Fig. 1

Illustration of IRM acquisition in a hematite and b magnetite. The small IRM acquisition below 300 mT in a and complete acquisition of SIRM below 300 mT in b explains the basis for use of a 300-mT cut-off field to discriminate between low-coercivity magnetite and high-coercivity hematite. The sample in a is red pigmentary Zebra rock sample Z539S (Abrajevitch et al. 2018) and the samples in b are synthetic nanoparticulate magnetites (“particles” are equidimensional magnetite (40–85 nm diameter); rods are elongated with length 250–300 nm and width 60–110 nm), which were synthesized following Hu et al. (2011). c First derivative curves of the IRM (i.e., gradient) for the three samples illustrated in a, b, which provide a measure of the respective coercivity distributions in relation to the 300 mT cut-off field. Dashed vertical lines represent the 300-mT cut-off field