melanterite

{{Infobox mineral | name = Melanterite | category = Sulfate mineral | boxwidth = | boxbgcolor = | image = Melanterite2 - Copperas Mountain, Paxton Township, Ross Co, Ohio, USA.jpg | alt = | caption = Melanterite as found in nature | formula = FeSO4·7H2O | IMAsymbol = Mln | 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 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.470 – 1.471 nβ = 1.477 – 1.480 nγ = 1.486 | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }}

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.