Siderotil

Siderotil is an iron(II) sulfate hydrate mineral with formula: FeSO₄·5H₂O which forms by the dehydration of melanterite.^[2] Copper commonly occurs substituting for iron in the structure. It typically occurs as fibrous or powdery encrustations, but may also occur as acicular triclinic crystals.^[3]

Siderotil
General
Category	Sulfate minerals
Formula	Fe2+SO4·5H2O
IMA symbol	Sdt[1]
Strunz classification	7.CB.20
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 6.26 Å, b = 10.63 Å c = 6.06 Å; α = 97.25° β = 109.67°, γ = 75°; Z = 2
Identification
Color	Pale green, yellowish, white
Crystal habit	Fibrous to powdery, rarely as acicular crystals
Mohs scale hardness	2.5
Luster	Vitreous or silky
Diaphaneity	Transparent to translucent
Specific gravity	2.1 - 2.2
Optical properties	Biaxial (-)
Refractive index	nα = 1.513 - 1.515 nβ = 1.525 - 1.526 nγ = 1.534 - 1.536
Birefringence	δ = 0.021
2V angle	Measured: 50°, calculated: 80° to 86°
Solubility	Soluble in water
References	[2][3][4]

It was first described in 1891 for an occurrence in the Idrija Mine, Idrija, Slovenia. Its name derives from the Greek sideros (iron) and tilos (fiber) in reference to its iron content and typical fibrous form.^[2] However, the material at the Idrija location may not be siderotil, but the mineral has been authenticated from a wide variety of worldwide locations.^[3]