Formula: K2Fe2+5Fe3+3Al(SO4)12.18H2O
Hydrated normal sulphate, voltaite group
Crystal System: Isometric
Specific gravity: 2.645 measured, 2.663 calculated
Hardness: 3 to 3½
Streak: Grey-green
Colour: Green to greenish-black; oil-green to pale green in transmitted light
Solubility: Soluble with difficulty in water, but readily soluble in dilute acid

Hydrothermal environments

Voltaite occurs in volcanic fumaroles, solfataras (fumeroles whose gases are characteristically sulphurous) and mud volcanos; it is an alteration product in mineral deposits rich in pyrite, especially in arid climates, where it may also be a post-mining product. Associated minerals include halotrichite, pickeringite, alunogen, metavoltine, römerite, hexahydrite, coquimbite, copiapite, rhomboclase, krausite, jarosite, goldichite, botryogen and melanterite (HOM).


At the Alcaparrosa Mine, Sierra Gorda, Antofagasta Province, Antofagasta, Chile, voltaite occurs as leek-green crushed fragments and as small crystals embedded in pickeringite and alum (AM 23.749).

At the Chuquicamata Mine, Chuquicamata District, Calama, El Loa Province, Antofagasta, Chile, voltaite is not an abundant mineral, being confined to the areas of massive pyrite veins and commonly associated with coquimbite. It appears to be earlier than metavoltine although the minerals have not been observed intimately associated. It is later than coquimbite (AM 23.749).

At the Queténa Mine, Toki Cu deposit, Chuquicamata District, Calama, El Loa Province, Antofagasta, Chile, massive voltaite occurs in massive coquimbite in the bottom of the pit (AM 23.749).

At the type locality, Solfatara di Pozzuoli, Pozzuoli, Naples, Campania, Italy, voltaite occurs in the solfatara associated with halotrichite (Mindat).

At the Zagh Mine, Haji Abad, Bandar Abbas, Bandar Abbas County, Hormozgan Province, Iran, voltaite occurs with metavoltine and römerite (Dana).

At the Magma mine, Pioneer District, Pinal county, Arizona, USA, voltaite has been found associated with rhomboclase and szomolnokite, probably as a result of acidic mine drainage (R&M 95.1.88).

At the United Verde mine, Jerome, Verde Mining District, Black Hills, Yavapai County, Arizona, USA, specimens of voltaite were formed in brecciated quartz immediately above burning sulphide ore on the upper levels of the mine. Voltaite when fresh occurs in small, black, opaque, resinous crystals varying in size from 1 mm to 5 mm. It alters rather quickly upon exposure to the air, forming a yellow powder (AM 12.7.287-289).

At the Huron Shale burn site, Huron River, Huron county, Ohio, USA, voltaite occurs as dark green to black, glossy isometric crystals to 1 mm, often forming continuous druses to 1 cm in diameter. Some hoppering of crystal faces is often seen (R&M 92-3.258).

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