The presence of metal Na nanoparticles causes the bright, thermally unstable colors of villiaumite (NaF) and halite (NaCl). These nanoparticles have been suspected for a long time to be caused by external irradiation. Metal nanoparticles, often referred to as metal colloids, cause surface plasmon resonance effects, characterized by a single Lorentzian-shaped absorption band. The color of these minerals is due to metal Na nanoparticles of 2.5–3 nm. A key point is that the resonance wavelength, which corresponds to the maximum of the absorption band, is inversely related to the value of the refractive index of the embedding mineral. This causes the position of the main absorption band to be offset downward by 140 nm in halite relative to villiaumite. As a consequence, the optical transmission window is shifted from the long to the short wavelength domain, explaining the color of blue halite and red villiaumite, respectively. Similar refractive index dependence may explain the purple color of fluorite caused by metallic Ca nanoparticles. Finally, the origin of the villiaumite irradiation may be the presence of Th-rich (about 8.8 wt% ThO2) nano-inclusions, about 500 nm large, illustrating the specific geochemistry of peralkaline rocks where villiaumite is found.

中文翻译：

---

碱金属卤化物矿物质中的钠纳米颗粒：为什么硅镁石是红色和岩盐是蓝色？

金属Na纳米粒子的存在会引起维柳石（NaF）和盐岩（NaCl）的明亮，热不稳定的颜色。长期以来，一直怀疑这些纳米颗粒是由外部照射引起的。金属纳米颗粒（通常称为金属胶体）会引起表面等离振子共振效应，其特征在于具有单个洛伦兹形吸收带。这些矿物的颜色归因于2.5–3 nm的金属Na纳米颗粒。一个关键点是，对应于吸收带最大值的共振波长与包埋矿物的折射率值成反比。这使得主吸收带的位置在盐岩中相对于硅镁石向下偏移了140 nm。作为结果，光学透射窗从长波长域转移到短波长域，分别解释了蓝色盐岩和红色堇青石的颜色。相似的折射率依赖性可以解释由金属Ca纳米颗粒引起的萤石的紫色。最后，铁矾的辐照起源可能是大约500 nm大的富Th（约8.8 wt％ThO2）纳米夹杂物的存在，这说明发现了钙铁矾的高碱性岩石的特殊地球化学。