GEOCHEMICAL UNDERSTANDING AS TO THE CRYSTAL-STRUCTURE OF THORTVEITITE- THE ULTIMATE IN REE FRACTIONATION AND THE END OF MAGMATISM

A new geochemical understanding as to the crystal structure of thortve itite in the granite pegmatite, Isanago Mine, Kyoto Prefecture, Japan, has been developed by refinement on the X-ray single-crystal diffract ion data and penetration into the genetic environment. The structure o f thortveitite is described as built up by the stacking of the hexagon al close packing Sc2O3 layers separated by a clue of Si2O7 composition . The larger isotropic temperature factor (2.473 Angstrom 2) and the o ver bonded state (2.124 vu: valence unit) for the bridging O(1) in Si2 O7 pyroanions of thortveitite may signify a ''soft'' mode of vibration , due to the repulsive power driving from the latter, for the ordered oxygen atoms containing a component of dynamic positional disorder. Cr ysal structure of thortveitite geochemically matches well with topolog ical combination of corundum and cristobalite in a polyhedral configur ation, because the former (corundum) is isostructural with the sheet o f Sc octahedra in thortveitite and the latter consists of the framewor k of only staggered Si2O7 groups. This consistency and the geochemical character of SC3+, inherent in thortveitite, have an implication for the ultimate in REE fractionation of magmatic process and the specific in genetic environment. In the process of igneous activity, the primi tive stage naturally suggests that crystallization of silicate mineral s poor in silica occurs in the primary melts, and then after a variety of igneous processes concentrating silica in the residues, the last s tage also unfolds to us the observation that silica-poor minerals such as thortveitite, garnet and fayalite crystallize in the silica-poor e nvironment within the igneous acid rocks. Magmatism comes to an end at the stage of returning to the originate crystallization of silica-poo r minerals.