A 'dry' condensation origin for circumstellar carbonates

The signature of carbonate minerals has long been suspected in the mid-infrared spectra of various astrophysical environments such as protostars(1). Abiogenic carbonates are considered as indicators of aqueous mineral alteration(2) in the presence of CO2-rich liquid water. The recent claimed detection of calcite associated with amorphous silicates in two planetary nebulae(3) and protostars(4,5) devoid of planetary bodies questions the relevance of this indicator; but in the absence of an alternative mode of formation under circumstellar conditions, this detection remains controversial(6-8). The main dust component observed in circumstellar envelopes is amorphous silicates(9), which are thought to have formed by non-equilibrium condensation(10). Here we report experiments demonstrating that carbonates can be formed with amorphous silicates during the non-equilibrium condensation of a silicate gas in a H2O-CO2-rich vapour. We propose that the observed astrophysical carbonates have condensed in H2O(g)-CO2(g)-rich, high-temperature and high-density regions such as evolved stellar winds, or those induced by grain sputtering upon shocks in protostellar outflows.