Muscovite (Var: Gieseckite)

Specimen size is 7.2 x 6.6 x 2.8 cm. Here is a recent acquisition, a really nice example of the Natural Bridge, New York pseudomorph known locally as gieseckite. Gieseckite is a muscovite varietal (in accordance with Mindat.org) and represents a pseudomorph after an unknown mineral, although other sources have it as illite (approved 1998). Some have proposed it after nepheline but given its association with quartz and the monoclinic (pseudo hexagonal) shape of the gieseckite crystals the evidence is against nepheline. The key, it has been said, is finding a gieseckite crystal with a remnant core. This specimen may fit the bill. Most examples of gieseckite are rather small. I personally acquired a few small specimens back in 2000 at a small show in W. Pierrepont, NY from the collector credited with the "rediscovery" of the once lost location, known to me only as Karl. This specimen, collected long ago before the location was lost, is actually quite large and appears to have a remnant core. Is this a call to research? Any takers, let me know. The largest crystal measures about 1.4 x 1.4 x 1.4 cm. Two old collector labels are present and pictured. The older label is incomplete, the collector's name is truncated, "L. A. L...." If anyone recognizes the name on this label or has any information concerning the label, I would appreciate any assistance offered. So, specimen is ex. L. A. L...(unknown) and ex. Clifford J. Awald. Additional close-ups and labels are also shown. This specimen was photographed by and is in the collection of Ron Gyllenhammer as M00942.

References:

George Robinson & Steven Chamberlain (2007) Gazetteer of major New York State mineral localities. Rocks & Minerals, 82, #6, 472-483.

REEXAMINATION OF THE CLASSIC GIESECKITE LOCALITY, NATURAL BRIDGE, NY
MCCLOSKEY, A., GREEN, J.S., and BAILEY, D.G., Dept. of Geosciences, Hamilton College, 198 College Hill Rd, Clinton, NY 13323, dbailey@hamilton.edu

REDISCOVERY OF THE CLASSIC LOCALITY FOR 'GIESECKITE' NEAR NATURAL BRIDGE, LEWIS COUNTY, NEW YORK
GREEN, Jeremiah S.1, CHAMBERLAIN, Steven C.2, ROBINSON, George W.3, and BAILEY, David G.1, (1) Hamilton College, 198 College Hill Rd, Clinton, NY 13323, jsgreen@Hamilton.edu, (2) Syracuse Univ, Syracuse, NY 13244, (3) Michigan Technical Univ, Houghton, MI 49931





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Photo added: 5th Jan 2009