CORUNDUM EXSOLUTION LAMELLAE IN ILMENITE FROM A WESTERN ADIRONDACK IRON AND EMERY PROSPECT

Ilmenite found at a former iron and emery prospect pit in the valley of Fish Creek in the town of Greig (western Adirondacks), contains corundum exsolution lamellae comprising up to 5 wt.%. Rocks found at the prospect pit are dominated by medium- to coarse-grained magnetite with lesser quantities of pyrite, corundum, ilmenite, hercynite, plagioclase, tourmaline, molybdenite, quartz and sillimanite. Corundum is in apparent textural equilibrium with ilmenite, magnetite, and hercynite, but thin (10 to 50 micrometer) rinds of sillimanite occur between corundum and quartz. The corundum exsolution lamellae are located on (0001) of ilmenite and were identified by clear color under plane polarized light of a petrographic microscope, a significantly higher polishing hardness than the host ilmenite under a reflected light petrographic microscope, and EDS analysis under the scanning electron microscope. We observed a 2-millimeter grain of ilmenite hosting corundum exsolution lamellae that are 30 to 90 micrometers wide and 200 to 700 micrometers in length.

Experimental work on the Al₂O₃-FeTiO₃ system is lacking. In the study of sapphires globally, empirical data on the Al₂O₃ side of the solvus suggests less than 1 wt.% TiO₂ + FeO is soluble in corundum (e.g. Grapes & Palmer, 1996, Jour. of Petrology). This report shows that significant amounts of corundum (up to 5 wt.%) are soluble in ilmenite on the FeTiO₃ side of the solvus. Metamorphic temperature estimates for rocks in the western Adirondacks is 800°C to 850°C, temperatures comparable to sapphire-forming conditions in other parts of the world. This indicates that the Al₂O₃-FeTiO₃ solvus is asymmetric and steepest towards the corundum side of the solvus.