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Formula: HgO
Simple oxide of mercury
Crystal System: Orthorhombic
Specific gravity: 11.23 measured, 11.22 calculated
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)
Environments
Montroydite occurs in hydrothermal mercury deposits, associated with
mercury, cinnabar,
metacinnabar, calomel,
eglestonite, terlinguaite,
mosesite, kleinite,
edgarbaileyite, gypsum,
calcite and dolomite
(HOM)
Localities
At Landsberg, Obermoschel, Nordpfälzer Land, Donnersberg District, Rhineland-Palatinate, Germany, montroydite is associated with
moschellandsbergite and cinnabar
(Dana).
At the Mariquita Mine (Sultana Mine), Usagre, Badajoz, Extremadura, Spain, montroydite is a rare mineral that
usually occurs as brush-shaped acicular or prismatic crystals forming spherical or ellipsoidal groups. It is probably
a product of the oxidation of native mercury at high pH (highly alkaline),
which explains its rarity. Montroydite maintains the original shape of the
mercury droplet which it replaces, thus representing a curious case of a
solid pseudomorph after a liquid. In such specimens the
montroydite is associated with dodecahedral crystals of
eglestonite and yellowish masses of
mosesite.
Less commonly, massive montroydite covers surfaces of several square centimetres. These masses show strong
exfoliation resulting in flexible and sectile sheets of an intense reddish brown colour and high lustre, intergrown
with terlinguaite,
edgarbaileyite and
eglestonite. Very rarely, montroydite forms well-individualised
reddish prismatic crystals to 1 mm associated with calcite
(MinRec 55.4.505).
Montroydite from the Mariquita Mine -
Image
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).
Montroydite from the Funderburk Prospect -
Image
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).
Montroydite from Clear Creek -
Image
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).
Montroydite from Terlingua - Image
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