Photoluminescence Spectra of Chrysoberyls from Various Origins
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摘要:
光致发光光谱因其无损、高灵敏度和高分辨率的特性，被广泛应用于宝石学和矿物学领域。采用无损激光激发光致发光光谱、激光烧蚀电感耦合等离子体质谱等技术手段对缅甸、巴西、斯里兰卡、坦桑尼亚等几个宝石产地的金绿宝石样品及其合成品进行了研究。结果表明，所有金绿宝石样品的光致发光光谱非常相似，它们主要是Cr离子发射，光谱均显示有两条狭窄的R线，可以帮助判断样品是天然或合成，也可以提供关于天然样品的地质来源的信息。在收集的光致发光光谱中，这些R线的峰值位置在很小的波长范围内变化，这些变化的来源依赖性可能有助于识别未知样本的地理来源。其中，合成金绿宝石样品的R2线位于最长的波长，天然金绿宝石样品则在更短的波长。通过R线峰位和相应的元素浓度的比较，R线的峰值位移与Al2O3浓度的增加有较强的相关性，这表明微量元素取代铝位点可能是导致R线峰位偏移的原因。
Abstract:Photoluminescence spectroscopy is widely applied in the fields of gemmology and mineralogy utilizing its non-destructive, high-sensitivity, and high-resolution features. Chrysoberyls from gem-mining regions such as Myanmar, Brazil, Sri Lanka, Tanzania and synthetic chrysoberyl samples were studied by non-destructive laser-induced photoluminescence spectroscopy and laser ablation-inductively coupled plasma-mass spectrometry. The experimental results demonstrate a high degree of similarity in the comparative photoluminescence spectra across all examined samples, manifested by the presence of two narrow R-lines that are predominantly attributed to the emissive properties of Cr ion. This characteristic signature not only facilitates the discrimination between natural and synthetic chrysoberyl samples but also provide information about natural samples' geological origin. In the collected spectra, peak positions of these R-lines varies across a small range of wavelengths; the origin-dependent nature of these variations may be helpful in identifying the geographic source of unknown samples. In particular, the R2 line of synthetic chrysoberyl is positioned at the longest wavelength, while for natural chrysoberyl this line is found at the shorter wavelength. Comparisons between R-line peak wavelengths and corresponding element concentrations of samples revealed strong correlations between peak shifts of the R-lines and increases in alumina concentrations. These correlations suggest that the substitution of trace elements for aluminum crystal sites responsible for the R-lines peak shifts.
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Keywords:
- chrysoberyl /
- photoluminescence spectra /
- origin
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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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图 2 缅甸、坦桑尼亚和斯里兰卡的宝石级绿色柱晶石样品.左边8颗斯里兰卡绿色样品及其4颗具有独特的猫眼效应；中间13颗坦桑尼亚苹果绿色样品原石及刻面型；右边3颗缅甸深绿色样品Figure 2. Faceted, cabochon and raw kornerupine samples from Myanmar, Tanzania and Sri Lanka. The eight green samples from Sri Lanka on the left of the photo, four of them showing distinctive chatoyancy effect. The thirteen samples with apple green colour in the center of the photo are from Tanzania.The three precious emerald-colored gemstones on the right are from Myanmar -
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