REDISCOVERY OF THE CLASSIC LOCALITY FOR 'GIESECKITE' NEAR NATURAL BRIDGE, LEWIS COUNTY, NEW YORK

In 1999 the classic locality for green, six-sided crystals of the mineral 'gieseckite' was rediscovered in Natural Bridge, NY, after being lost for over ninety years. The only published description of gieseckite from this locality was done by George J. Brush in 1858, who concluded that it was a pseudomorph of illite after nepheline. Brush also noted that augite, calcite, and magnetic pyrites were found at this locality. The crystals from this locality occur in a calc-silicate rock on the contact between Grenville marble and augite syenite.

A number of rock specimens collected at the locality in 1999 were analyzed for this study. The research done on these samples yielded a more complete list of associated minerals. In order of decreasing abundance these include: calcite, diopside, phlogopite, albite, microcline, quartz, goethite, and brookite.

The study also confirmed that gieseckite is a pseudomorph composed of very fine-grained illite, commonly rimmed with fine-grained muscovite. No indication of an original mineral was found in the samples, although evidence against it being nepheline include: 1)its association with quartz, and 2)the monoclinic (pseudohexagonal) shape of the gieseckite crystals. Further study to determine the original mineral is underway.