Gemmological Characteristic of Purple-Brownish Red Garnet from Zambia

The Magodi mining area in Zambia is a new source of purple-brownish red garnets, but there are only a few studies on this locality. In this paper, the garnets from Zambia were systematically studied by conventional gemmological tests, electron probe micro-analysis (EPMA), Raman spectrometer (Raman) and ultraviolet-visible absorption spectrometer (UV-Vis). The refractive index of garnets from Zambia is about 1.750-1.772, and the relative density is about 3.77-3.92. The garnets correspond to the almandine-pyrope series, which contain minor mineral components like grossularite and spessartite. Besides, inclusions are rich in variety, such as transparent euhedral to semi-euhedral mineral inclusions, round ablation inclusions, dense rod or granular inclusions, long needle-like inclusions with parallel arrangement, "fingerprint-like" healed fractures, etc. Raman spectra showed that the mineral inclusions include rutile, zircon and anatase. The ultraviolet-visible absorption spectra are mainly related to the transition of d-d orbitals of Fe²⁺, Fe³⁺ and Mn²⁺ ions. High content of Fe²⁺ produces the main absorption in yellow-green spectrum region, leading to higher transmittance in red region and blue-violet region, which mix into the purple hue of some samples. The other samples with brownish-red hue produce stronger absorptions at 368 nm and 425 nm that related to Fe³⁺ than the purple samples, which decreases the transmission of blue and violet light, thus relatively more light transmits through the red region, causing the brownish-red hue. The chemical composition, absorption spectrum and inclusions characteristics provide references for studying the deposit genesis and geological background of garnets from Zambia, as well as the basis for determination of provenance.