[Rockhounds] Coating on rocks near Globe, AZ

[Al Balmer]

On the west side of Globe, there's an area on the edge of a mine dump
which has common stone covered with some green copper mineral. This is
evidently a short-term process, since rocks half-buried in the dirt
have the coating only on the covered part.

I'm curious as to what the coating is, and what the mechanism is.
Anyone know?

-- 
Al Balmer
Sun City, AZ