Chalcanthite is a natural, water-soluble copper sulphate. It is a secondary mineral that is formed in arid climates or in rapidly oxidizing copper deposits, usually of post-mining formation, forming on mine walls and by the action of acidic surface waters on copper veins.
So it IS a real thing!
However, I am adding it to the Fakes and Forgeries page as a caution. Many, usually well-crystallised chalcanthite specimens offered for sale (e.g.on-line auctions), are artificially grown WITHOUT full disclosure of this fact.
This is also toxic. You do not want to lick it, handle it and then place your fingers in your mouth, eyes, etc.