Hexasulfur is an inorganic chemical with the chemical formula S6. This allotrope was first prepared by M. R. Engel in 1891 by treating thiosulfate with HCl. Cyclo-S6 is orange-red and forms a rhombohedral crystal. It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. Another method of preparation involves the reaction of a polysulfane with sulfur monochloride: