Fig. 5

IRM acquisition and backfield demagnetization curves for hematite samples. a IRM acquisition curves for known hematite size fractions, where finer sizes are more resistant to acquisition (and vice versa). Redrawn from Thompson (1986). b Backfield demagnetization curves for SD hematite samples measured at the Australian National University (where DC demagnetization curves are equivalent to IRM acquisition curves with twice the amplitude and opposite slope for non-interacting SD particles). Two samples (particles, zebra white) have steep initial slopes due to magnetically soft hematite near the superparamagnetic/SD threshold size and are then dominated by higher coercivities. Zebra samples are pigmentary hematite from Abrajevitch et al. (2018). LAN samples are natural nanocrystalline hematite samples from deeply weathered regolith from Lancefield South gold mine, Western Australia. ‘Particles’ are equidimensional hematite nanoparticles (45–85 nm particle sizes) and ‘rods’ are nanorods (250–350 nm (length) × 50–100 nm (width)) that were synthesized hydrothermally (Islam et al. 2012). Dashed vertical lines represent the 300-mT cut-off field