High-pressure and high-temperature Raman spectra of synthetic FeCO3 siderite and Mg0.76Fe0.24CO3 magnesite were measured across the spin transition up to nearly 60 GPa and 700 K. In pure siderite the spin transition is sharp and observed between 44 and 46 GPa, with no discernible temperature dependence up to 700 K. The spin transition in Fe-bearing magnesite (“ferromagnesite”) is also sharp and takes place between 45 and 47 GPa at ambient temperature, whereas the transition pressure range broadens significantly at about 600 K (45–52 GPa). Our results suggest that the onset pressure of the spin transition in the siderite–magnesite solid solution series is independent of temperature and composition up to 700 K, whereas the broadening of the spin transition range at higher temperature is driven by the Mg content of the sample. Finally, comparison of the (Mg,Fe)CO3 and the (Mg,Fe)O systems indicates that the onset pressure of the spin transition is temperature-independent in both cases, which is rationalized in terms of the FeO6 octahedral compression.

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