ELECTRON AND X-RAY PETROGRAPHY OF AN UNUSUAL SERPENTINE FROM THE TILLY FOSTER MINE, BREWSTER, NEW-YORK

The pseudocubic cleavage of an unusual serpentine from the Tilly Foste r mine, Brewster, New York, was first investigated by J.D. Dana in 187 4, who concluded this serpentine to be a pseudomorph. Investigators si nce have suggested that this pseudocubic serpentine does not fit into the standard classification of serpentine (chrysotile, lizardite, anti gorite) and have variously argued that the pseudocubic structure belon gs to the serpentine itself, is partly crystalline and partly amorphou s, or is a new type of serpentine. We have investigated the microfabri c of Tilly Foster serpentine using SEM, TEM and XRD methods. High-reso lution SEM imaging has proved to be essential in the elucidation of th e true nature of this material. The Tilly Foster serpentine is compose d of large (but very thin) single-crystal plates of antigorite interle aved with composite matted-fiber layers of chrysotile. Antigorite-chry sotile layered units occur in mutually perpendicular array in differen t blocks throughout the ''crystal'', giving rise to the morphological pseudocubic cleavage. We believe that interpretation of the growth pro cess of this serpentine composite involves the pseudomorphic replaceme nt of a precursor with orthogonal cleavages. The replacement may have taken place in three stages: (1) nucleation and oriented growth of pla tes of antigorite on an orthogonal array of cleavage surfaces of the p recursor, (2) nucleation and oriented growth of chrysotile fibers on t he antigorite, (3) wholesale replacement of the precursor by chrysotil e. Chrysotile fibers intergrown with antigorite are observed to be ori ented with their X fiber axes either parallel to Y of antigorite or ro tated approximately +/-60-degrees from X of antigorite, but rarely wit h X fiber axes parallel to X of antigorite.