Characterization of hematite (a-Fe2O3), goethite (a-FeOOH), greigite (Fe3S4), and pyrrhotite (Fe7S8) using first-order reversal curve diagrams

[1] First-order reversal curve (FORC) diagrams have become a standard tool in rock magnetism, yet magnetite is the only magnetic mineral that is well characterized using FORC diagrams. We present FORC diagrams for predominantly single-domain (SD) synthetic aluminous hematite (a-Fe2-xAlxO3) and goethite (a-(FeAl)OOH) and natural greigite (Fe3S4) and pyrrhotite (Fe7S8) to constrain interpretation of FORC diagrams from natural samples. Hematite and goethite have low spontaneous magnetizations and negligible magnetic interactions, while greigite and pyrrhotite have higher spontaneous magnetizations and can have strong magnetic interactions. The coercivity of hematite systematically increases with Al content only for samples produced using the same synthesis method, but it is variable for samples produced with different methods even for similar Al content. This precludes use of magnetic coercivity alone to quantify the Al content of natural hematites. Goethite has much higher coercivity than hematite for all measured samples. SD and superparamagnetic (SP) behavior is common in natural greigite samples, with peak coercivities ranging from 70 mT (SD) to zero (SP). This range overlaps with that of lower-coercivity minerals, which can make greigite identification ambiguous at room temperature. Fine-grained SD pyrrhotite has slightly higher coercivities than greigite, which progressively decreases with increasing grain size within the SD size range and overlaps the range for greigite. While FORC diagrams are useful for magnetic characterization, care is needed in interpretation because of overlaps in the broad range of magnetic properties, which result from variations in domain state, for any magnetic mineral with respect to other minerals.