Melanterite
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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. Wikipedia
Cleavage: Perfect,, Distinct
Color: Green, pale green, greenish blue, bluish green, colorless
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Encrustations and capillary efflorescences; rarely as equant pseudo-octahedral, prismatic or tabular crystals
Crystal system: Monoclinic
Formula (repeating unit): FeSO4·7H2O
Optical properties: Biaxial (+)