STRUCTURE AND TWINNING IN MG-AND MN-RICH ZONED GARNET FROM THE TOCANTINS RHODOLITE GARNETS, TOCANTINS STATE, BRAZIL

The Tocantins area of North-East Brazil produces both gem quality and highly included rhodolite garnets from a working mine on the Rodolita Farm. The garnets occur in mylonitic schist lenses of kyanite-staurolite grade in migmatitic gneiss. These garnets contain high Mn cores and low Mn rims and increase in Mg from core to rim creating a color change from red in the core to purple in the rims. Fe content is within error from core to rim for all garnets analyzed. Preliminary analysis of the inclusions in the garnets range from aligned quartz, and Fe-Ti oxides. The substitution of Mg for Mn in the rims of the garnets not only produces a color change but cause fracturing perpendicular to the chemical zoning.

Mineral crystal structure is a primary tool used in understanding the intricate history of geologic environments. Decoding the crystal structure of garnets enables the determination of thermal and chemical conditions in both igneous and metamorphic systems that created them. We present an examination of the crystal structure and garnet mineral chemistry on the internal structure of almandine-spessartine-pyrope garnets. By using X-ray diffraction and secondary electron microscopy we have determined that twinning is the dominant control in fracture and crystal shape. Twinning occurs as pseudomerohedral ferroelastic twin lamellae that have preferred orientation that of twin domain boundaries parallel to (101).