Abstract
Garnet has many species because of its common isomorphism. In this study, a suite of 25 natural gem-quality garnets, including pyrope, almandine, spessartine, grossular, and andradite, were examined by standard gemological testing, LA-ICP-MS, FTIR, and Raman analysis. Internal stretching and bending vibrations of the SiO4-tetrahedra of garnet exhibit correlate with the type of cations in garnet’s dodecahedral position (A site) and octahedral position (B site). FTIR and Raman spectra showed that with the increase of the radius of Mg2+, Fe2+, Mn2+, and Ca2+ in A site, or the unit cell volumes of pyrope, almandine, spessartine, and grossular, the spectral peaks of Si–Ostr and Si–Obend modes shift to low wavenumber. Because of the largest cations both in A site (Ca2+) and in B site (Fe3+), andradite exhibited the lowest wavenumber of Si–Ostr and Si–Obend modes of the five garnet species. Therefore, garnet has correlations between chemical composition and vibration spectroscopy, and Raman or IR spectroscopy can be used to precisely identify garnet species.
Highlights
The popular garnet family is one of the most prolific mineral groups and branches into a spectrum of colors and gem types [1,2]
The four green samples did not appear red through the Chelsea filter
Analysis, can provide precise specifications of types and series according to a full compositional characterization
Summary
The popular garnet family is one of the most prolific mineral groups and branches into a spectrum of colors and gem types [1,2]. The garnets used in gems are the common silicate garnets with relatively simple chemical compositions [3]. The garnets belong to two subgroups and six end-members (species), known as the pyralspite subgroup (pyrope-almandine-spessartine; B = Al3+ and A = Mg2+ , Fe2+ , and Mn2+ , respectively) and the ugrandite subgroup Because of complex isomorphous substitution, the RI and SG values of some garnets deviate from the range of relative endmembers Such garnets cannot be determined by standard gemological methods [1]. Grossular end-members or species of garnets by vibrational spectra has not been proposed so far. Grossular, and andradite, were studied for the correlations among gemological orange-brown properties, composition, and spectroscopic features. It provides a detailed set of IF spectroscopic light) help the precise identification of garnet species without composition analysis
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