Experimental Determination of the Upper Thermal Stability of Fe-Staurolite+Quartz at Medium Pressures

The thermal equilibrium for the reaction Fe-staurolite+quartz=almandine+sillimanite+H₂Ohas been reversed at 3?25 and 5?00 kb pressure using a well-characterizednatural Fe-rich staurolite. Long run times of 100 days at 3?25kb and 60 days at 5 kb, in addition to pretreatments of thealmandine+sillimanite+quartz by annealing at the experimentalP and T for 30 days prior to the experimental run, increasedthe probability that equilibrium was attained and that sufficientamounts of reaction had occurred to allow its detection. Reaction direction was determined by directly observing surfacemorphologies of the staurolites with the Scanning Electron Microscope(SEM), a technique that permits evaluation of stability andinstability even when the extent of reaction is minor. Growthand dissolution appear to be crystallographically controlledbut produce distinct morphologies that allow mterpretation ofthe reaction direction. Growth of staurolite develops by a face-selectiveprocess, such that small step-like features overgrow the originalseed staurolite surface. Dissolution produces simpler, blockierforms locally transected by etch pits. Based on textural criteria for staurolite stability and instability,the equilibrium boundary is located between 643' and 658?C at3?25 kb and between 673 and 688?C at 5 kb. This phase boundaryhas a shallower dP/dT slope and lies {small tilde}25?C lowerthan the previous experimental investigation at low pressure(Richardson, 1968). However, this study has not solved the apparentdiscrepancy between the experimentally determined thermal stabilityof staurolite and natural occurrences of staurolite (the stauroliteproblem). For the experimentally determined staurolite curveto agree with natural staurolite occurrences, the experimentalequilibrium boundary would have to be {small tilde}50? lowerthan that indicated by the results of this study. Additionalthermochemical discrepancies are most likely related to thecomplex crystal chemistry of staurolite. ^*Present address: Department of Geology & Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803