{{Short description|Highly hydrated sulfate of aluminium and iron}} {{Infobox mineral | name = Halotrichite | boxwidth = | boxbgcolor = | image = Mineraly.sk - halotrichit.jpg | imagesize = 280px | alt = | caption = A sample of halotrichite | category = Sulfate minerals | formula = FeAl<sub>2</sub>(SO<sub>4</sub>)<sub>4</sub>·22H<sub>2</sub>O | IMAsymbol = Hth<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 7.CB.85 | dana = | system = Monoclinic | class = Prismatic (2m) <br/><small>(same H-M symbol)</small> | symmetry = ''P2''<sub>1</sub>/c | unit cell = a = 20.51, b = 24.29 <br/>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 = n<sub>α</sub> = 1.480 n<sub>β</sub> = 1.486 n<sub>γ</sub> = 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 = <ref name=HBM>[https://rruff.info/doclib/hom/halotrichite.pdf Handbook of Mineralogy]</ref><ref name=Mindat>[http://www.mindat.org/min-1809.html Halotrichite on Mindat.org]</ref><ref name=Webmin>[https://webmineral.com/data/Halotrichite.shtml Halotrichite data on Webmineral]</ref> }}
'''Halotrichite''', also known as '''feather alum''', is a highly hydrated sulfate of aluminium and iron. Its chemical formula is {{chem2|FeAl2(SO4)4*22H2O}}. It forms fibrous monoclinic crystals. The crystals are water-soluble.
It is formed by the weathering and decomposition of pyrite commonly near or in volcanic vents. The locations of natural occurrences include: the Atacama Desert, Chile; Dresden in Saxony, Germany; San Juan County, Utah; Iceland; Idrija, Slovenia; and Mont Saint-Hilaire, Canada.
The name is from Latin: ''halotrichum'' for salt hair which accurately describes the precipitate/evaporite mineral.<ref name=Webmin/>
;Gallery <Gallery> File:Halotrichite Hydrous iron aluminum sulfate Corral, California 3009.jpg|Halotrichite from California File:Halotrichite-179634.jpg|Halotrichite from the abandoned Golden Queen mine on Soledad Mountain south of Mojave, California </Gallery>
==References== {{reflist}} *[https://www.saint-hilaire.ca/en/halotri.htm Saint-Hilaire] *{{usurped|1=[https://web.archive.org/web/20061124193345/http://www.mineralatlas.com/mineral%20general%20descriptions/H/halotrichitepcd.htm Mineral Atlas]}}
Category:Iron(II) minerals Category:Aluminium minerals Category:Sulfate minerals Category:Monoclinic minerals Category:Minerals in space group 14
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