Summary
Massive, nearly ‘pure’, haemo‐ilmenite layers from historic ore deposits in Rogaland, Norway contain very few silicates or other oxides and typically produce remanence‐dominated magnetic anomalies. These rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains. A typical bulk composition, Ilm 84, exsolved at high temperature to produce host ilmenite Ilm 94 and micron‐sized haematite lamellae Ilm 23 as measured by electron microprobe (EMP). Subsequent undercooling of the ilmenite and the micron‐scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit‐cell scale, leaving depleted hosts between lamellae with compositions of Ilm 98 and Ilm 15–13 as measured by TEM–EDX. Samples have high coercivities, and average NRM values of 25 A m−1, which typically show ∼2 per cent saturation in the NRM state. The amount of magnetization in these samples is too high to be solely accounted for by a spin‐canted AF moment in the haematite. Based on Monte Carlo simulations of haematite–ilmenite interfaces at the atomic scale and on measured rock‐magnetic properties, we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine‐scale titanohaematite and ilmenite exsolution lamellae.