Study on Spectral Characteristics and Color Origin of Scheelite from Xuebaoding, Pingwu County, Sichuan Province, P.R. China

The Xuebaoding deposit, which is located 14.5 km northwest of Huya Town, Pingwu County, Mianyang City, Sichuan Province, China, produces a kind of yellow-orange-hued scheelite with ideal crystal shapes, large-grain crystals, and high market value which is favored by gem and mineral crystal collectors. In this article, five Xuebaoding scheelite samples are used as research objects. The infrared absorption in the fingerprint region (2000 to 400 cm−1) of scheelite is at 440 cm−1 and 800 to 900 cm−1, which shows the out-of-plane bending vibration and asymmetric stretching vibration attributed to the [WO4]2− tetrahedral group, respectively. The Raman shift at 911 cm−1 is assigned to the ν1 symmetric stretching vibration of [WO4]2−; the Raman spectra scattering peak at 797 cm−1 belongs to the ν3 asymmetric stretching vibration of [WO4]2−; the Raman shift at 332 cm−1 and the low-intensity Raman scattering peak near 400 cm−1 belong to the ν2 out-of-plane bending vibration of [WO4]2−. Furthermore, the low-intensity Raman shift around 211 cm−1 is caused by the transitional mode of (Ca–O). The UV-Vis-NIR absorption is attributed to the existence of “didymium”, a mixture of the rare earth elements Pr and Nd, and the absorption at 584 and 803 nm is assigned to Nd, which may be related to origin of the color of scheelite. The 3D fluorescence spectra show that the colorless and colored scheelite samples produce the same number of main fluorescence peaks with similar positions. Those 3D fluorescence peaks are located near λex235 nm/λem455 nm, λex250 nm/λem490 nm, and λex265 nm/λem523 nm. In addition to the above-mentioned main fluorescence peaks, the pale-yellow-colored samples also produced fluorescence peaks near λex250 nm/λem425 nm, which may be associated with the rare earth elements in scheelite. Combined with the test results of LA-ICP-MS, the yellow-orange hue of Xuebaoding scheelite is caused by the isomorphic rare earth elements, such as La, Ce, Pr and Nd ions that replace Ca2+.