# Is there a gas with metallic luster?

We know that both solids and liquids can exhibit metallic luster. I wonder whether there is a gas that when filling a transparent envelope (such as a glass tube) would make it appear with metallic luster?

• You certainly have big imagination :) – Mithoron Jul 19 '15 at 16:35
• I thought that metallic luster should require coherent reflection of light, which itself should require a substance which is (quasi-)regular/periodic/ordered at least at a scale a bit larger than a few atoms. A gas is none of these. However, glass can reflect some light quite well without having a crystalline structure, so that complicates my argument. – Nicolau Saker Neto Jul 19 '15 at 20:03
• One other thing that came to mind is that gasses are much more sparse than solids or liquids; there's so much space between atoms in the average gas that light should often permeate the interior without coming close to more than a handful of atoms simultaneously. So you're basically asking a few completely uncorrelated atoms to reflect visible light, with wavelength over 100 times their dimension, coherently. Sounds like an extremely tough proposition. – Nicolau Saker Neto Jul 19 '15 at 20:10
• @Nicolau Saker Neto what about a metal in supercritical state? – Anixx Jul 20 '15 at 9:23
• Supercritical fluids can have arbitrarily close similarity to a gas or a liquid, so it would probably work, given that liquid metals have metallic luster (I don't know why I hadn't thought of liquid metals in my previous comments...). However, I think you'd have to be quite far from the gaseous region of a state diagram for a supercritical fluid to develop lustre, so it would be unwise to call it a gas. – Nicolau Saker Neto Jul 20 '15 at 11:59

Good question... would a plasma qualify? Metallic luster is due to a sea of electrons, free to move and to reflect (well, re-radiate) incident radiation. A plasma can be created in a gas, such as the interstellar medium, or in Earth's atmosphere. The Kennelly-Heaviside layer is such a "shiny", reflective layer, causing skip signal propagation to extend the range of radio communication. So if you saw in radio wavelenghts, rather than the visual spectrum, plasma is shiny!

Another possibility would be a Bose-Einstein condensate... but the act of illuminating it tends to destroy it.

• What about supercritical metal such as mercury? – Anixx Jul 20 '15 at 9:24

Luster is an optical property of minerals, and a mineral is defined as a solid inorganic substance that occurs naturally (that includes metals, of course). The Mineralogic Society of America defines metallic luster (see link) as something that occurs in minerals which have a refractive index greater than 3.

I wasn't able to find any gases with a refractive index even close to the cutoff value. I'd say the answer is no, and that what might appear to be luster is instead scattering of incident light (not refraction).

• @Anixx Yes, but then you're dealing with supercritical mercury, which regardless of its properties seems to me to go against the spirit of your original question. SC $\ce{Hg}$ would not fill the tube like a gas... first, it would settle to the bottom like a liquid; second, it would melt/explode the tube! ($T_c=1478\ ^\circ\mathrm{C}$, $P_c=1673\mathrm{\ bar}$). – hBy2Py Jul 27 '15 at 11:18