Crystal chemistry of Ti-bearing andradites
Armbruster, Thomas; Birrer, Julia; Libowitzky, Eugen; Beran, Anton
European Journal of Mineralogy Volume 10 Number 5 (1998), p. 907 - 922
published: Oct 5, 1998
manuscript accepted: Apr 27, 1998
manuscript received: Sep 23, 1997
ArtNo. ESP147051005006, Price: 29.00 €
Abstract Ti-bearing andradites (space group la3̄d) were investigated by single-crystal and powder X-ray diffraction methods, IR spectroscopy, optical microscopy and electron microprobe analyses. X-ray site population refinements and IR spectroscopy indicate that three of the structurally investigated crystals (two from Kaiserstuhl, Germany and one from Atlas mountains, Morocco) are andradites characterized by the schorlomite substitution Ti4+ → Si4+, where Ti4+ preferentially occupies the octahedral site and mainly Fe3+ occupies the tetrahedral vacancies. These garnets show a very low hydrogarnet substitution of the type O4H4 → SiO4 giving rise to only weak IR absorptions between 3400 and 3700 cm-1. Ti-bearing andradites, e.g., from the Zermatt area (Valais, Switzerland) and from San Benito Co. (Ca, USA), have the morimotoite substitution, Fe2+ + Ti4+ → 2 Fe3+, in the octahedral site coupled with a pronounced hydrogarnet substitution (O4H4 → SiO4) affecting the tetrahedral site. Comparison of electron microprobe analyses with site population refinements (X-ray data) indicates that in all analyzed Ti-bearing andradites Al is concentrated on the octahedral site. The increased cell dimension observed for all Ti-bearing andradites compared to endmember andradite is explained by repulsion between Ca (dodecahedron) and Ti4+ (octahedron) decreasing the shared octahedral-dodecahedral edge and increasing the shared tetrahedral-dodecahedral edge. This expansion of the tetrahedron leads to structural stress which is released by incorporation of large cations on the tetrahedral site. Depending on the geochemical environment, oxygen fugacity, and p,T conditions either the hydrogarnet (O4H4 → SiO4) substitution operates or tetrahedral Fe3+, Fe2+ substitutes for Si4+.