HR: 11:50h
AN: V12A-07    [Abstracts]
TI: Growth of Fibrous Talc and Anthophyllite in the Hydrothermal Diamond Anvil Cell (HDAC)
AU: * Kerrigan, R J
EM: kerrigan@geol.umd.edu
AF: University of Maryland, Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742,
AU: Candela, P A
EM: candela@geol.umd.edu
AF: University of Maryland, Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742,
AU: Piccoli, P M
EM: piccoli@geol.umd.edu
AF: University of Maryland, Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742,
AU: Wylie, A G
EM: awylie@geol.umd.edu
AF: University of Maryland, Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, MD 20742,
AB: Talc deposits are formed by metasomatism of dolostone or ultramafic rocks during metamorphism or hydrothermal alteration (Van Gosen et al., 2004: Env. Geo., v. 45, p. 920). Although talc is common, fibrous (asbestiform) talc is rare and its origin has been explained by the replacement of asbestiform amphiboles during retrograde metamorphism. The effects of fibrous talc on the human respiratory system remain unresolved. In order to elucidate the formation of asbestiform talc in natural systems, kinetics experiments in the MgO-FeO- SiO₂-H₂O-HCl system were performed by using an hydrothermal diamond anvil cell (HDAC) apparatus. Two different chamber sizes have been used, with either 650 μm or 300 μm diameter holes in 250 μm thick rhenium foil. We have observed that at elevated pressures and temperatures, deformation of the gasket occurs and the chamber volume is reduced by up to 25%. Forsterite (Fo₉₅) + quartz + 0.1 mM HCl solution (pH = 4 @ 25^oC and 0.1 MPa) were added to the chamber and run at temperatures of 560 - 660^oC and at pressures up to 1 GPa for a duration of 1 - 8 hours. The experiments were run isothermally (+/- 3^oC) at the target temperature. The experiments were recorded by a video camera mounted on a polarized light microscope. Experiments have been performed at pressures and temperatures near the talc + enstatite = anthophyllite equilibrium. Run products consist of talc or anthophyllite with a fibrous habit. Vapor-liquid homogenization temperatures before and after the experiment were used to determine the prograde and retrograde isochores, respectively. Quartz was dissolved upon heating by as much as 5%; dissolution occurred on a time scale of ~ 30 seconds. Whiskers of fibrous anthophyllite crystallized on the high temperature side of the talc + enstatite = anthophyllite equilibrium whereas fibrous talc crystallized on the low temperature side. Composition of the whiskers was determined by using a spindle stage and immersion oils. The talc and anthophyllite whiskers share similar morphological characteristics (aspect ratios ~25:1, average lengths ~40 um) and grew on the forsterite surface and/or on the gasket walls. Growth rates for both minerals are on the order of ~ 0.03 μm/s in the long dimension. Growth of fibrous talc and fibrous anthophyllite may be due to: (1) strong spatial gradients in the aqueous concentration of SiO₂ during growth; (2) fluctuations in pressure due to changes in cell volume brought on by gasket deformation. The production of fibrous talc in these experiments demonstrates that growth of talc after amphibole is not necessary for the production of a fibrous habit. The results of this study may be used to determine the conditions under which fibrous talc may form in some deposits.
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3630 Experimental mineralogy and petrology
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting