| Citation: |
ZHAO Siyi, XU Bo. Gemmological and Mineralogical Characteristics of Gem-Quality Green Kornerupine from Tanzania, Myanmar and Sri Lanka[J]. Journal of Gems & Gemmology, 2024, 26(S1): 96-98.
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Kornerupine is a complex hydrous magnesia-iron-aluminum borosilicate that ranges in colour from colourless to yellow, green, brown and blue, etc.The gem-grade green kornerupine is the most precious and popular in the gemstone market now. Nonetheless, due to its rarity, the information about the origins and gemmological characteristics of the gem-quality green kornerupine is little known. In this paper, the gemmological and mineralogical characteristics of the kornerupine samples from Tanzania, Myanmar, and Sri Lanka respectively were studied by conventional gemmological methods, UV-Vis-NIR, Raman spectroscopy, EMPA and LA-ICP-MS. Kornerupine samples from the three origins exhibited similar gemmological characteristics, except for the inert ultraviolet fluorescence response of the Myanmar samples, which is attributed to higher iron content. Besides, all the kornerupine samples from the three origins displayed abundant fluid inclusions, needle-like inclusions and graphite inclusions with varying degrees of crystallization, indicating their metamorphic origin. The B element apfu (atoms per formula unit) serves as a crucial criterion for the classification of kornerupine mineral species. The testing results of B element using LA-ICP-MS indicate that kornerupine samples from Myanmar are predominantly classified as kornerupine, whereas those samples from Tanzania and Sri Lanka are primarily identified as prismatine. Due to differences of trace element compositions, the gemmological properties of kornerupine samples from Myanmar were different compared to the other two origins. The V-Cr-Mn and Fe-Mn-V are trace elements "fingerprints" that provide a useful tool for identifying the provenance of kornerupines from Tanzania, Myanmar, and Sri Lanka. V3+ is the predominant colouring element of kornerupines from Tanzania, and Sri Lanka, giving rise to more yellowish green colour. Chromium is the predominant chromophore of kornerupine from Myanmar, according to the combined observations from UV-Vis-NIR spectroscopy and trace element analyses.
柱晶石(Kornerupine)是一种复杂的含水镁-铁-铝硼硅酸盐，其颜色从无色到黄色、绿色、棕色和蓝色不等，其中，以绿色的柱晶石最为珍贵。柱晶石中根据硼含量可以分为柱晶石(kornerupine)和硼柱晶石(prismatine)，一般化学式为X(Mg, Fe)M(Al, Mg, Fe)9T(Si, Al, B)5(O, OH, F)22 (X: 立方位; M: 八面体位; T: 四面体位), 属于kornerupine-prismatine系列的晶体均呈现正交结构，其空间群命名为Cmcm[1]。复杂的晶体晶格具有三个标记为T1到T3的四面体位点，五个不同的八面体位点，标记为M1到M5，以及一个扭曲的、8配位的立方位点，标记为X。化学成分上，kornerupine-prismatine的不同主要在于镁、铁、铝和氟的含量。值得注意的是，当硼存在时，硼有序地位于T3位点，硼含量从0到1 apfu(每个分子式单位的原子数)[1]，这是矿物种类分类的重要因素: 硼含量低于0.5 apfu的柱晶石被归类为kornerupine，而硼含量超过0.5 apfu的柱晶石被称为prismatine[2]。在本研究中，为了简化，我们使用“kornerupine”作为kornerupine和prismatine硼硅酸盐矿物系列中所有矿物组的组名。
据报道，在世界上至少84个地区的角闪岩和麻粒岩相岩石中发现了柱晶石及硼柱晶石[2-3]。然而，在世界范围内，宝石级柱晶石通常是小批量开采，主要来自斯里兰卡、缅甸和东非(图 1)。宝石级的柱晶石因其鲜艳的绿色和优秀的透明度而受到珠宝设计师和更广泛的宝石市场的珍视和追捧。然而到目前为止，关于宝石级绿色柱晶石的产地来源信息并不全面。在本文，我们对来自坦桑尼亚、缅甸和斯里兰卡的宝石级绿色柱晶石样品进行了表征。虽然本文研究的柱晶石样品都有一个充满活力的绿色色调，但它们在色调和饱和度上表现出显著的变化。坦桑尼亚柱晶石样品呈苹果绿色(图 2)，与薄荷绿色的莎弗莱石非常相似，但由于前者具有强烈的多色性，很容易区分。来自斯里兰卡和坦桑尼亚的柱晶石样品具有相似的绿色，值得注意的是斯里兰卡的4颗柱晶石样品显示出猫眼效应，源于排列成平行纹理的针状金红石内含物[4]。最珍贵的“祖母绿色”柱晶石样品产自缅甸，缅甸是世界上历史上最重要的宝石开采地之一(图 2)。毫无疑问，颜色的差异是由不同的地质环境遗传下来的，这些地质环境导致了三个产地的宝石级柱晶石的形成。需要进一步研究柱晶石的宝石矿物学以及产地信息，以充分了解在自然界中创造这些特殊宝石的地质过程。
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