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Melanterite2 - Copperas Mountain, Paxton Township, Ross Co, Ohio, USA.jpg
Melanterite as found in nature.
Category Sulfate mineral
(repeating unit)
Strunz classification 7.CB.35
Dana classification
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group 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
Crystal habit Encrustations and capillary efflorescences; rarely as equant pseudo-octahedral, prismatic or tabular crystals
Cleavage {001} Perfect, {110} Distinct
Fracture Conchoidal
Mohs scale hardness 2
Luster Vitreous
Streak White
Diaphaneity Subtransparent to translucent
Specific gravity 1.89 - 1.9
Optical properties Biaxial (+)
Refractive index nα = 1.470 - 1.471 nβ = 1.477 - 1.480 nγ = 1.486
References [1][2][3][4]

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[2] and as a rare sublimate phase around volcanic fumaroles.[4] Associated minerals include pisanite, chalcanthite, epsomite, pickeringite, halotrichite and other sulfate minerals.[4]

It was first described in 1850.[4]