Formula: Fe3+2O(SO4)2.8H2O
Hydrated sulphate
Crystal System: Triclinic
Specific gravity: 2.255 measured, 2.250 calculated
Hardness: 3
Streak: Orange-yellow
Colour: Brown to burnt orange, light amaranth-red
Luminescence: Not fluorescent under UV
Solubility: Decomposed by hot water but virtually insoluble in cold water, readily soluble in hydrochloric acid

Hydrothermal environments

Hohmannite is a low-temperature precipitate in oxidised iron sulphide deposits (Webmin); it rapidly dehydrates to metahohmannite on exposure to air (AM 23.746).


At the type locality, Sierra Gorda, Antofagasta Province, Antofagasta, Chile, hohmannite is always intimately associated with chalcanthite and picromerite. The chalcanthite is an earlier mineral and is believed to be formed from an earlier iron-chalcanthite (copper-bearing melanterite?). Hohmannite is later than both fibroferrite and copiapite and earlier than amarantite (AM 23.746). Sideronatrite is another associated mineral (Mindat).

At the Redington mine, Knoxville, Knoxville Mining District, Napa county, California, USA, hohmannite occurs in opaline. Opal is the principal constituent with some secondary chalcedony, disseminated pyrite and marcasite, possibly millerite, and relict grains of picotite from the peridotite stage.
All specimens contain minute well-formed sulphur crystals implanted on the hohmannite and a few of the specimens show cinnabar that occurs as a coating on the opaline fragments and seems to be earlier than the hohmannite. The following sulphate minerals have been found at this mine: copiapite, coquimbite, botryogen, redingtonite and knoxvillite.
The hohmannite consists of a massive aggregate of minute subhedral crystals out of which have grown clusters and single prismatic crystals. The massive hohmannite is burnt-orange in colour and the crystals are dark brown. Hohmannite is unstable above 27"C. Its ultimate source is probably the marcasite and pyrite that are prominent in some parts of the mine (AM 16.396-404 as castanite).

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