Siderotil

(repeating unit)Fe²⁺SO₄·5H₂OIMA symbolSdt^[1]Strunz classification7.CB.20Crystal systemTriclinicCrystal classPinacoidal (1)
(same H-M symbol)Space groupP1Unit cella = 6.26 Å, b = 10.63 Å
c = 6.06 Å; α = 97.25°
β = 109.67°, γ = 75°; Z = 2IdentificationColorPale green, yellowish, whiteCrystal habitFibrous to powdery, rarely as acicular crystalsMohs scale hardness2.5LusterVitreous or silkyDiaphaneityTransparent to translucentSpecific gravity2.1 - 2.2Optical propertiesBiaxial (-)Refractive indexn_α = 1.513 - 1.515 n_β = 1.525 - 1.526 n_γ = 1.534 - 1.536Birefringenceδ = 0.0212V angleMeasured: 50°, calculated: 80° to 86°SolubilitySoluble in waterReferences^[2][3][4]

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]

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]