Original paper

New typology and origin of tsavorite based on trace-element chemistry

Feneyrol, Julien; Giuliani, Gaston; Ohnenstetter, Daniel; Rondeau, Benjamin; Fritsch, Emmanuel; Fallick, Anthony E.; Ichang'i, Daniel; Omito, Edward; Rakotondrazafy, Michel; Ranatsenho, Madison; Lallier, Florent


New electron-microprobe analyses of 'tsavorites' from the Neoproterozoic Metamorphic Mozambique Belt deposits allow the characterization of green grossular according to its trace-element chemistry (V, Cr, Mn). Five chemical types are defined: type 1, vanadian grossular with V > Cr > Mn (in atoms per formula unit); type 2, vanadian grossular with V > Mn > Cr; type 3, Mn-bearing vanadian grossular with Mn > V > Cr; type 4, Mn-bearing chromian grossular with Mn > Cr > V; and type 5, Cr- and Mn-bearing grossular with Cr > Mn > V. These types are also characterized by different absorption spectra in the ultraviolet–visible–near infrared. Type 1 tsavorite spectra show a total absorption below 430 nm due to the high vanadium contents. Type 2 tsavorite spectra present the classical absorption bands of V. Types 3 and 4 tsavorite spectra display additional shoulders at 407 and 408 nm due to Mn2+, whereas spectra of Cr-bearing types 4 and 5 tsavorite show the two additional bands of Cr3+ at 697 and 701 nm. The different absorption spectra also indicate Fe2+-Ti4+ charge transfer. We measured OH equivalent to 0.08 to 0.38 wt% eq. H2O within the structure. Concentrations of vanadium, chromium and manganese are good chemical ''fingerprints'' for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar.


chemical typologyelectron microprobe analysisgarnetgemmologygeographic origingreen grossulartsavoriteuv-vis-nir absorption spectrav/cr ratio