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International audience; 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.

Translucent pink opals from Mexico (states of Mapimi and Michoacan) and Peru (Acari area, near Arequipa) are opal- CT, containing from 10 to 40 % palygorskite, as demonstrated by XRD, infrared absorption and specific gravity measurements. Their nanostructure is unusual, with bunches of fibres 20 to 30 nm in minimum diameter, related to the fibrous nature of paly- gorskite crystals, as demonstrated by electron microscopy. A complex absorption centred at about 500 nm is the cause of the pink colour. It is proposed that the absorption is due to quinone fossil products associated with the phyllosilicate fibres. The Raman spectrum of monoclinic palygorskite is deduced from that of its mixture with opal. The opal-CT-palygorskite-quinone associa- tion is a geological marker of a specific environment, presumably of a fossil lake environment in a volcanic region.

Raman spectroscopy provides structural data useful for opal classification. Because it is sensitive to short-range order and the material water content, this non-destructive method offers more information than the classical classification of Jones and Segnit (1971), based on X-ray diffraction. The position of the apparent maximum of the main Raman band at low wavenumbers can be used to classify opals on the basis of their degree of crystallinity. The most amorphous opals (those from Australia in our study) have a maximum beyond 400 cm (super -1) , the best crystallized ones (those from Mexico in our study) around 325 cm (super -1) . Brazilian opals occupy an intermediate position. Two preliminary results need to be verified on a much larger sampling including all types of known opal deposits: the position of the various bands of Raman spectrum seems to be characteristic of the geographical origin of the sample (at the scale of the geological province), and also of the type of geological origin (volcanic versus sedimentary).