The Bureau of Mines examined a fibrous talc sample from the Gouverneur Talc District of New York by transmission electron microscopy (tem) and polarized light microscopy to determine the mineralogical relationship of the fibrous talc to the amphibole present in the sample. Two amphiboles, anthophyllite and tremolite, were present in the sample. Tremolite occurred as a separate mineral phase, which was blocky in habit. Only a few composite tremolite-talc grains were observed. Anthophyllite, however, was present only in the fibrous talc grains. Microdiffraction study of the fibrous talc grains containing anthophyllite showed that the anthophyllite was intermixed with the talc on a fine scale and that there was a crystallographic relationship between the talc and anthophyllite lattices in the fibrous talc grains. A mechanism similar to the process that forms biopyriboles could explain the structural defects, the fibrous habit of the talc, and the structural relationship between the talc and anthophyllite in the fibrous talc grains. Because of these characteristics, phase contrast microscopy and a provisional tem technique for monitoring asbestos exposure would not distinguish between fibrous talc and fibrous amphiboles. Tem techniques employing electron diffraction and energy-dispersive x-ray analysis are recommended to positively identify the fibrous phases for regulatory purposes.