INTERFACES OF MAGNETITE WITH RHOMBOHEDRAL FE-TI OXIDES: MAGNETITE EXSOLVED FROM SYNTHETIC TITANOHEMATITE (ILM40) OF MAGNETITE, AND NATURAL ILMENITE SUZANNE MCENROE 1,2 , NOBUYOSHI MIYAJIMA 1 , PETER ROBINSON 2 , KARL FABIAN 2 , PHILLIP SCHMIDT 3 , DAVE CLARK 3 , TIZIANA BOFFA BALLARAN 2 AND BENJAMIN BURTON 4 1 Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany (Suzanne.mcenroe@ngu.no) 2 Geological Survey of Norway, 7091 Trondheim Norway 3 CSIRO, Earth Science & Resource Engineering, North Ryde NSW A ustralia 1670 4 NIST, Gaithersburg, MD USA 20899 A synthetic sample of X FeTiO 3 = 0.40 was annealed for 7 days at 887 °C in a matrix of sodium borate flux. The run product consists of crystals in a glass matrix. In reflected light, some grains show two or three areas of different reflectivity along very sharp contacts, possibly due to twinned crystal orientation. Optical observations indicated only one phase and no exsolution was evident. The X-ray diffraction pattern shows no line broadening that would indicate significant variation in composition, nor evidence for other phases. Lattice parameters: a = 5.0557 Å, c = 13.8528 Å, V = 306.646 Å 3 indicate a composition XFeTiO 3 = 0.405, extremely close to the nominal composition. Magnetic hysteresis experiments from 300 to 10K, showed two-phase behavior and below 35K a large increase in coercivity and a strong shift in remanence. Exchange coupling across interfaces of two phases with different magnetic properties appears necessary to explain this behavior. Though the second phase was not detected by XRD, a high-resolution transmission electron microscopy (HRTEM) demonstrated the presence of extremely fine magnetite lamellae parallel to (001) of the host rhombohedral phase, ~6-8 nm thick, or approximately 4-6 unit cells of the rhombohedral oxide host. Electron diffraction patterns of host areas with lamellae indicated the rhombohedral titanohematite coexists with a spinel- structured phase, together with a pattern of moiré effects. The parallel orientations of c* of the hematite and 111* of the spinel-structure phase indicate that the prominent lamellar interfaces are on the (001) basal plane of the host and on the (111) octahedral plane of the lamellae, a well known interface in natural magnetite-ilmenite intergrowths. Naturally occurring oxidation exsolution lamellae of ilmenite in magnetite is common in many rock types and these magnetites are typical carriers of the paleomagnetic signal. The ilmenite lamellae create smaller magnetic domains within a large magnetite grain enhancing the magnetic stability. Rarely reported however are magnetite lamellae in ilmenite. Here we discuss the mineralogy of a natural sample from the Black Hill Norite from southern Australia with discrete grains of magnetite and ilmenite. The ilmenite has abundant fine exsolution lamellae. A TEM study demonstrates that the fine lamellae are magnetite, and the interface is parallel to (001) of the host ilmenite. This norite has a remanence-dominated magnetic anomaly and a very stable paleomagnetic direction (Rajagopalan et al., 1993; 1995). The Black Hill Norite is unusal in that it contains both magnetite with oxy- exsolution of ilmenite, and a coexisting ilmenite with magnetite exsolution produced by reduction. A proper balance of these two reactions within the rock as a whole could explain these conflicting trends. References Rajagopalan S., Schmidt P. and Clark D., 1993. Rock magnetism and geophysical interpretation of the Black Hill Norite, South Australia. Explor. Geophys., 24(2), 209-212. Rajagopalan S., Clark D. and Schmidt P., 1995. Magnetic mineralogy of the Black Hill Norite and its aeromagnetic and palaeomagnetic implications. Explor. Geophys., 26(3), 215-220. View publication stats View publication stats