Abstract
Garnets (19 pieces) of Late Antique S-fibulae from the archaeological site at Lajh-Kranj (Slovenia) were analysed with Raman microspectroscopy to obtain their mineral characteristic, including inclusion assemblage. Most garnets were determined as almandines Type I of pyralspite solid solution series; however, three garnets showed a higher Mg, Mn and Ca contents and were determined as almandines Type II. Most significant Raman bands were determined in the range of 169–173 cm−1 (T(X2+)), 346–352 cm−1 (R(SiO4)), 557–559 cm−1 (ν2), 633–637 cm−1 (ν4), 917–919 cm−1 (ν1), and 1042–1045 cm−1 (ν3). Shifting of certain Raman bands toward higher frequencies was the result of an increase of the Mg content in the garnet composition, which also indicates the presence of pyrope end member in solid garnet solutions. Inclusions of apatite, quartz, mica, magnetite, ilmenite, as well as inclusions with pleochroic or radiation halo and tension fissures (zircon), were found in most of the garnets. Rutile and sillimanite were found only in garnets with the highest pyrope content. Spherical inclusions were also observed in two garnets, which may indicate the presence of melt or gas residues. The determined inclusion assemblage indicates the formation of garnets during medium- to high-grade metamorphism of amphibolite or granulite facies. According to earlier investigations of the garnets from Late Antique jewellery, the investigated garnets are believed to originate from India.
Highlights
The production of ornamented metal jewellery decorated with red garnets has a long history in both European and Eastern cultures, dating back to the 3rd century BC
Shifts of the Raman bands to higher frequencies, e.g., modes assigned to the translation mode of X2+, the rotation mode of SiO4 tetrahedron and the Si–O stretching mode, indicate the presence of higher pyrope content in the solid solution for the three garnets
According to the observed trends in Raman shifts of vibrational modes in the obtained Raman spectra, the presence of three groups of almandines on the examined S-fibulae was determined, each of which differs in its pyrope content, while spessartine and grossular components were present in minor amounts
Summary
The production of ornamented metal jewellery decorated with red garnets has a long history in both European and Eastern cultures, dating back to the 3rd century BC. The investigation of geographical and geological origin of garnets using chemical and mineralogical characterisation is and remains the subject of many studies, from the perspective of understanding the deposits of minerals and trade routes that linked them to the rest of the world, since large amounts of gemstones have been used in jewellery production [2,3,4,5,6,7,8,9,10]. The determination of the structure, composition and physical properties of minerals from archaeological artefacts is difficult since they should not be compromised. In this case, non-destructive analytical methods can be used.
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