Formula: HgO
Simple oxide of mercury
Specific gravity: 11.23
Hardness: 1½ to 2
Streak: Yellow-brown
Colour: Deep red, brownish red to brown, Orange-red to pale yellow in transmitted light, with decreasing thickness
Solubility: Soluble in cold hydrochloric or nitric acid, and in alkali chloride solutions (Dana)

Hydrothermal environments

Montroydite occurs in hydrothermal mercury deposits, associated with mercury, cinnabar, metacinnabar, calomel, eglestonite, terlinguaite, mosesite, kleinite, edgarbaileyite, gypsum, calcite and dolomite (HOM)


At Landsberg, Obermoschel, Nordpfälzer Land, Donnersberg District, Rhineland-Palatinate, Germany, montroydite is associated with moschellandsbergite and cinnabar (Dana).

At the Funderburk prospect, Pike county, Arkansas, USA, montroydite occurs in fractures in sandstone with artsmithite; other minerals in the fractures include cinnabar, quartz, dickite, calomel, eglestonite, mercury, metacinnabar and terlinguaite (Minrec 35.4.344).

At the Clear Creek claim, Goat Mountain, New Idria Mining District, San Benito county, California, USA, montroydite is common in all of the workings as pear-shaped crystals and hair-like mats of needles, as radiating groups of crystals and as blocky masses of fine-grained crystals. It is most commonly observed covering or attached to wattersite, native mercury, edgarbaileyite, eglestonite and cinnabar. Occasionally found as hard shells that are epimorphous after native mercury (Minrec 36.4.348).

Two miles west of Redwood City, San Mateo county, California, USA, montroydite occurs with eglestonite, calomel, mercury and cinnabar (Dana).

At the type locality, the Terlingua Mining District, Texas, USA, montroydite occurs in veins with calcite, terlinguaite, mercury and eglestonite (Dana).

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