Nano-Mineralogical Study on the Colour Mechanism of Sunstone from Oregon
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摘要:
俄勒冈太阳石的颜色成因是矿物学中一个经典而有争议的话题，这是由于它具有在同一长石晶体中各向异性(绿红色)和各向同性(红色)颜色区域共同存的特殊光学性质(图 1)。经过近50年的研究，迄今为止的研究中尚没有达成共识的解释模型被提出。在本研究中，受到纳米科学中局域表面等离子体共振(LSPR)理论(图 2)的启发，我们对红色、绿红色、绿色3颗俄勒冈太阳石沿特定晶体方向通过聚焦离子束提取制备样片，并进行了高分辨率透射电子显微镜分析，并配合LA-ICP-MS和偏振紫外-可见光谱分析，以及光谱模拟计算，对这一问题展开研究。结果表明，在各向异性和各向同性色区中，我们观察到斜长石中包含的Cu纳米粒子具有不同的几何形状。在各向同性(红色)区域，纳米粒子是随机分布的纳米球体或纳米椭球体(直径8.7~12.0 nm)，纵横比为1.0~1.3(图 3)。相比之下，在二向色(绿色/红色)区域，纳米粒子是定向排列的纳米棒(沿长轴8.5~21.0 nm)，纵横比约为2.5(图 4)。我们应用LSPR理论来模拟吸收光谱(图 5)，并通过模拟计算来解释观察到的光学特性。本研究系统地揭示了长石晶体中不同形状金属纳米粒子包裹体的存在及其光学影响。此外，它还表明了将LSPR纳入矿物致色理论的必要性。此外，含Cu纳米粒子拉长石已被证明具有三阶非线性光学性质，结合纳米粒子的形状和尺寸将有助于设计具有定制光学行为的纳米粒子嵌入光学材料。
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关键词:
- Cu纳米粒子 /
- 俄勒冈太阳石 /
- 纳米夹杂物 /
- 局域表面等离子体共振 /
- 高分辨率透射电子显微镜 /
- 斜长石
Abstract:The colouration mechanism of Oregon sunstone is a classic and controversial topic in mineralogy because of the unique co-existence of anisotropic (green-red) and isotropic (red) colour zones within single feldspar crystals (Fig. 1). After nearly 50 years of research, no models proposed to date have satisfactorily accounted for all observed optical phenomena. In this paper, we present high-resolution transmission electron microscopy analyses of samples prepared by focused ion beam extraction along specific crystal directions on three Oregon sunstone samples (red, green-red and green, respectively). In both the anisotropic and the isotropic colour zones, we observed Cu nanoparticles (NPs) included within plagioclase but with different geometries. In the isotropic (red) zone, NPs were randomly distributed nano-spheres or nano-ellipsoids (8.7-12.0 nm in diameter) with an aspect ratio of 1.0-1.3 (Fig. 2). In contrast, in dichroic (green/red) zones, NPs were directionally-aligned nano-rods (8.5-21.0 nm along the long axis) with an aspect ratio of -2.5 (Fig. 3). We applied localized surface plasmon resonance (LSPR) theory (Fig. 4) to simulate absorption spectra (Fig. 5), and we developed a model to explain the observed optical properties. LA-ICP-MS and polarized UV-Vis spectroscopy were also performed to confirm our conclusions. This study systematically reveals the existence and optical influence of variably shaped metal-NP inclusions in feldspar crystals. Furthermore, it demonstrates the necessity of including LSPR in the canon of mineral coloration mechanisms. Cu-NP-bearing labradorite has been shown to exhibit third-order non-linear optical properties, and approaches that incorporate NP shapes as well as sizes will assist in the design of NP-embedded optical materials with tailored optical behaviors.
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图 1 同时具有各向同性色区和各向异性色区的俄勒冈太阳石Figure 1. Sunstone samples from Oregon with both isotropic and anisotropic colour zones -
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图 1 同时具有各向同性色区和各向异性色区的俄勒冈太阳石
Figure 1. Sunstone samples from Oregon with both isotropic and anisotropic colour zones
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[1] Hofmeister A M, Rossman G R. Exsolution of metallic copper from Lake County labradorite[J]. Geology, 1985, 3(9): 644-647.
[2] Rossman G R. Optical spectroscopy[J]. Reviews in Mineralogy and Geochemistry, 2014(78): 371-398.
[3] Kiefert L, Wang C, Sintayehu T, et al. Sunstone labradorite-bytownite from Ethiopia[J]. Journal of Gemmology, 2019(36): 694-695.
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