Radtkeite
Radtkeite is an IMA-recognized mineral species in the sulfide class, with the chemical formula Hg₃S₂ClI.
It crystallizes in the monoclinic crystal system (space group C2/m).
Key properties: Mohs hardness 2–3; specific gravity 7; Yellow-orange, blackening on exposure to light streak; Nearly adamantine (synthetic) luster; optically biaxial.
IMA symbol Rdk, IMA status: approved, named in 1989.
Found in tuffaceous rhyolitic lake-bed sediments, formed as a reaction product between halide-bearing hydrothermal solutions and cinnabar or corderoite.
It is found associated with Cinnabar, corderoite, quartz, gypsum.
The mineral was named to honor Arthur Sears Radtke (1936-2004), American mineralogist and geochemist, U.S. Geological Survey, Palo Alto, California, USA.