A DETAILED STUDY OF MORPHOLOGY AND CHEMISTRY OF GARNET CRYSTALS WITH SUGGESTION OF NEW SUBDIVISIONS: DATA FROM PELITIC SCHISTS, HORNFELSES AND APLITES OF HAMEDAN REGION, IRAN*

Document Type: Regular Paper

Author

Department of Geology, Faculty of Sciences, Bu Ali Sina University, Hamedan, I. R. of Iran

Abstract

The study area is a part of the Sanandaj-Sirjan metamorphic belt comprising low- to high-grade
regional and contact metamorphic rocks that are intruded by various plutonic bodies. Garnet crystals are
abundant in pelitic schists, amphibole schists, migmatites, hornfelses and aplites. They are mostly almandinerich
(XAlm>60) pyralspite garnets and show dodecahedron, trapezohedron and mixed (dodecahedrontrapezohedron)
forms. Garnets in pelitic schists have different forms, but in hornfelses and aplites they have a
mostly trapezohedron form. Sometimes, in some layers adjacent to each other that have experienced equal
pressure and temperature, observed crystal forms are various, which may be due to the changes in the
chemical composition of the mineral and its host rock. Absolute temperature may not be so important for
form variations but temperature gradients during crystal growth may have affected the crystal forms. In more
rapid temperature gradients the trapezohedron form is more common, but in slow gradients the dodecahedron
form is more common. Mn-rich/Ca-poor crystals have a pure trapezohedron form; Mn-poor/Ca-rich crystals
have a dodecahedron form and others have mixed forms.

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