Publication details
Characterization of new gem Fe-rich tsavorite from Tanzania
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| Year of publication | 2022 |
| Type | Conference abstract |
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| Description | The colour of grossular garnet depends on elements that replace Al at octahedral site; depending on contents of Cr, V and Fe, it may gain colours in shades of green, yellow, brown, or colourless to pale yellow [1]. A highly valued conspicuously green grossular variety called tsavorite is produced from deposits in Kenya and Tanzania [2]. New type of gem tsavorite with atypical zonal development (colourless with a pale yellowish tinge to intense green – figure 1a) was discovered in the area of Lelatema Mountains, Tanzania [3]. To better understand the nature of its colour and determine its correct classification, we studied the material using EPMA, LA-ICP-MS, Raman and UV-vis-NIR spectroscopy. The studied tsavorite is grossular with 16-41 mol% of andradite component, high Cr and negligible V contents; this is in contrast to typical tsavorite characterized by very low Fe-contents and V»Cr [4]. EPMA and LA-ICP-MS results (figure 1b) show that the andradite component contents are distinct for each of the bicoloured grains (avg. ca. 17, 20, and 37 mol%); variably coloured zones differ mainly in trace element contents (Cr <5000 ppm, V <190 ppm, Ti <3500 for the green zones, and Cr <1100 ppm, V <110 ppm, Ti <3000 for the colourless). All zones show identical grossular Raman spectra with minimal fluorescence under 532 and 473 nm lasers. Under LW- and SW-UV radiations it shows deep red and brownish yellow fluorescence, respectively. UV-vis-NIR spectra confirm Fe3+ and Cr3+ (absorption bands at 451, 609, and 889 nm) as main chromophores. The studied material is the only occurrence of Fe-rich Cr-dominant gem tsavorite [4]. |