See more from this Division: Topical Sessions
See more from this Session: Discovering Petrologic Truth in Minerals II: In Honor of Bernard W. Evans
Abstract:
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.
See more from this Division: Topical Sessions
See more from this Session: Discovering Petrologic Truth in Minerals II: In Honor of Bernard W. Evans