A comparative study of the ARM and Isr of some natural magnetites of MD and PSD grain size

Abstract

Summary. ARM and Isr stability with respect to alternating field and low‐temperature treatment were measured for a set of artificial rock specimens. Each of the specimens contained a known amount of a natural magnetic mineral of a known grain size. The following minerals were used: a homogeneous natural magnetite, a natural titanomagnetite with submicroscopic Fe‐rich inclusions, and a natural titanomagnetite and a natural titano‐maghemite that both show exsolution phenomena. The grain size of the fractions varied from < 5 to 250 μm. It is found that the stability of ARM and Irs of multi‐domain grains with respect to alternating fields and low‐temperature treatment is similar, whereas for the smallest grains studied (< 5 μm) ARM is more stable than Isr. Multi‐domain ARM data from this study are in agreement with current multi‐domain ARM theory. Low‐temperature behaviour of ARM and Isr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains. Copyright © 1982, Wiley Blackwell. All rights reserved