Chemical analyses provide very little information about the site distribution and valence of titanium in garnet. Mossbauer spectra of titanian garnet specimens show that as titanium enters the structure of andradite. ferric iron is forced into the tetrahedral site, displacing silicon. The spectra also suggest that a small amount of ferrous iron is in tetrahedral coordination. The amount of tetrahedral iron is not sufficient, however, to account for all of the deficiency in silicon, and although one would suppose that tetrahedral aluminum accounts for most of the remainder, the possibility of some tetrahedral titanium remains open.

The widely-quoted arguments of Tarte (1961), supposedly demonstrating the existence of tetrahedral titanium from IR spectra, are refuted by reference to spectra of synthetic garnet containing tetrahedral Fe3+ and Al: the features in the titanian garnet spectra allegedly due to tetrahedral titanium are better explained as due to Fe3+ and Al.

Optical spectra of Ti-garnet from all but one locality, in which the presence of an interfering contaminant is suspected, show no evidence for the presence of Ti3+. The dark brown color of titanian garnet is due to charge-transfer absorptions. A band in the near infrared can be assigned to tetrahedral ferrous iron, tending to corroborate the Mössbauer evidence.

The preference for Fe3+ over Ti4+ in tetrahedral coordination may be ascribed to covalency factors. Values of isomer shift for the tetrahedral site, measured in Mossbauer spectra, suggest that the bonding of this site is more covalent than normal for tetrahedral sites in oxides and silicates.

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