Formula: Hg3S2ClI
Sulphide, mercury-bearing mineral
Crystal System: Monoclinic
Specific gravity: 7.0 measured, 7.05 calculated
Hardness: 2 to 3
Streak: Yellow-orange
Colour: Yellow-orange when fresh, but highly sensitive to sunlight; during exposure it darkens within minutes through shades of yellow-brown to black (Mindat).

Sedimentary environments
Hydrothermal environments

Although radtkeite was approved in 1989, to date (September 2023) it has been reported only from the type locality.


At the type locality, the McDermitt Mine, Opalite Mining District, Humboldt County, Nevada, USA, radtkeite formed during reaction of hydrothermal solutions with lake sediments in the McDermitt caldera complex. Rhyolitic tuffaceous sediments accumulated in an ancient lake; those sediments are now strongly altered in places to mixtures of montmorillonite, kaolinite, adularia, opal, cristobalite and clinoptilolite. Radtkeite was found closely associated with quartz, cinnabar and corderoite at one location on the south margin of the present open pit. Other rare mercury minerals that occur at the McDermitt mine are calomel, kleinite, eglestonite, native mercury and possibly mosesite.
Radtkeite occurs as submicrometer grains, irregular masses, finely dispersed grains in the silica-clay matrix, coatings on cinnabar and corderoite, and euhedral crystals up to 10 microns wide and 30 microns long, most commonly, crystals are tabular prismatic.
The sequence of deposition of sulphides began with early pyrite with stibnite, followed by cinnabar. Corderoite was formed when dilute solutions with relatively low chlorine and sulphur contents reacted with cinnabar. Some corderoite replaced cinnabar volume for volume.
The radtkeite may have formed by the reaction of chloride-iodide solutions with cinnabar, but some was by reaction of iodine with corderoite as indicated by the common occurrence of radtkeite with corderoite (AM 76.1715-1721).

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