Gemmological and Spectroscopic Characteristics of A Type of Trapiche Garnet

As gemmological and mineralogical research progresses, the trapiche or trapiche-like phenomenon is increasingly discovered in various gemstones, which enriches the gemstone market and promotes understanding of crystal growth and related mineral geological origins. While the concept of trapiche garnet is occasionally mentioned, detailed information on its specific characteristics remains scarce. In this study, the authors comprehensively investigated the gemmological and mineralogical characteristics of a group of trapiche garnets utilizing conventional gemmological methods, Fourier transform infrared spectrometer (FTIR), Raman spectrometer, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), electron probe microanalysis (EPMA), ultraviolet-visible (UV-Vis) absorption spectrophotometer and X-BOX1.0 scanner. Based on EPMA testing results and anion method calculations, the chemical formula of the samples is determined to be Ca_2.97(Al_1.97Ti_0.02Fe_0.02Mg_0.01)_2.02(Si_2.97Al_0.03)_3.00O₁₂ and Ca_2.95(Al_1.98Ti_0.01Fe_0.02Mg_0.01)_2.02(Si_2.98Al_0.02)_3.00O₁₂. Combined FTIR and Raman analyses, it was confirmed that the samples are as grossular garnets containing a small amount of structural water. The LA-ICP-MS testing results showed that the samples mainly contained the trace elements such as Na, Mg, K, Cr, and Fe. The UV-Vis spectrum shows absorption at 315, 370, 410 nm. Based on trace element analysis, UV-Vis spectrum data, and previous studies on the colour of garnet, it is preliminarily speculated that Fe, Cr, and Mn are responsible for the light pink colour of the crystal matrix. The black inclusions of graphite determined by Raman spectrometer are controlled by the crystal structure of calcium-aluminite and distributed on the rhombohedral symmetry surface. Diamond View^TM observation confirms the entire grossular garnet to be a single crystal. When cut perpendicular to 〈111〉 (the L³ axis direction), the graphite inclusions exhibit the six-armed trapiche phenomenon. The low content of heavy elements and lighter impurities such as graphite in grossular garnet match its relatively low refractive index (1.728-1.730) and slightly lower relative density (3.52-3.59).