Kleinite is a rare mineral that has only been found in the United States and Germany that occurs in hydrothermal mercury deposits.

[2] It occurs associated with calcite, gypsum and (rarely) barite or calomel.

[3] Its color can range from pale yellow/canary yellow to orange, and it is transparent to translucent.

[2] As a photosensitive mineral, its coloration darkens when exposed to light.

[4] Kleinite is named after Carl Klein (1842–1907), who was a professor of mineralogy at the University of Berlin.