Figure 4. Selected synchrotron Mössbauer spectra from high P-T treatment of FeCO₃.

(a–d) Spectra collected at the indicated pressures at room temperature before heating; (e–h) Spectra collected during or after heating at the indicated temperatures. Subspectra are shaded as follows: blue doublet—high-spin ferrous iron, red singlet—low-spin ferrous iron; green sextet—α-Fe₂O₃ and orange sextet—HP-Fe₃O₄ (Supplementary Fig. 5). Note that in a the intensity (amount) of high-spin ferrous iron increases with increasing temperature. The appearance of HP-Fe₃O₄ in g and h is the result of progressive decomposition of FeCO₃ upon melting at high pressure. Continuous lines are fitted with the full transmission integral (Methods section). Percentages on the left of each spectrum indicate the relative absorption.