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 sam-ples 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 i-sotropic 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 opti-cal behaviors.
Cu nanoparticlesunstone from Oregonnano-inclusionlocalized surface plas-mon resonancehigh-resolution transmission electron microscopyplagioclase
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