Copper( II) sulfate is the chemical compound with the formula CuSO4. This salt exists as a series of compounds that differ in their degree of hydration. The anhydrous form is a pale green or gray-white powder, whereas the pentahydrate ( CuSO4· 5H2O) , the most commonly encountered salt, is bright blue. The anhydrous form occurs as a rare mineral known as chalcocyanite. The hydrated copper sulfate occurs in nature as chalcanthite ( pentahydrate) , and two more rare ones: bonattite ( trihydrate) and boothite ( heptahydrate) . Archaic names for copper( II) sulfate are " blue vitriol" and " bluestone" .[ 1]