莫桑比克含铜和锰的紫色碧玺的谱学特征及产地判别

Spectral Characteristic and Provenance Discrimination of Cu- and Mn-Bearing Purple Tourmaline from Mozambique

  • 摘要: 莫桑比克是全球最重要的碧玺产区之一,以产出晶体粗大、颜色鲜艳且净度高的帕拉伊巴及紫色-蓝紫色碧玺著称。本研究采用常规宝石学测试、傅里叶变换红外光谱、激光共聚焦显微拉曼光谱、紫外-可见吸收光谱、激光剥蚀电感耦合等离子体质谱以及电子探针等方法,对7颗刻面型紫粉色及蓝紫色莫桑比克碧玺样品展开研究。结果显示,碧玺样品折射率为1.620~1.640,双折射率为0.018~0.020,相对密度为3.01~3.04,均具有强二色性;红外光谱与拉曼光谱中均检测到表征羟基(OH) 振动的峰位,表明样品属于热液成因型锂碧玺;紫外-可见吸收光谱显示了Mn和Cu致吸收光谱特征。其中,490、540 nm处的吸收峰由Mn3+5Eg5T2g分裂所致,690 nm处的吸收宽带则归因于Cu2+2Eg2T2g分裂;成分分析结果显示,样品的颜色受Mn/Cu质量分数比值影响,比值越大紫色调越明显,反之则蓝色调明显。晶体化学分类结果表明,6颗样品(A1-A6)属于碱金属2族中的Elbaite(锂电气石),样品A7属于空位2族中的Rossmanite(罗斯曼石)。SPSS判别分析表明,基于氧化物质量分数的模型可有效区分巴西、莫桑比克和尼日利亚碧玺,其效果优于微量元素模型,且具有一定的稳定性。

     

    Abstract: Mozambique stands as one of the world's most significant tourmaline producing regions, particularly renowned for its large, vivid-coloured, and high-clarity crystals, notably Paraíba-type and purple to purplish-blue tourmalines. This study investigated 7 faceted pink-purple and blue-purple tourmaline samples from Mozambique using conventional gemmological tests, Fourier-transform infrared (FTIR) spectroscopy, laser confocal micro-Raman spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and electron probe microanalysis (EPMA). The results indicate that the samples exhibit refractive indices ranging from 1.620 to 1.640, birefringence from 0.018 to 0.020, and relative densities from 3.01 to 3.04, with all samples displaying strong dichroism. Hydroxyl (OH) vibration peaks were detected in both infrared and Raman spectra, indicating the hydrothermal origin of these lithium tourmalines. UV-Vis absorption spectra reveal that this type of tourmaline primarily exhibits absorption characteristics attributable to Mn and Cu ions: absorption peaks at 490 nm and 540 nm are assigned to the 5Eg5T2g splitting of Mn3+, whereas a broad absorption band at 690 nm is attributed to the 2Eg2T2g splitting of Cu2+. Compositional analysis further demonstrates that the colour of the samples is controlled by the Mn/Cu mass fraction ratio: a larger ratio corresponds to a more pronounced purple hue, while a smaller ratio results in a more distinct blue hue. Crystal-chemical classification indicates that 6 samples (A1-A6) are classified as Elbaite (a lithium tourmaline) of alkali group 2, while sample A7 is assigned to the Rossmanite subgroup of vacancy group 2.SPSS discriminant analysis demonstrates that a model based on oxide mass fractions effectively distinguishes tourmaline samples from Brazil, Mozambique, and Nigeria, outperforming a trace-element model and exhibiting a certain degree of stability.

     

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