Melanterite

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]

Melanterite
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]

It was first described in 1850.[4]