Crystal chemistry of Ti-bearing garnets with volcanic origin

The crystal chemistry of Ti-bearing andradites ("melanites") belonging to different pyroclastic units of Albani Hills (Lazio, Italy) has been investigated electron probe microanalysis (EPMA), single crystal X-ray diffraction (SCXRD), Mössbauer spectroscopy (MS) and X-ray Photoelectron Spectroscopy (XPS). The aim is to determine the correct cations distribution over the X(8-fold), Y(6-fold) and Z(4-fold) sites and accurate values of cations site populations. The analysed samples are characterized by a low Ti-content ( TiO_2 in the range 1.76-3.59 wt%) and cell edges in the range 11.996(3)-12.014(1). MS investigation reveald Fe to be present predominantly as Fe³⁺(Y) and subordinately as Fe²⁺(X). XPS analysis was accomplished according to the procedure developed by Malitesta et al.(1989). The fitting of Ti2p signals seem to indicate that two (octahedral Ti³⁺ and Ti⁴⁺) or three (octahedral Ti³⁺ and Ti⁴⁺, tetrahedral Ti⁴⁺) Ti species may occur in these specimen. From the combination of the results from different techniques the distribution of Al over the Y and Z sites has been deduced. By comparing the Albani Hills samples crystal chemistry with that of melanites from Mt. Vulture (Scordari et al., 1999), it has been found that in the latter samples the Z sites is occupied by Si, Fe³⁺ and Ti⁴⁺, whereas in the former by Si,Ti⁴⁺ and Al³⁺. Substitution mechanisms through which Fe and Ti enter the garnet structure are expected to be related to the geologic environment in which they occurr. It has been recently proposed that in samples from volcanic rocks schorlomite substitution, Ti⁴⁺(Y) + Fe³⁺(Z) leftrightarrow Si⁴⁺(Z) + Fe³⁺(Y) is predominant, associated to a low degree of hydrogarnet component, (SiO_4)^4- leftrightarrow (O_4H_4)^4-; on the contrary Ti-garnets from metamorphic rocks should be affected by morimotoite substitution Ti⁴⁺(Y) + Fe²⁺(Y) leftrightarrow 2Fe³⁺(Y) and a high degree of hydrogarnet component (Armbruster et al., 1998). The result obtained in the present work are in partial agreement with those of Armbruster et al.(1998). References Armbruster, T., Birrer, J., Libowitzky, Beran, A.(1998). Eur.J.Mineral., 10, 907. Malitesta, C., Losito, I., Scordari, F., Schingaro, E. (1995). Eur. J. Mineral., 7, 847-858. Scordari, F., Schingaro, E., Pedrazzi, G. (1999). Eur. J. Mineral., 11, 855-869.