Experiments designed to elucidate the nature and source of high-stability remanence in igneous rocks have been carried out. Many of the pyroxene grains of the samples used (from the Modipe gabbro of southern Africa) contain small (from a few microns down to the optical limit) magnetite inclusions, some of which are very probably single-domained by virtue of their size and shape. It is shown that these inclusions contribute significantly to the TRM, and may, in fact, account for virtually all of the remanence. Measured coercivities as high as 1800 œ are attributed to the observed shape anisotropy of some of the magnetite inclusions. A discussion of the various theories of coercive force indicates that this is the only mechanism which enables magnetite to possess coercivities in excess of 1000 oersteds.
The conclusions reached can be strictly applied only to this particular rock; however, it is felt that the natural occurrence of single-domained magnetite is more common than previously supposed. The reason for the comparative rarity of similar observations may well be the minute size and very small total quantity of magnetite necessary to account for typical TRM's in igneous rocks.