Yankuang ceshi (Nov 2020)
Study on REE Distribution and Mineralogical Characteristics of Different Garnets by Electron Probe and Inductively Coupled Plasma-Mass Spectrometry
Abstract
BACKGROUND Garnet is a common silicate mineral in metamorphic and magmatic rocks, and its isomorphism is very common. The existing data show that the color of garnet with different composition is quite different, but the relationship between the composition and color of garnet has not been systematically studied. OBJECTIVES To reveal the internal relationship and variation law of garnet composition, structure and color, and provide a basis for the summary and geological application of the mineralogical characteristics of garnet in different geological environments. METHODS Common red (G1), orange (G2), green (G3) and maroon (G4) garnet have been tested systematically by electron microprobe, inductively coupled plasma-mass spectrometry, X-ray powder crystal diffraction, Raman spectroscopy, infrared spectroscopy and ultraviolet-visible absorption spectroscopy. RESULTS The results showed that the samples of G1 and G4 contained more Fe (Fe3+:0.24%, 0.24%, Fe2+:1.01%, 0.89%). The samples of G2 contained higher Mn (2.76%), whereas the samples of G3 have higher Cr and V contents of 3453×10-6 and 1458×10-6, respectively. Isomorphic substitution greatly affected the crystal structure of garnet. The cell parameters were a=11.530nm(G1), 11.563nm(G2), 11.849nm(G3) and 11.470nm(G4). Trace and rare earth elements in garnet can be used to indicate the source and formation process. The rare earth element analysis showed that the total rare earth elements of garnet were distributed unevenly, and the ratio of LREE/HREE was less than 1, with enriched heavy rare earth elements. The Eu/Eu* ratio was less than 1, which was a negative Eu anomaly. Ce abnormalities of all samples were not obvious. G1 and G4 have Fe3+ electronic transition absorption peak at 570nm. G2 has Mn2+ electronic transition absorption peak near 460nm and 520nm, whereas G3 has Cr3+ electronic transition absorption peaks at 690nm. The Raman spectra of garnet samples showed obvious differences in peak intensity and position, which also reflected the ubiquitous existence of isomorphism in these garnets. The ultraviolet-visible absorption spectra of these garnets showed high consistency with its color and characteristic elements. The absorption peaks of Fe3+ in red and maroon samples at 570nm were related to the high content of Fe, while the characteristic absorption peaks of orange sample near 460 and 520nm belong to Mn2+, corresponding to the large amount of Mn (2.76%). A strong absorption peak was observed at 690nm in the green sample, which was caused by the transition of Cr3+ and the presence of trace element Cr (3453×10-6). The results showed that the color of garnet had a good correspondence with its composition and structure. CONCLUSIONS The color characteristics of garnet can be used as a typomorphic feature of minerals to indicate the existence of different characteristic elements. These methods can be used to study the isomorphism and color origin of garnet effectively.
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