montroydite

{{Infobox mineral | name = Montroydite | boxwidth = | boxbgcolor = | image = Montroydite-mf17c.jpg | imagesize = | alt = | caption = Vein of dark red montroydite and orange kleinite in a matrix of white calcite | category = Oxide mineral | formula = HgO | IMAsymbol = Mtyd | strunz = 4.AC.15 | dana = | system = Orthorhombic | class = Dipyramidal (mmm)H-M Symbol: (2/m 2/m 2/m) | symmetry = | unit cell = a = 5.52 Å, b = 6.6 Å, c = 3.52 Å; Z=4 | color = Deep red, brownish red to brown | habit = Long prismatic, equant, rarely flattened; striated; massive to vermicular clusters | twinning = | cleavage = Perfect {010} | fracture = | tenacity = Sectile | mohs = 1.5 - 2.0 | luster = Sub-adamantine, vitreous | streak = Yellow brown | diaphaneity = Transparent to translucent | gravity = 11.23 | polish = | opticalprop = Biaxial (+) | refractive = nα = 2.370 nβ = 2.500 nγ = 2.650 | birefringence = δ = 0.280 | pleochroism = Deep red-orange to yellowish brown (visible in thick sections) | 2V = Large | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }} Montroydite is the mineral form of mercury(II) oxide with formula HgO. It is a rare mercury mineral. It was first described for an occurrence in the mercury deposit at Terlingua, Texas and named for Montroyd Sharp who was an owner of the deposit.

Montroydite occurs in mercury deposits of hydrothermal origin. Associated minerals include: native mercury, cinnabar, metacinnabar, calomel, eglestonite, terlinguaite, mosesite, kleinite, edgarbaileyite, gypsum, calcite and dolomite.