Geikielite and Coupled Spinel-Rutile Exsolution from Titanohematite in the Mesoproterozoic Granulite Facies, South Norway

Sizes and thermal evolution of exsolution in rhombohedral Fe-Ti oxides is a key to studies of remanence and lamellar magnetism of planetary crusts. Extensive negative aeromagnetic anomalies in the Modum area derive from rocks containing ilmenite with hematite exsolution or hematite with ilmenite exsolution, carrying strong/stable reversed remanence. Rocks include recrystallized gabbros, metamorphosed hydrothermally altered volcanics, and metamorphosed oxidized sediments, some possibly evaporitic, with Cl-scapolite. Magnetic properties at room-, high- and very-low-T aid phase identification before mineralogic study.

One rock contains a 3-cm-diameter titanohematite in a vein. Reflected-light and EMP analyses showed the titanohematite (8%R²⁺TiO₃, 2%MgTiO₃) contains three types of exsolution: spinel plates on (0001) of the host; rutile rod satellites on spinel parallel to host rhombohedral face edges; and lamellae 0.1-0.3 µm thick, also parallel to (0001), but too fine for EMP analyses. Overlap analyses showed enrichment in MgO, TiO₂ and lack of Al₂O₃, indicating a mixture of titanohematite and geikielite-rich solid solution. Powder XRD gave a=5.0393, c=13.7687, V=302.81 for titanohematite (≈Ilm9), and unrefined reflections of rutile and geikielite.

Spinel and rutile, analyzed by EMP, formed earlier than geikielite. Spinel gave 96%MgAl₂O₄, 3%FeFe₂O₄, Mg/ total R²⁺ = 0.98. How did magnesian/aluminous spinel lacking Ti⁴⁺ ions exsolve from titanohematite? The answer is in coupled exsolution with ferrian rutile, where combined components were in solution in high-T titanohematite as corundum/geikielite. Phase separation lowered geikielite, and depleted the corundum component.

TEM-EDS analyses showed hematite is ≈6%R²⁺TiO₃ (2%MgTiO₃); geikielite is ≈100%R²⁺TiO₃(70%MgTiO₃). MgTiO₃ is important because Mg²⁺ has no magnetic moment, but breaks up linkages between Fe atoms, lowers Néel temperatures, and produces unusual low-T properties. This sample shows hematite Néel T, 878K, geikielite, 34K (Fe₂O₃ 953K, FeTiO₃ 57K), with possible spin-glass below 13K, and points toward magnetic study of the systems FeTiO₃-MgTiO₃ and Fe₂O₃-MgTiO₃.