For 10 points, name this alkali element, slightly less reactive to air and water than sodium, whose chemical symbol is K.
Orthoclase is a type of feldspar that contains this element, whose isotope-40 is used with argon to date rock formations. This element's chloride is used in executions, and nitrates of it are found in saltpeter and fertilizer. This element emits a (*) lilac color in a flame test, and a water-soluble form of this element is found in potash. For 10 points, name this alkali element, slightly less reactive to air and water than sodium, whose chemical symbol is K.
Orthoclase is a type of feldspar that contains this element, whose isotope-40 is used with argon to date rock formations.
Potassium (accept K before mentioned)