赞比亚紫红色—棕红色石榴石的宝石学特征

Gemmological Characteristic of Purple-Brownish Red Garnet from Zambia

赞比亚Magodi矿区是紫红色-棕红色石榴石的新产地,目前关于该产地的研究较少。通过常规宝石学测试、电子探针、拉曼光谱和紫外-可见吸收光谱测试对赞比亚的紫红色-棕红色石榴石进行了系统的研究。赞比亚紫红色-棕红色石榴石的折射率约为1.750~1.772,相对密度约为3.77~3.92;属于铁铝-镁铝榴石系列,含有少量钙铝榴石、锰铝榴石等;具有种类丰富的内含物,包括自形-半自形的透明晶体包裹体、浑圆状熔蚀包裹体、密集的短棒状和粒状包裹体、平行排列的长针状包裹体、“指纹状”的愈合裂隙等,拉曼光谱表明矿物包裹体以金红石、锆石和锐钛矿为主。紫外-可见吸收光谱显示,赞比亚紫红色-棕红色石榴石吸收峰主要与Fe^(2+)、Fe^(3+)和Mn^(2+)离子d-d轨道的跃迁有关,含量较多的Fe^(2+)在黄绿光区产生了最主要的吸收,反衬出红光区和蓝紫光区较高的透过率,一部分样品呈紫红色的色调,另一部分样品在368、425 nm处(与Fe^(3+)有关的峰位)产生了更强的吸收,蓝紫光的透过减少,样品偏向棕红色调;Mn^(2+)含量较低,吸收较弱,对石榴石颜色的影响不显著。赞比亚石榴石的化学成分、吸收光谱和内含物等特征可作为产地溯源的依据,也可为其矿床成因、地质背景等研究提供参考。

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 Fe^(2+),Fe^(3+) and Mn^(2+) ions.High content of Fe^(2+) 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 Fe^(3+) 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.