Variety in mode of occurrence of staurolite and characteristics of chemical composition

This paper summarizes the variety in the mode of occurrence of staurolite and compiles its compositional features. Staurolite generally occurs in pelitic schists that underwent medium pressure (P) and medium-temperature (T) metamorphism (about 0.5–1.0 GPa, 500–600°C), which is represented by the presence of the staurolite zone in the Barrovian region. However, in not a few cases, staurolite is stable even in eclogite- and granulite-facies conditions, and also in andalusite-stable low-P conditions. Mineral parageneses are important to give a constraint to P-T conditions; i.e. staurolite itself cannot be an indicator for P- T conditions. Especially, the most important point is whether the staurolite-forming environments are SiO₂-saturated or not. In the SiO₂-undersaturated environments, Mg-rich staurolite [Mg/(Fe+Mg)=0.30] is stable over a wide range of P-T conditions, and is commonly associated with aluminous phases such as spinel and corundum. For example, Mg-rich staurolite is known to be stable even in ultrahigh pressure conditions, which was shown by the synthetic experiments and observation of natural rocks. Also in the granulite-facies rocks, staurolite occurs as inclusions in garnet, cordierite and plagioclase or as a matrix phase associated with spinel and corundum. Stability P-T field of staurolite is not known, but staurolite itself should be stable even in high-T conditions (=700°C). Compilation of natural staurolite compositions revealed that exchange between bivalent and trivalent cations should be considered for formula expression. In addition, comparison of staurolites formed under high-P and medium- or low-P conditions did not show a clear relationship between staurolite compositions and pressures. We cannot give a constraint to pressure conditions only from the staurolite compositions. Geothermometer using Fe-Mg exchange reaction between garnet and staurolite has been proposed, but Fe-Mg distribution coefficients are widely scattered even in the rocks that experienced mostly the same temperature metamorphism. Solid-solution properties of staurolite are still not well known.