Leightonite, hydrous Cu, K, Ca sulfate, found only at the Chuquicamata mine, Atacama province, Chile, was originally described by Palache in 1938 as triclinic. It is a pale blue-green translucent mineral, occurring in crystals and, more commonly, as cross-fiber veinlets. Study of a specimen in the Royal Ontario Museum shows that leightonite is orthorhombic, a = 11.67, b = 16.52, c = 7.492 A, a:b:c = 0.7064:1:0.4535. Space group is Fmmm; unit-cell content 4[K 2 C 2 Cu(SO 4 ) 4 .2H 2 O]. Gypsum, quartz, atacamite, bloedite, and natrochalcite occur associated with leightonite. Polyhalite, K 2 Ca 2 Mg(SO 4 ) 4 .2H 2 O, is a chemical homologue, but is triclinic.