Crystal Chemistry of Ti-bearing andradites
- Autor(en)
- Thomas Armbruster, Julia Birrer, Eugen Libowitzky, Anton Beran
- Abstrakt
Ti-bearing andradites (space group Ia.hivin.3d) 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 3 of the structurally investigated crystals (2 from Kaiserstuhl, Germany and one from Atlas mountains, Marocco) 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 concd. on the octahedral site. The increased cell dimension obsd. for all Ti-bearing andradites compared to end-member 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 geochem. environment, O fugacity, and p,T conditions either the hydrogarnet (O4H4 ® SiO4) substitution operates or tetrahedral Fe3+, Fe2+ substitute for Si4+.
- Organisation(en)
- Institut für Mineralogie und Kristallographie
- Externe Organisation(en)
- Universität Bern
- Journal
- European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
- Seiten
- 907-921
- Anzahl der Seiten
- 15
- ISSN
- 0935-1221
- Publikationsdatum
- 1998
- Peer-reviewed
- Ja
- ÖFOS 2012
- 105116 Mineralogie, 104011 Materialchemie
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/crystal-chemistry-of-tibearing-andradites(91c2a4e2-f7b8-4325-a377-799ff97bb10d).html