| Halotrichite | |
| Category: | Sulfate minerals |
| Formula: | FeAl2(SO4)4·22H2O |
| Imasymbol: | Hth[1] |
| Strunz: | 7.CB.85 |
| 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 |
| Habit: | Acicular to asbestiform clusters, incrustations and efflorescences |
| Cleavage: | Poor on |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Mohs: | 1.5–2 |
| Luster: | Vitreous |
| Diaphaneity: | Transparent, translucent |
| Gravity: | 1.89 |
| Opticalprop: | Biaxial (−) |
| Refractive: | nα = 1.480 nβ = 1.486 nγ = 1.490 |
| Birefringence: | δ = 0.010 |
| 2V: | Measured: 35° |
| Solubility: | Soluble in water |
| Other: | Astringent taste |
| References: | [2] [3] [4] |
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 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 and Mont Saint-Hilaire, Canada.
The name is from Latin: halotrichum for salt hair which accurately describes the precipitate/evaporite mineral.[4]