十字石の産状の多様性と化学組成上の特徴

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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.
日本鉱物科学会の論文
2004-11-30

十字石の産状の多様性と化学組成上の特徴

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