We analyzed the chemical composition and observed morphological relationships of mineral phases in a thin section and grain mount of a talc-bearing amphibole schist collected at a former talc mine near Talcville, New York at scales ranging from hand sample to crystal lattice. The main mineral phases are tremolite, anthophyllite, and talc. Tremolite exists as unaltered crystals throughout the sample. Anthophyllite has an acicular habit and a series of fractures running perpendicular to crystal elongation (i.e., its c crystallographic axis). Talc occurs as a fine-grained mass in anthophyllite fractures, between acicular anthophyllite partings, and as pseudomorphs after anthophyllite, with some occurring with an asbestiform habit. Thus, the talc alterations occur both parallel and perpendicular to the elongate anthophyllite, with the latter producing very high-aspect ratio (i.e., greater than 20 times longer than wide) non-asbestiform anthophyllite particles. Based on wavelength-dispersive spectroscopy (WDS) analysis the three phases have near endmember formulas: tremolite [K0.03Na0.17Ca1.74Mn0.01Fe0.01Mg5.21Al0.03Si7.94O22(OH1.65F0.35)], anthophyllite [Na0.04Ca0.04 Mn0.04Fe0.02Mg6.96Al0.02Si7.95O22(OH1.82F0.18)], fracture-filling talc [Mg3.10Mn0.02Fe0.01Al0.01Si3.93O10(OH1.92F0.08)], and talc pseudomorph [Mg3.03Mn0.02Fe0.01Al0.01Si3.96O10(OH1.92F0.08)]. Microscopic examination in light and electron microscopes showed intergrowths of talc and anthophyllite, but it was only with the aid of high-resolution lattice imaging perpendicular to the c crystallographic axis where the formation mechanism of asbestiform talc could be ascertained.
- © 2015 Mineralogical Association of Canada