halotrichite

{{Infobox mineral | name = Halotrichite | boxwidth = | boxbgcolor = | image = Mineraly.sk - halotrichit.jpg | imagesize = 280px | alt = | caption = A sample of halotrichite | category = Sulfate minerals | formula = FeAl2(SO4)4·22H2O | IMAsymbol = Hth | molweight = | strunz = 7.CB.85 | dana = | system = Monoclinic | class = Prismatic (2m) (same H-M symbol) | symmetry = P21/c | unit cell = a = 20.51, b = 24.29 c = 6.18 [Å]; β = 100.99°; Z = 4 | color = Colorless to white, yellowish, greenish | colour = | habit = Acicular to asbestiform clusters, incrustations and efflorescences | twinning = | cleavage = Poor on {010} | fracture = Conchoidal | tenacity = Brittle | mohs = 1.5–2 | luster = Vitreous | streak = | diaphaneity = Transparent, translucent | gravity = 1.89 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.480 nβ = 1.486 nγ = 1.490 | birefringence = δ = 0.010 | pleochroism = | 2V = Measured: 35° | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = Soluble in water | impurities = | alteration = | other = Astringent taste | prop1 = | prop1text = | references = }}

Halotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium and iron. Its chemical formula is . It forms fibrous monoclinic crystals. The crystals are water-soluble.