Title

Experimental determination of the upper thermal stability of fe-staurolite+quartz at medium pressures

Document Type

Article

Publication Date

2-1-1989

Abstract

The thermal equilibrium for the reaction Fe-staurolite+quartz=almandine+sillimanite+H2O has been reversed at 3·25 and 5·00 kb pressure using a well-characterized natural Fe-rich staurolite. Long run times of 100 days at 3·25 kb and 60 days at 5 kb, in addition to pretreatments of the almandine+sillimanite+quartz by annealing at the experimental P and T for 30 days prior to the experimental run, increased the probability that equilibrium was attained and that sufficient amounts of reaction had occurred to allow its detection.Reaction direction was determined by directly observing surface morphologies of the staurolites with the Scanning Electron Microscope (SEM), a technique that permits evaluation of stability and instability even when the extent of reaction is minor. Growth and dissolution appear to be crystallographically controlled but produce distinct morphologies that allow mterpretation of the reaction direction. Growth of staurolite develops by a face-selective process, such that small step-like features overgrow the original seed staurolite surface. Dissolution produces simpler, blockier forms locally transected by etch pits.Based on textural criteria for staurolite stability and instability, the equilibrium boundary is located between 643' and 658°C at 3·25 kb and between 673 and 688°C at 5 kb. This phase boundary has a shallower dP/dT slope and lies ̃25°C lower than the previous experimental investigation at low pressure (Richardson, 1968). However, this study has not solved the apparent discrepancy between the experimentally determined thermal stability of staurolite and natural occurrences of staurolite (the staurolite problem). For the experimentally determined staurolite curve to agree with natural staurolite occurrences, the experimental equilibrium boundary would have to be ̃50° lower than that indicated by the results of this study. Additional thermochemical discrepancies are most likely related to the complex crystal chemistry of staurolite. © 1989 Oxford University Press.

Publication Source (Journal or Book title)

Journal of Petrology

First Page

229

Last Page

248

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