Abstract
Chloritoid stability limits relative to coexisting minerals of regionally metamorphosed pelitic rocks have been investigated chiefly as a function of the total or solid pressure, temperature, and oxygen fugacity. Excess silica has a very small effect on the breakdown temperature of the mineral. Variation of oxygen fugacity within the field of magnetite changes the stability limit of the mineral. The breakdown equilibria are relatively insensitive to changes in total pressure. Chloritoid cannot be stable above 575-600 degrees C, the transformation to staurolite taking place between 500 to 575 degrees C. The assemblage chloritoid, diaspore, and kaolinite can be stable at oxygen fugacities within the field of hematite, less than 375 degrees to 400 degrees C. Petrographic evidence suggests the Al-silicate triple point should be between the equilibria Chd + Al 2 SiO 5 <--> St + Q + H 2 O and Chd <--> Alm + Cor + H 2 O.
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