Theory and application of the Day plot (Mrs/MsversusHcr/Hc) 2. Application to data for rocks, sediments, and soils

New theoretical curves relating the hysteresis parameters M rs / M s and H cr / H c for single‐domain (SD), superparamagnetic (SP), pseudo‐single‐domain (PSD), and multidomain (MD) grains and their mixtures are applied to published data for natural materials. The Day plot of M rs / M s versus H cr / H c has been used to crudely classify samples into box‐like SD, PSD, and MD (or sometimes incorrectly, MD + SP) regions with arbitrary boundaries. New type curves for MD, PSD/SD + MD, and SD + SP grains and mixtures permit more subtle and precise modeling. The predicted MD trend and its junction with the PSD trend are observed in two data sets: for magnetite spherules from carbonate rocks and for temperature‐varying hysteresis results spanning the Verwey transition. The latter data are the basis of a suggested new method for pinpointing the PSD‐MD threshold size. Selected data for pottery clays, soils, and paleosols generally follow SD + MD type curves and indicate intermediate‐size PSD magnetite with narrow to broad size distributions. A lake sediment section with known grain‐size progression tracks in the predicted sense along the SD + MD trend. Selected data for glaciomarine and pelagic sediments are also generally compatible with SD + MD trends. Examples of remagnetized carbonate rocks, submarine basaltic glasses, and glassy rims of pillow basalts all follow predicted SP + SD or SP + PSD mixing curves, with a large range in volume fraction of SP grains (0–75%) but a narrow range of SP particle sizes: 10 ± 2 nm. Larger SP grains spanning the range to SD size (25–30 nm) are absent for unknown reasons. Oceanic dolerites, gabbros, and serpentinized peridotites in some cases fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.