How much aluminum is in Earth's crust

[Lord Jubjub]

If you purified the aluminum that is in Earth's top couple of miles of crust, how deep a layer could you cover the Earth with it?

[Hornblower]

I would estimate several hundred feet. The crust is about 8% aluminum. The metal would be dirt cheap if it was not so difficult to extract from the complex silicates that are everywhere.

http://www.webelements.com/aluminium/geology.html