Nano-Mineralogical Study on the Colour Mechanism of Sunstone from Oregon

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.