Gemmological Characteristic and Cause of Iridescence of Garnet from Balin Left Banner, Inner Mongolia, China

The Baiyinnuoer lead-zinc deposit of Balin Left Banner, Inner Mongolia, China, produces a kind of large garnet crystal with well-developed crystal surface and strong iridescent effect. It is favored by gem and mineral collectors. Seven iridescent garnets from Baiyinnuoer were selected as the research objects for clarifying the gemmological characteristics and iridescent effect mechanism of the mineral. Fourier transform infrared spectroscopy, ultraviolet-visible spectrscopy, laser microscope confocal Raman spectroscopy, electron probes and scanning electron microscope were used in this study. The infrared spectra showed that the absorption of iridescent garnet is mainly caused by the vibration Si-O tetrahedrons and in the fingerprint area absorption peak at 435 cm^-1 belongs to ν₂ symmetric bending vibration, and the absorption peak at 470~590 cm^-1 are assigned to ν₄ asymmetric bending vibration, while the absorption peak at 810~935 cm^-1 is caused by ν₃ asymmetric stretching vibration. Raman spectra showed that the Raman shifts of 700~1 000 cm^-1 are classified as (Si-O)_str vibration, 400~600 cm^-1 are classified as (Si-O)_bend, 300~400 cm^-1 are caused by R(Si-O)₄, 150~300 cm^-1 is related to T(Ca), and the remaining Raman shifts are caused by T(SiO₄). The UV-Visible spectra only showed two absorption peaks, located at 408 nm and 438 nm, respectively, related to the transition of Mn²⁺ to the first field independent state ⁴A₁⁴E and the d-d electron transition of Fe³⁺ in the octahedral field ⁶A₁→⁴A₁⁴E(G). The electron probe testing results showed that the chemical formula calculation result of garnet is (Ca_3.011Mn_0.035Mg_0.006)(Fe_1.796Al_0.188)Si_2.977O₄₃, which belongs to the series of calcium iron garnet to calcium aluminum garnet. Under the backscattered electron, there are stripes intersecting light and dark related to Fe³⁺ and Al³⁺, and the average content of Fe³⁺ in the light area is about 2% higher than that in the dark area, while Al³⁺ is correspondingly about 2% lower. Under scanning electron microscopy, it was found that the thickness of layered structure of the sheet (211)_⊥ and sheet (110)_⊥ were different. The the former was 400~600 nm on average, while the latter was uneven, with the maximum thickness of about 24 μm. This indicates that the iridescence effect of the iridescent garnets from Baiyinnuoer is caused by thin-film interference. According to the optical path difference formula combined and the refractive index of iridescent garnet, it was calculated that the theoretical layer thickness range of iridescent garnet that can produce thin film interference effects is between 438.08 nm and 766.65 nm.