Halotrichite
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Halotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium and iron. Its chemical formula is FeAl₂(SO₄)₄·22H₂O. It forms fibrous monoclinic crystals. The crystals are water-soluble.
It is formed by the weathering and... Wikipedia
Crystal habit: Acicular to asbestiform clusters, incrustations and efflorescences
Formula (repeating unit): FeAl2(SO4)4·22H2O
Other characteristics: Astringent taste
Space group: P21/c