Melanterite

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Melanterite
Melanterite2 - Copperas Mountain, Paxton Township, Ross Co, Ohio, USA.jpg
Melanterite as found in nature.
General
Category Sulfate mineral
Formula
(repeating unit)
FeSO4·7H2O
Strunz classification 7.CB.35
Dana classification 29.06.10.01
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
Identification
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]

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