Formula: (Hg₂N)(Cl,SO₄) · nH₂O

Species:

Colour: Light to canary-yellow, orange; darkens to reddish yellow or orange in daylight and reverts to original color in darkness. Yellow to colourless in transmitted light.

Lustre: Adamantine, Greasy

Hardness: 3½

Crystal System: Hexagonal

Name: Named in 1905 by Arthur Sachs in honor of Johann Friedrich Carl Klein (1842–1907), German Professor of Mineralogy and curator of the Museum für Naturkunde, University of Berlin. Klein was a petrographer who made major contributions to understanding mineral optics and the identification of minerals in thin sections. He was a pioneer in studying meteorites in thin section.

Type Locality: Terlingua Mining District, Texas – USA

Dimorph of: Mosesite