Halotrichite Locality: Golden Queen Mine, Soledad Mt., Golden Queen District, Kern County , California , USA ( Locality at mindat.org ) Size: 5.2 x 4.2 x 3.3 cm. Most likely collected in the late 1960s or early 70s, this is a gorgeous specimen of acicular (hair-like) crystals of halotrichite - here represented in a tuft perfectly set atop matrix! Halotrichite (a hydrated iron aluminum sulfate, usually a product of pyrite oxidation). — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Halotrichite
'Feather alum' — a highly hydrated iron-aluminium sulfate forming silky water-soluble fibers on mine walls.
Intro adapted from Wikipedia (CC BY-SA).
Halotrichite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Fe²⁺Al₂(SO₄)₄·22H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c). It is a member of the Halotrichite structural group.
Key properties: Mohs hardness 1.5; specific gravity 1.89; Vitreous luster; refractive index 1.485; optically biaxial.
IMA symbol Hth, grandfathered status — known before the IMA’s founding, named in 1777.
Occurs as efflorescences in weathering sulfide deposits and oxidizing pyritic coals, with persistent accumulations in arid climates; as a precipitate around volcanic fumaroles and hot springs.
It is found associated with Melanterite, copiapite, gypsum, epsomite, alunogen.
The name comes from the Latin, halotrichum, derived from the Old German, Haarsalz, for hair salts, in allusion to its common hairlike habit.