赞比亚、坦桑尼亚和澳大利亚紫红-棕红色石榴石的成分和光谱学对比

doi:10.3964/j.issn.1000-0593(2022)01-0184-07

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Abstract Purple-brownish red garnet is the best-selling garnet variety, that from different origins often have various prices owing to particular hues and clarity, so it is necessary to determine the origins. Tanzania and Australia are two main provenances of purple-brownish red garnets, while Magodi, Zambia, is a newly discovered locality. We compared the chemical composition and spectroscopic characteristics of samples from these localities. LA-ICP-MS testing shows that all samples belong to the pyrope-almandine series. The linear discriminant analysis based on 17 chemical components can distinguish different localities with 96.7% cross-validation accuracy. MgO, FeO, MnO, Co, etc., have larger weights in discrimination than other components based on standardized canonical discriminant function coefficients. According to the distribution curve of rare earth elements, Australian garnets perform an obvious upward trend in heavy rare earth elements. The ratio between the total content of heavy rare earth elements and light rare earth elements was various among localities, 9-175 in Zambia, 50-164 in Tanzania and 191-334 in Australia. With the increase of MgO and the decrease of FeO, Raman peaks related to Si-O stretching vibration, Si—O bending vibration and [SiO₄] rotation shift to the direction of high wavenumber. The offset of wavenumber is linearly correlated to the content of Mg and Fe. Thus, the Australian samples can be roughly distinguished by Raman peaks due to lower MgO content. Garnets from three origins also have a difference in hue. Ultraviolet-visible absorption spectra show that the hues are mainly affected by the absorption strength at 425 nm. The ratio between the absorbance at 368 and 503 nm can be used to distinguish three localities, of which Australia is more significant than 1.3, Zambia is between 0.8 and 1.3, and Tanzania is less than 0.8.

Key words：Garnet; LA-ICP-MS; Linear discriminant analysis; Raman spectrum; UV-Vis absorption

Received: 2020-11-23 Accepted: 2021-03-02

ZTFLH:

P575.4

Corresponding Authors: Andy Hsitien Shen E-mail: ahshen@foxmail.com

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http://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2022)01-0184-07 OR http://www.gpxygpfx.com/EN/Y2022/V42/I01/184