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

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

来源：European Journal of Mineralogy, 2014, 26(2): 293-308.

DOI：10.1127/0935-1221/2014/0026-2367

• tsavorite
• green grossular
• electron microprobe analysis
• UV-Vis-NIR absorption spectra
• chemical typology
• V/Cr ratio
• garnet
• geographic origin
• gemmology

New electron-microprobe analyses of %26apos;tsavorites%26apos; 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 %26gt; Cr %26gt; Mn (in atoms per formula unit); type 2, vanadian grossular with V %26gt; Mn %26gt; Cr; type 3, Mn-bearing vanadian grossular with Mn %26gt; V %26gt; Cr; type 4, Mn-bearing chromian grossular with Mn %26gt; Cr %26gt; V; and type 5, Cr- and Mn-bearing grossular with Cr %26gt; Mn %26gt; 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 %26quot;fingerprints%26quot; for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar.

• 出版日期2014-3