Melanterite

Melanterite
Melanterite as found in nature
General
Category	Sulfate mineral
Formula (repeating unit)	FeSO4·7H2O
IMA symbol	Mln
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

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.

Gallery

• 
  Crystal structure of melanterite
• 
  Cuprian melanterite