Zircon from syenitic ejecta of Vesuvius (Campania, Italy) is unusually blue, a property shared with gem zircon from Ratanakiri province (Cambodia), which turns from natural reddish-brown to blue when heated under reducing conditions. Here, the origins of these unusual crystals were traced through geochronology, trace elements, and O-Hf isotopic compositions. The causes of its colour were investigated through optical and electron microscopy, optical absorption spectroscopy, and Raman microspectroscopy. Colour stability upon heating and ultraviolet light (UV) exposure was tested using Ratanakiri zircon as a control. Vesuvius zircon contains vesiculated zones with abundant inclusions ~2.5 μm to <100 nm in diameter (mostly U-rich thorianite and pyrochlore-group minerals), while homogeneous zircon domains are high in Th and U (up to 5.9 and 1.8 wt%, respectively). Its blue colouration is stable under UV radiation, as well as heat-treatment under reducing conditions (1000 °C; >15 h). Turbid domains rich in large inclusions change to yellow-brown after heating under oxidizing conditions, while transparent domains remain pale blue or colourless. Optical absorption spectra display sharp absorption lines attributed to U 4+ , and slightly elevated absorption towards shorter wavelengths. The ~1007 cm −1 ν 3 (SiO 4 ) Raman band is broadened due to lattice distortion by non-stoichiometric elements in high-Th/-U zircon, whereas narrow bands in inclusion-rich domains indicate a decrease in lattice strain due to inclusion precipitation. Blue colouration in Vesuvius zircon is explained by the effect of light scattering (Rayleigh and/or Mie scattering) on highly refractive actinide-rich inclusions ranging in size from <1/10 to few multiples of the wavelengths of visible light. Inclusions likely formed during fluid-mediated coupled dissolution-reprecipitation that locally transformed lattice-strained actinide-rich zircon within several hundreds of years prior to eruption.

中文翻译：

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来自维苏威火山的天然蓝色锆石

来自维苏威火山（意大利坎帕尼亚）正长岩喷出物的锆石呈异常蓝色，与腊塔纳基里省（柬埔寨）的宝石锆石具有相同的特性，在还原条件下加热时，锆石会从天然红棕色变为蓝色。在这里，通过地质年代学、微量元素和 O-Hf 同位素组成追踪了这些不寻常晶体的起源。通过光学和电子显微镜、光吸收光谱和拉曼显微光谱研究了其颜色的原因。使用 Ratanakiri 锆石作为对照测试加热和紫外线 (UV) 暴露时的颜色稳定性。维苏威锆石包含具有丰富包裹体的气泡带，直径约 2.5 μm 至 <100 nm（主要是富含 U 的钍钍和烧绿石族矿物），而均质锆石域的 Th 和 U 含量很高（高达 5.9 和 1.8 wt%，分别）。它的蓝色在紫外线辐射下以及在还原条件（1000 °C；> 15 小时）下的热处理下是稳定的。在氧化条件下加热后，富含大夹杂物的混浊区域变为黄褐色，而透明区域保持淡蓝色或无色。光吸收光谱显示出归因于 U 4+ 的尖锐吸收线，并且对较短波长的吸收略有增加。~1007 cm -1 ν 3 (SiO 4 ) 拉曼谱带由于高 Th/-U 锆石中非化学计量元素的晶格畸变而加宽，而富含夹杂物域中的窄谱带表明由于夹杂物沉淀。维苏威锆石呈蓝色的原因是光散射（瑞利和/或米氏散射）对高折射富含锕系元素的内含物的影响，这些内含物的尺寸范围从 <1/10 到可见光波长的几倍。包裹体可能在流体介导的溶解-再沉淀耦合过程中形成，这些包裹体在喷发前的数百年内局部转化了晶格应变的富含锕系元素的锆石。