A series of powders were synthesized along the (Fe2+–Mg2+) ilmenite–geikielite solid solution to constrain their crystal structures and establish a compositional calibration to determine the Fe# (=100 × Fe2+/(Fe2+ + Mg2+) molar ratio) of ilmenite in ilmenite-bearing rocks. Using X-ray powder diffraction analyses, we characterized the effect of Fe# on unit-cell parameters and the intensity and position of primary reflections. We used Rietveld and Lebail refinement modeling to correlate Fe# with microprobe analyses and assess two methods for estimating the Fe# in ilmenite based on (1) the unit-cell parameters a and c and (2) by the relative difference between select peak reflections (Δ2θ). Both methods were tested against five Apollo 17 high-Ti lunar basalts, and one synthetic ilmenite-bearing basalt. A strong correlation is observed between the Fe# of the ilmenites and the XRD parameters for all samples, and a compositional calibration is presented based on results for the synthesized ilmenite–geikielite suite. The XRD parameters are sensitive to minor changes (±2 in Fe#) in cation composition, thus the compositional calibration provides a new and sensitive method for estimating Fe# in ilmenite.

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