Publication details

Characterization of new gem Fe-rich tsavorite from Tanzania



Year of publication 2022
Type Conference abstract
MU Faculty or unit

Faculty of Science

Attached files
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].

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