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.^[2]

Montroydite
Vein of dark red montroydite and orange kleinite in a matrix of white calcite
General
Category	Oxide mineral
Formula (repeating unit)	HgO
IMA symbol	Mtyd[1]
Strunz classification	4.AC.15
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M Symbol: (2/m 2/m 2/m)
Unit cell	a = 5.52 Å, b = 6.6 Å, c = 3.52 Å; Z=4
Identification
Color	Deep red, brownish red to brown
Crystal habit	Long prismatic, equant, rarely flattened; striated; massive to vermicular clusters
Cleavage	Perfect {010}
Tenacity	Sectile
Mohs scale hardness	1.5 - 2.0
Luster	Sub-adamantine, vitreous
Streak	Yellow brown
Diaphaneity	Transparent to translucent
Specific gravity	11.23
Optical properties	Biaxial (+)
Refractive index	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 angle	Large
References	[2][3]

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.^[3]