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

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 almandine- rich (XAlm > 60) pyralspite garnets and show dodecahedron, trapezohedron and mixed (dodecahedron- trapezohedron) 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.