Potassium sulfide KFeS2 (hanswilkeite) has been identified in polymineralic inclusions a diamond from the Udachnaya kimberlite pipe (Siberian craton, Yakutia). This is second occurrence of hanswilkeite nature and first one mantle-derived samples. Sulfide monoclinic, space group—C 2/c. Its crystal structure consists chains with K interstices. The tetrahedra are centered by iron ions linked edges...