Ti-RICH ANDRADITES: CHEMISTRY, STRUCTURE, MULTI-PHASES, OPTICAL ANISOTROPY, AND OSCILLATORY ZONING

Electron probe microanalyzer (EPMA) data were obtained for 11 Ti-rich andradite garnet, ([8])X(3)([6])Y(2)([4])Z(3)([4])O(12), samples from Magnet Cove, Arkansas, USA. The crystal structures of nine of these samples were refined with the Rietveld method, space group la (3) over bard, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. One sample (MC9) contains two distinct compositions corresponding to different phases and subtle compositional differences are observed in two other samples (MC8 and MC10). The EPMA data falls on a straight line representing solid solutions from the end-member andradite, Ca3Fe23+Si3O12, to a composition, Ca-3(Ti4+ Fe-1.5(0.5)2+)[Fe3+Si2]O-12, with the coupled substitution: Fe3+ + Si4+ <-> Ti-1.5(4+) + Fe-0.5(2+). The Y site contains Ti4+ cations (up to approximate to 1.1 atom per formula unit, apfu). A few samples show oscillatory zoning that arises mainly from the distribution of Ti atoms coupled with subtle variations in Fe, Al, and Mg atoms. The HRPXRD results show that seven samples contain two or three cubic phases intergrown together. These seven samples exhibit birefringence because of strain arising from structural mismatch. There are three Ti-rich schorlomite samples (MC1, MC5, and MC7) of which MC1 and MC7 are single cubic phases and are isotropic. The bond distances (average < X-O >, Y-O, Z-O, and mean < D-O >) change linearly with increasing a unit-cell parameter. The Y-O distance is nearly constant. The Z-O distance increases most because of Fe3+ (0.49 angstrom) substituting for Si4+ (0.26 angstrom) cations at the Z site.