Spectra Characterization of Cr-Grossular (Tsavorite) With “Frogspawn” Color Zoning From Canada
MAO Xiao-tian1, CHEN Chang2, YIN Zuo-wei1*, WANG Zi-min1
1. Gemmological Institute, China University of Geosciences (Wuhan), Wuhan 430074, China
2. Yunnan Land and ResourcesVocational College, Kunming 652501，China
Abstract：The gem Cr-grossular (also called tsavorite) from Canada often has a special green color zoning. The black or dark green core is often surrounded by an emerald green edge, which looks like a “frogspawn”. To study the gemological and spectral characteristics of the Canadian tsavorite with “frogspawn” color zoning, the samples from this area were systematically studied by standard gemological methods, as well as LA-ICP-MS, UV-Vis-NIR spectrum and Raman spectrum, which reveal the causes of the color zoning and the changes of composition and spectroscopy in different color zones. The chemical composition analysis shows that the Canadian tsavorite is mainly composed of grossular (Core: Gro>55.64 mol%; rim: Gro>83.90 mol%), but the content of Cr2O3 varies significantly in different color zones. The deep-colored core in the center shows high uvarovite content (Uva average 21.49 mol%). The black core also has high Ti content (TiO2>1.9 Wt%). In addition, the samples also contain a small amount of Fe and a trace amount of V, Mg and Mn. UV-Vis-NIR spectra show that Cr3+ is the main ion causing green color and Fe mainly induces yellow color in samples. The absorption bands at about 435nm in the blue violet region and 603 nm in the red region are mainly due to Cr3+. The double peak at about 700 nm of Cr3+ can be used as the characteristic peak to distinguish the presence of V3+. Fe3+ causes an absorption peak at about 370 nm and contributes to the absorption band at 435 nm in the blue-violet region. Fe2+ causes the broad and weak absorption band at 1 200 nm. By analyzing the ratio of Fe2O3 to Cr2O3+V2O3, color tones of grossular can be effectively distinguished. When the ratio is less than or near 1.61, it often shows pure emerald green; When the ratio is near 2.71, it often shows yellowish-green; When the ratio is around 4.38, it often shows greenish-yellow. Raman spectrum analysis shows that the sample’s yellowish-green to emerald green regions are typical grossular. The Raman peaks between 800 and 1 100 cm-1 are mainly caused by the stretching vibration of [SiO4] tetrahedron; The Raman peaks between 400 and 700 cm-1 are mainly caused by the bending vibration of [SiO4] tetrahedron; The Raman peaks below 400 cm-1 are mainly caused by lattice vibration. Based on the comparison between the Raman spectra of color regions of the sample and natural uvarovite, the increase of Cr3+ content in the mineral structureleads to regular Raman peak shift, as the testing point moves from the edge of the sample to the black core.
Key words：Cr-grossular; Tsavorite; Color zoning; Color genesis; UV-Vis-NIR spectrum; Raman spectrum
毛晓天，陈 畅，尹作为，王梓民. 加拿大具“蛙卵”状色带铬钙铝榴石（沙弗莱石）的谱学特征[J]. 光谱学与光谱分析, 2023, 43(02): 520-525.
MAO Xiao-tian, CHEN Chang, YIN Zuo-wei, WANG Zi-min. Spectra Characterization of Cr-Grossular (Tsavorite) With “Frogspawn” Color Zoning From Canada. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2023, 43(02): 520-525.
