REEXAMINATION OF THE CLASSIC GIESECKITE LOCALITY, NATURAL BRIDGE, NY

Gieseckite is the name originally given to pseudomorphs of illite after nepheline. The original gieseckite was collected by Charles Giesecke in Greenland, and first described and named by Allan (1813). The second major reference to gieseckite was in an article by Brush (1858) that described green-brown, prismatic crystals from a popular collecting site in Natural Bridge, NY. The Natural Bridge locality was lost in the beginning of the 20th century, but then rediscovered in 1999 by a local resident. Crystallographic analysis of new and old gieseckite specimens by Chamberlain (pers. comm.) suggested that they were pseudo-hexagonal, and therefore, were probably not pseudomorphs after nepheline. The purpose of this study was to reexamine the gieseckite specimens and locality in order to determine the original mineral species. The new gieseckite specimens are smaller and lighter in color than the 19th century specimens, yet all exhibit similar morphologies and are composed almost entirely of illite. The new samples are found in a coarse-grained marble and are associated with calcite, diopside, phlogopite, perthitic microcline, albite, tremolite and minor graphite, titanite, and quartz. The only other lithology exposed in the immediate area is a highly deformed, quartzofelsdpathic gneiss. The geologic setting and presence of quartz in the marble argue against the original mineral being nepheline. A more detailed crystallographic examination of both the old and new gieseckite specimens revealed that the morphology is, however, consistent with that of nepheline, although a few other hexagonal and pseudo-hexagonal minerals could not be excluded as possible parent phases. In over thirty thin sections of the gieseckite marble, only one gieseckite crystal exhibited what appears to be a relict core. EDS analyses of this core material were inconclusive, yet indicated a parent phase composed primarily of Mg, Al, and Si. Until a well-preserved core is found within one of the gieseckite specimens, the original mineral species will remain uncertain.