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Iron makes up around 5 wt% percent of the Earth’s crust and is second in abundance to aluminium among the metals and fourth in abundance behind oxygen, silicon, and aluminium among the elements. Both in its ferrous (Fe2+) and ferric (Fe3+) forms, iron can be involved in direct or coupled substitutions with a variety of divalent, trivalent, and tetravalent cations in 4-, 6-, and 8-coordinated environments. It is consequently a major component in natural oxide and silicate solid solutions. Iron has a partially-filled 3d shell and occurs principally in a high-spin state, giving it a permanent magnetic moment. The presence of a magnetic species in a solid solution not only changes its macroscopic properties in fundamental ways but also provides a sensitive probe of the local chemical environment (Harrison & Putnis, 1999a).

Despite the abundance of Fe in the crust, only a small number of naturally occurring oxide, hydroxide, and sulphide minerals display magnetic ordering phenomena at room temperature (Dunlop & Özdemir, 1997). The most important of these are the titanomagnetite forumla and titanohematite forumla solid solutions. These oxides are the dominant carriers of natural remanent magnetisation (NRM) in rocks, and display diverse and complex behaviour resulting from the interaction between chemical and magnetic ordering (Burton, 1991; Harrison, 2000). No silicates display ordering at room temperature, but several (e.g. pyroxenes, amphiboles, micas, olivines, garnets) do show strong antiferromagnetic ordering at temperatures below 100 Κ (Coey & Ghose, 1987). The presence of magnetic ordering at such low temperatures can still have a significant impact on the thermodynamic properties of the solid solution at high temperatures.

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