Caracterização cristaloquímica de minerais ricos em titânio: perovskita e ti-granada da região de Tapira, MG

Abstract

Perovskite and Ti-garnet crystals of the region of the Tapira Alkaline Complex, localized in the city of Tapira, Minas Gerais State, have been investigated. Both minerals are of centimetric size, of grey to black color and the crystals are well formed, with octahedral and rhombohedral form, in its majority. The perovskite has been characterized by physical-chemistry analyzes (X-ray Fluorescence, Inductively coupled plasma-optical emission spectrometry and Electron microprobe), Infraredspectroscopy and X-ray powder and single crystal diffraction. Through chemical analyzes it was observed that the mineral is basically the perovskite sensu stricto (CaTiO3), however, some substitutes had been considered for the structure refinement, as Na+, Ce3+ and Fe3+. Theallocation of these minority cations in the structure was based on the loparite (Na0,5Ce0,5TiO3) and latrappite (CaNb0,5Fe0,5O3) structures, which are minerals from the perovskite group. The Infrared Spectroscopy indicated that the perovskite of Tapira is an anidrous mineral and,crystallochemical analyses show that the co-ordinated polyhedra for TiO6 are octahedron without distortions while the polyhedra for the CaO12 are totally distorted. The point group for each one was established as being Oh and C1, respectively. The refinements of the structure had been carried through in the space group Pbnm and the established final chemical formula for the perovskite is: (Ca2+0,95(1) Ce3+0,011(2) Na1+0,010(4))( Fe3+0,022(2) Ti4+0,98(1))O2-3. The schorlomite has been characterized by physical-chemistry analyzes (Electron microprobe), Mössbauer spectroscopy, Infrared spectroscopy, Termogravimetric analysis and X-ray powder and single crystal diffraction. The chemical analyzes had indicated great isomorphic substitutions in the structure of the mineral, the majority ions are Ca2+, Fe3+, Fe2+, Ti4+ and Si4+; and the minority ions are Mg2+, Mn2+, Al3+ and Zr4+. However, Mn2+ and Zr4+, as adding less than 1% wt of oxides, had been ignored for the cationic distribution in the structure refinement. Correlations established for the chemical analyzes had indicated that Mg2+, Fe2+ and Fe3+ occupy the same site. The Mössbauer spectroscopy showed that Fe2+ occupies the dodecahedron and octahedron sites while Fe3+ occupies the octahedron and tetrahedron sites. The Infrared spectroscopy had been useful in the allocation of the Ti4+, with referring bands for the 6 co-ordination number of this ion; and together with the Termogravimetric analysis it showed that this mineral is anidrous. The structure refinement on X ray single crystal diffraction had been made considering the space group Ia d3 and the final chemical formula:{Ca2+2,98(2) Fe2+0,04(1)}3,02 [Mg2+0,16(2) Ti4+1,00(1) Fe3+0,65(2) Fe2+0,20(1) Al3+0,13(1)]2,14(Si4+2,49 (1)Fe3+0,49(6))2,98 O2-12.