Mössbauer and magnetic measurements of iron phase distributions in apollo lunar samples

Abstract

Knowledge of the distribution and valence of iron is required for meaningful interpretation of the basic physical properties of lunar surface samples (magnetic remanence, electrical conductivity, optical behavior, etc.) and also has important consequences for discussions of lunar regolith processes. The amounts of metallic Fe and ferrous Fe in various mineral phases are related to surface processes involving localized or wide‐spread reheating with moltenstate of subsolidus reduction, and the metallic‐ferrous ratio (Fe°/Fe2+) appears to be a useful classification parameter for lunar samples. When combined with other data (e.g., chemical and petrographic analyses), the Fe phase distributions (the percentages of Fe contained in various phases) of lunar rocks may provide useful constraints concerning original source materials and thermal histories. Here we report Fe phase distributions as determined by Mossbauer spectroscopy and Fe°/Fe2+ ratios as determined from both Mossbauer and magnetic measurements for a large suite of lunar samples. The (Fe°/Fe2+) values determined by the two techniques agree reasonably well with the exception of a number of samples which contain large amounts of Fe in olivine; for these samples, the magnetic analysis gave higher (Fe°/Fe2+) values than did the Mossbauer analysis. The most likely origins of the discrepancies are antiferromagnetic clustering in olivine and/or an abundance of large grain size metallic Fe (≳50μm).The percentage of Fe in olivine is found to decrease markedly with increaisng wt.% FeO, which is believed to reflect higher crystallization temperatures for lunar highlands material. At 4°K, Mossbauer spectra of most high olivine samples exhibit a well‐resolved hyperfine spectrum believed to arise from antiferromagnetic clusters of Fe2+ spins in olivine mirror sites. Some high olivine samples do not show this behavior and petrographic studies indicate that these samples underwent relatively rapid cooling. Presumably, this would allow less time for preferential Fe2+ occupation of mirror sites, which are slightly larger and more strongly coupled magnetically than the inversion sites in the olivine structure.