Sodium nanoparticles in alkali halide minerals: Why is villiaumite red and halite blue?

Abstract

The presence of metal Na nanoparticles causes the bright, thermally unstable colors of villiaumite, NaF, and halite, NaCl. These nanoparticles have been suspected since 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 downwards 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 This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Innovative Structural Characterization of Natural Villiaumite Crystal

“…Only after three days at 430 • C or several hours of heating at 450 • C did all crystals become colorless. Perhaps Calas et al [3] had a similar experience, because they wrote in their article: "The tested samples have an intense red color that disappears within a few minutes at 500 • C and after 2 h at 400 • C".…”

“…Only after three days at 430 °C or several hours of heating at 450 °C did all crystals become colorless. Perhaps Calas et al [3] had a similar experience, because they wrote in their article: "The tested samples have an intense red color that disappears within a few minutes at 500 °C and after 2 h at 400 °C". Even though the crystals became colorless, no changes in the far infrared spectra were observed, except for a small broadening of the band.…”

“…The most well-known places where it occurs are as follows: Guinea-Los Islands, where it was first discovered by Maxime Villiaume; Russia-Khibiny and Lovozero massive, Kola Peninsula; Canada-Mon Saint Hilaire in Quebec; Kenya-Magadi Lake; Namibia-Aris; Greenland-Kvanefjeld; Brazil-Minas Gerais; and USA-Point of Rocks, New Mexico, and Porphyry Mountain, Colorado [1]. The thermally unstable color of villiaumite, which includes various shades of red and pink, is not caused by impurities [2] but comes from metal Na nanoparticles (2.5-3 nm in diameter) [3]. Despite its impressive appearance, it is practically not used in the jewelry industry due to its high solubility in water [4] and its toxicity [5].…”

“…The aim of this work was to use far infrared spectroscopy to fill the gap in the characterization of villiaumite in an innovative way. Due to the change in the color of the mineral from red to colorless when heated [2,3,33], the far infrared spectra were also measured for samples in the temperature range of 20-500 • C. Additionally, the IR spectrum was calculated for the sodium fluoride structure model (125 atoms) using density functional theory (DFT). The theoretical results were compared with experimental data.…”