Cadwaladerite, Al2(H2O)(OH)4·n(Cl,OH–,H2O), from Cerros Pintados, Chile, defined as a valid mineral species and the discreditation of lesukite

Abstract

Cadwaladerite, described in 1941 as Al(OH)2Cl·4H2O, and lesukite, described in 1997 as Al2(OH)5Cl·2H2O, are very closely related chemically and structurally, but are found in very different environments. Cadwaladerite was found at the edge of a salar in Chile. Lesukite has been described from a volcanic fumarole and from burning coal seams. Both materials have cubic symmetry with a = 19.788 to 19.859Å. The crystal structure, common to both, consists of a rigid three-dimensional framework of edge- and corner-sharing Al(OH,H2O)6 octahedra that contains large interconnected cavities where loosely held Cl, OH, and H2O are located. The fact that Cl is loosely held within the structure is demonstrated by a dramatic reduction in Cl content after washing the material in distilled water, while the structural integrity is maintained. Herein, cadwaladerite is confirmed as a valid mineral species and lesukite is discredited because the only difference between the two materials is the loosely held extra-framework Cl, OH, and H2O. Cadwaladerite, Al2(H2O)(OH)4·n(Cl,OH,H2O) (Z = 48) takes precedence over lesukite based on the date of description. Material similar to cadwaladerite is found as a corrosion product on some types of nuclear fuel elements and is also closely related to the molecular species used in antiperspirant and water filtration.