montroydite

{{Short description|Mercury(II) oxide mineral, HgO}}

{{Infobox mineral

| name = Montroydite

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| image = Montroydite-mf17c.jpg

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| caption = Vein of dark red montroydite and orange kleinite in a matrix of white calcite

| category = Oxide mineral

| formula = HgO

| IMAsymbol = Mtyd{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3 |pages=291–320|doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}

| 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

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| cleavage = Perfect {010}

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| 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 = n_α = 2.370 n_β = 2.500 n_γ = 2.650

| birefringence = δ = 0.280

| pleochroism = Deep red-orange to yellowish brown (visible in thick sections)

| 2V = Large

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| references = [https://www.mindat.org/min-2771.html Mindat.org]{{Cite web |url=http://www.handbookofmineralogy.com/pdfs/montroydite.pdf |title=Handbook of Mineralogy |access-date=2017-08-04 |archive-date=2022-07-02 |archive-url=https://web.archive.org/web/20220702191750/http://www.handbookofmineralogy.com/pdfs/montroydite.pdf |url-status=dead }}

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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.