To make a blue moon, what element does it need?

Question

We know that Mars has a reddish color, because its ground consists of iron-based compounds. The moon is grey-whiteish because it consists mostly of silicon-based compounds.

What element should the moon in my world have in order to have a blue color?

I thought about cobalt-based compounds, but naturally this element is silver-grey.

Answer 1

You don't need an element, you need a mineral.

Certain elements do tend to make things a certain colour, for example nickel will make minerals green, manganese pink, and cobalt purple.

The fact that cobalt itself is a metal is irrelevant. Whatever "element" you have will not be metallic, it will be as a cation (or anion) in the silicates.

So your question should be rephrased to:

What element do I need to make the silicates the moon is made of, blue?

You have several options.

1. Add sodium and chlorine, in order to make sodalite: Na₈Al₆Si₆O₂₄Cl₂ (with the other elements already abundantly present on the moon).

2. Sodium and water, at high pressure and then somehow expose those rocks on the surface. You will have glaucophane: Na₂(Mg₃Al₂)Si₈O₂₂(OH)2. This is the main ingredient of the terrestrial rocks known as blueschists:

3. Tons more aluminium, so then you can stabilise corundum (Al₂O₃). When combined with the already abundant iron and titanium as trace elements, you end up with blue corundum. Also known as sapphire when in gem quality:

This should get you started. Other things you might consider are potassium and whatever makes amazonite (potassium feldspar) green-blue. Copper is also a good one, but the blue requires water that isn't common on the Moon.