Titanium-containing silicate garnets; II, The crystal chemistry of melanites and schorlomites Available to Purchase

A suite of eleven melanite and schorlomite garnets, mostly from alkaline igneous rocks, has beens tudied by means of the electron microprobe and Mössbauer spectroscopy. The TiO₂ contents of these garnets ranged from 2.5 to 16.5 weight percent. The Mössbauer spectra showed the presence of up to five distinct kinds of iron absorption doublets within the garnet structure: ferric iron in octahedral and tetrahedral coordination, and ferrous iron in dodecahedral, octahedral, and tetrahedral coordination, Fe²⁺in octahedral or tetrahedral coordination, rather than Ti, is responsible for the deep red-black color and correlated features in the optical absorption spectrum of these garnets. The Mössbauer parameters of the absorption due to tetrahedral ferrous iron are unusually small and are consistent with electron-hoppingp henomena. The coexistence of ferrous and ferric iron on both the octahedral and the tetrahedral sites is unusual for the garnet structure, and is believed to be suggestive of low oxygen fugacities at the time of formation of the garnets. The Fe³⁺/∑Fe as determined by Mössbauer spectroscopy varied from 0.00 to 0.26 and agreed generally with the total number of reducing cations as determined by wet chemistry or by calculation from the electron microprobe analysis, but in some of the samples, small amounts of other reducing species, presumably Ti³⁺, were also required to account for significant differences between the methods. Estimates of Ti³⁺/∑Ti for these garnets exceeded 0.10 in only one sample, for which the ratio was as high as 0.25.

Ti⁴⁺ ⇄ Si⁴⁺ or M²⁺ + Ti⁴⁺ ⇄ 2M³⁺ or Ti³⁺ = M³⁺.

The distribution of iron between the octahedral sites and those tetrahedral sites not occupied by Si is compatible with the scheme established for Fe³⁺ in synthetic garnets only if the oxidation states of the iron cations are disregarded. This implies that, at high temperatures, extensive electron-hopping between Fe cations and presumably Ti cations occurs rapidly so that no distinction of different valence states of the Fe and Ti cations can be made with respect to cation exchange