俄罗斯翠榴石的化学成分及光谱学表征

doi:10.3964/j.issn.1000-0593(2019)12-3849-06

Abstract
References
Related Citation (15)

Download: PDF (2562 KB)
Export: BibTeX | EndNote (RIS)

Abstract Garnet is the most valuable subspecies in garnet family, and is popular for its beautiful appearance and rarity. In particular, demantoid from Russia is the most sought after object of domestic and foreign collectors. Previous studies were more on garnet minerals from different perspectives, but there are few studies on the demantoid. To study the chemical composition and spectroscopic characteristics of the Russian demantoid, the systematic research is conducted by using LA-ICP-MS, IR spectrum, Raman spectrum, UV-Vis absorption spectrum, so as to obtain the chemical components, especially rare earth elements and spectroscopic characteristics, and to analyze the causes of colour, and provide important data for its variety identification and traceability of origin. Chemical research shows that the demantoid is almost pure Andradite (Andradite>96.39 Mol.%). Among the secondary components, Cr₂O₃ content is relatively high, with an average of 0.502 Wt%. Cr and V are chromophore which causes the green color in garnet. Content of rare earth elements generally is low, ∑REE with an average of 4.85 μg·g^-1; And the light rare earth elements are significantly enriched, ∑LREE with an average of 4.56 μg·g^-1; Heavy rare earth elements relative loss, ∑HREE with an average of 0.29 μg·g^-1, ∑LREE/∑HREE=5.35~100.48. Most samples show Eu positive anomaly. The main Raman shifts are 995, 874, 841, 815, 576, 552, 515, 492, 451, 369, 351, 323, 311, 295, 263, 235, and 172 cm^-1. Raman spectra can only be used as one of the methods for the identification of variety, and have little effect on determination of its origin. The infrared spectroscopy studies show that the infrared spectra of the fingerprint region can be used to identify the demantoid, and the functional region shows the absorption peak of the structure water, which indicates that the Russian garnet contains a small amount of structural water, which is relate to its formation with hydrothermal process. Studies on UV-Vis absorption spectra show that Russian demantoid has an obvious absorption peak at 384 and 440 nm, a weak absorption peak at 436 nm, a wide absorption band near 620 nm, and a strong absorption from 500 nm to the ultraviolet region. The 440 nm absorption band was attributed to the ⁶A₁ to ⁴A_1g+⁴E_g(G) d—d transition of Fe³⁺ in octahedral site. The 620 nm absorption band was attributed to the ⁴A_2g(F) to ⁴T_2g(F) d—d inhibiting transition of Cr³⁺ in octahedral site. Fe and Cr are both chromogenic elements, and the O-Fe Charge transfer band and the 440 nm strong absorption band produce yellow and yellow-green garnet. The addition of Cr³⁺ produce a 620 nm wide absorption band, which absorbs orange light and makes the gem color shift to the green, producing the pure green demantoid. The fingerprint characteristics of Raman spectrum and infrared spectrum can be used for the accurate identification of demantoid. The characteristics of rare earth elements and the structural water features of functional groups in the middle infrared spectrum can provide important information for its origin determination.

Key words：Demantoid; Russia; REE; Raman spectrum; Infrared spectrum; UV-Vis spectrum; Origin traceability

Received: 2019-03-04 Accepted: 2019-07-15

ZTFLH:

P575.4

Cite this article:

URL:

https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2019)12-3849-06 OR https://www.gpxygpfx.com/EN/Y2019/V39/I12/3849