AN OCCURRANCE OF MICROSCOPIC SAPPHIRE (BLUE CORUNDUM) IN THE WESTERN ADIRONDACKS

Microscopic grains of sapphire (blue corundum) have been identified in a small (1 – 2 cm wide), discontinuous, dike of nelsonite hosted by aluminous feldspathic gneiss. The gneiss was excavated during construction of a hydroelectric plant on the Black River at Port Leyden, NY (western Adirondack Highlands). The location is 250 m NE of the Port Leyden nelsonite (Darling and Florence, 1995, Econ. Geol., v. 90, p. 964-968).

The small nelsonite dike is fine-grained and comprises magnetite, rutile (after ilmenite?), apatite, pyrite, Ce-monazite, corundum, hercynite, and zircon. The host gneiss contains medium-grained plagioclase, biotite, garnet, and sillimanite. The small dike may represent a separate intrusion of nelsonite or one sheared from the main nelsonite orebody during Ottawan (ca. 1050 Ma) deformation and metamorphism.

The sapphires range in size from 0.1 – 1.0 mm, and commonly show parting, pleochroism, and hexagonal oscillatory zoning (from deep blue to clear). Electron microprobe analysis shows comparable levels of Fe in both clear (0.69-0.86 wt.%) and blue (0.51-0.69 wt.%) sections of crystals, but clear sections have significantly lower Ti levels (0.003-0.03 wt.%) compared to blue sections (0.11-0.22 wt.%). The blue color is attributed to Fe²⁺-Ti⁴⁺ acceptor-donor pairs, located on nearest-neighbor Al sites (Emmett et al., 2003, Gems & Gemology, vol. 39, pp. 84–135).

It is unclear whether the sapphire is igneous or metamorphic in origin. An igneous origin is supported by its occurrence in a nelsonite dike, although sapphire has not been described from the main nelsonite orebody. The presence of fine oscillatory zoning in sapphire also suggests an igneous origin, yet oscillatory zoning has been described in corundum (Burmese rubies) of unequivocal metamorphic origin (Mittermayr et al., 2008, Geophys. Res. Abs., vol. 10, EGU2008-A-10706). Regardless of its origin, this occurrence of sapphire is not totally unexpected given the low silica activity and high Fe and Ti abundances of the nelsonite host rock.