picromerite

from The Century Dictionary.

• noun A hydrous sulphate of magnesium and potassium, obtained in monoclinic crystals and in crystalline crusts at the salt-mines of Stassfurt in Prussia.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A saline evaporite, consisting of a mixed potassium and magnesium sulphate, with the chemical formula K2Mg(SO4)2·6(H2O).