Raman Microspectroscopy of Garnets from S-Fibulae from the Archaeological Site Lajh (Slovenia)

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.

Editorial for the Special Issue “Modern Raman Spectroscopy of Minerals”

“…In the same context of characterisation and conservation of historical objects and materials, the comprehensive study of garnets in late-antique jewellery from the Roman, Lombardic, and Ostrogothic archaeological site Lajh (Kranj, Slovenia) by S. Kos et al [7] has to be mentioned. Raman spectroscopy of garnets and their inclusions indicated their assignment to type I and II garnets (almandine) [31] most likely originating from India, thus excluding other potential provenances, such as Sri Lanka, Bohemia, Portugal, or East Africa [10]. We would like to also mention the article on the CaSO4-H2O system in this context [7], because it is partially based on previous research on high-fired medieval gypsum mortars by the same team [1,34,35].…”

“…At least in parts of six out of seven studies published in this Special Issue, Raman micro-spectroscopy instruments according to the principle shown in Figure 1b were used [5][6][7][8][9][10]. Although only N. Böhme et al made use of the full imaging capabilities of such instruments [9], some groups employed Raman microscopes for performing in situ measurements in heating stages developed for microscopic observation [7][8][9], and others needed the spatial resolution for specifically choosing spots for single-point measurements within complex samples [6], including positioning in three-dimensional space for identifying mineral inclusions [10]. These possibilities demonstrate why microspectroscopy instruments have become relatively widespread in research laboratories specialising in Raman spectroscopy.…”

“…Especially in the cases of bassanite and anhydrite III, the presented spectra are the most complete examples found in the literature so far, which can be explained by difficulties in their measurements arising from auto-fluorescence emission and the instability of the latter with respect to its reaction with air humidity, whereas for comprehensive theoretical and practical investigations of the Raman spectra of gypsum and anhydrite II, we would like to draw the reader's attention also to the pioneering studies of B. J. Berenblut et al from the early 1970s [29,30]. The article by S. Kos et al provides interesting reference data for Raman studies of garnets [10]. Its introduction section is our recommended read for researchers interested in the different types of garnets [31,32] found in late-antique to early-medieval jewellery, and the results section provides Raman spectra of different compositional groups of almandine (Fe 3 Al 2 [SiO 4 ] 3 ) garnets (types I and II according to [31], see Figure 4b) and their inclusions.…”

“…This Special Issue includes technological developments and applications in the field of modern Raman spectroscopy of minerals in a broad sense, from natural mineral deposits and archaeological objects to inorganic phases in man-made materials. The studied minerals include fossil resins [4], typical rock-forming minerals (calcite, quartz, forsterite) [5], iron-sulphur species (e.g., mackinawite) [6], a range of sulphates (gypsum, bassanite, anhydrite III, anhydrite II [7]; celestine, barite [8]; ternesite [9]), as well as silicate minerals like garnets (e.g., almandine) [10].…”

Defects, Diffusion and Dopants in Sillimanite

“…This mineral is mainly found in metamorphosed rocks and its formation is dependent on temperature and pressure [6]. It is an economically important mineral in the industrial preparation of bricks, cement, ceramics, jewelry (e.g., sillimanite gold ring) and fine porcelain (e.g., table top) [7][8][9]. The industrial use of sillimanite is related to its unique chemical composition, thermal stability and the formation of mullite-rich aggregates [10].…”