[Gemology and spectra characterization of gem garnet from Muling City, Heilongjiang Province]

Cenozoic basalts gem-garnets from Muling City, Heilongjiang Province were studied by using standard gemological methods, electron microprobe, Raman spectroscopy, infrared spectroscopy, and ultraviolet-visible spectroscopy to obtain the gemology and spectra characterization. Chemical composition analysis indicates that the garnets are pyropes with some impurity element Fe, Ca, Mn, Cr, and Ti. The average chemical structure formula of the gem-garnet is (Mn0.022 Ca0.455, Fe(2+)0.720, Mg1.793) sigma = 2.990 (Ti0.003 Cr0.009 Fe(3+)0.062 Al1.951) sigma = 2.025 (SiO4)3. Roman spectrum analysis suggests that there are mixed-phases in the garnet, which can be shown by the Roman shift which is caused by bridging oxygen vibration of garnet. The Roman shifts of bridging oxygen bending vibration of pyrope are at 560 cm(-1) (A(1g)), and 641 cm(-1) (E(g) + F(2g)), while the Roman shifts of bridging oxygen bending vibration caused by E(g) + F(2g) of almandine and grossular are at 507 and 486 cm(-1). IR functional group area indicates that the pyropes have no molecules water, but seldom pyropes have a little structure water, which forms three stepped weak absorption peaks at 3 585, 3 566 and 3 544 cm(-1) respectively. Most pyropes are brown-red, which is caused by electronic transitions of impurity ions Cr3+, Fe3+ and Mn2+. UV-Vis spectra show that absorption peaks caused by electron transition of Fe3+ are at 570, 521 and 502 nm, while absorption peaks caused by electron transition of Mn2+ are at 460 and 430 nm, and absorption peaks caused by electron transition of Cr3+ are at 690 and 367 nm.