Magnetic mineralogy
Strongly magnetic minerals
Many of the most important magnetic minerals on Earth are oxides of iron and titanium. Their compositions are conveniently represented on a ternary plot with axes corresponding to the proportions of Ti4+, Fe2+, and Fe3+. Important regions on the diagram include the titanomagnetites, which form a line of compositions Fe3-xTixO4 for x between 0 and 1. At the x=0 end is magnetite, while the x=1 composition is ulvöspinel. The titanomagnetites have an inverse spinel crystal structure and at high temperatures are a solid solution series. Crystals formed from titanomagnetites by cation-deficient oxidation are called titanomaghemites, an important example of which is maghemite. Another series, the titanohematites, have hematite and ilmenite as their end members, and so are also called hemoilmenites. The crystal structure of hematite is trigonal-hexagonal. It has the same composition as maghemite; to distinguish between them, their chemical formulae are generally given as γFe2O3 for hematite and αFe2O3 for maghemite.
Magnetic mineralogy-4