melanterite

{{Short description|Heptahydrated iron(II) sulfate}}

{{Infobox mineral

| name = Melanterite

| category = Sulfate mineral

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| image = Melanterite2 - Copperas Mountain, Paxton Township, Ross Co, Ohio, USA.jpg

| imagesize = 260px

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| caption = Melanterite as found in nature

| formula = FeSO₄·7H₂O

| IMAsymbol = Mln{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|url=https://www.cambridge.org/core/journals/mineralogical-magazine/article/imacnmnc-approved-mineral-symbols/62311F45ED37831D78603C6E6B25EE0A|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 = 7.CB.35

| dana = 29.06.10.01

| system = Monoclinic

| class = Prismatic (2/m)
(same H-M symbol)

| symmetry = P2₁/c

| unit cell = a = 14.077 Å, b = 6.509 Å,
c = 11.054 Å; β = 105.6°; Z = 4

| color = Green, pale green, greenish blue, bluish green, colorless

| colour =

| habit = Encrustations and capillary efflorescences; rarely as equant pseudo-octahedral, prismatic or tabular crystals

| twinning =

| cleavage = {001} Perfect, {110} Distinct

| fracture = Conchoidal

| tenacity =

| mohs = 2

| luster = Vitreous

| streak = White

| diaphaneity = Subtransparent to translucent

| gravity = 1.89 – 1.9

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| opticalprop = Biaxial (+)

| refractive = nα = 1.470 – 1.471 nβ = 1.477 – 1.480 nγ = 1.486

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| references = [https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Melanterite Mineralienatlas][http://www.mindat.org/min-2633.html Mindat][http://webmineral.com/data/Melanterite.shtml Webmineral data][http://rruff.geo.arizona.edu/doclib/hom/melanterite.pdf Handbook of Mineralogy]

}}

Melanterite is a mineral form of hydrous iron(II) sulfate: FeSO₄·7H₂O. It is the iron analogue of the copper sulfate chalcanthite. It alters to siderotil by loss of water. It is a secondary sulfate mineral which forms from the oxidation of primary sulfide minerals such as pyrite and marcasite in the near-surface environment. It often occurs as a post mine encrustation on old underground mine surfaces. It also occurs in coal and lignite seams exposed to humid air and as a rare sublimate phase around volcanic fumaroles. Associated minerals include pisanite, chalcanthite, epsomite, pickeringite, halotrichite and other sulfate minerals.

It was first described in 1850.