shows clearly that two different carbonate group environments are present in the liquid: a weak vibration is observed near 1070 cm−1 and a stronger peak near 970 cm−1. Of these two peaks, the one with lower amplitude is indicative of normal, symmetric carbonate units (with partial double C-O bonds), while the higher amplitude feature is associated with single-bonded C-O units, and thus highly distorted, yet structurally discrete, carbonate units. This diversity of structural environments is likely associated with the high affinity of incompatible elements, such as the rare earths, for carbonatite liquids. These structural observations provide a microscopic rationale for the dramatic difference in element partitioning between carbonatite and silicate liquids.