Voltaite

Chemical formula: K₂Fe₂+₅Fe₃+₃Al(S₆+O₄)₁₂·₁₈H₂O

Voltaite is a rare, hydrated iron and potassium sulfate, forming dark, isometric crystals in the oxidation zones of sulfide deposits.

## Characteristics Voltaite is a complex, hydrated iron, potassium, and aluminum sulfate, belonging to the voltaite group. It typically forms well-developed, isometric crystals, most often as octahedra, rhombic dodecahedra, or combinations thereof. Less frequently, it occurs as granular aggregates or crusts. Its crystals are usually opaque, with a dark, almost black color with a greenish or olive hue. It is a secondary mineral, unstable under normal atmospheric conditions. ## Physical Properties The hardness of voltaite on the Mohs scale is 2.5, making it a relatively soft mineral. It has a luster described as resinous to greasy. It is brittle, and its fracture is conchoidal to uneven. The mineral's density ranges from 2.59 to 2.77 g/cm³. Voltaite is soluble in water, leaving a yellow precipitate upon dissolution. ## Colors and Varieties This mineral is characterized by a fairly uniform coloration. Its color is most often dark green, olive-green, brown, or black. The streak is greenish-gray. No distinct color varieties or commercial varieties are recognized. ## History and Name Voltaite was first described in 1841 by Arcangelo Scacchi. The mineral's name honors the Italian physicist Alessandro Volta (1745–1827), a pioneer in electricity research. It was discovered in the fumaroles of Mount Vesuvius in Italy. ## Uses Voltaite has no industrial applications. Due to its rarity, attractive crystal forms, and genesis, it is a prized collector's mineral.

Properties

Mohs hardness: 2.5
Luster: Resinous to greasy
Streak: Greenish-gray
Density: 2.59-2.77
Cleavage: None
Fracture: Conchoidal to uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Characteristic features of voltaite include its regular, most often octahedral crystals, dark green to black color, resinous luster, and low hardness. A key diagnostic feature is its water solubility. ## Distinguishing from Similar Minerals Voltaite is sometimes confused with other dark sulfate minerals or oxides found in similar environments. It differs from chalcanthite by its color (chalcanthite is blue) and crystal form. From tenorite or goethite, it differs significantly in its much lower hardness, luster, and water solubility. ## Crystal Forms Voltaite crystallizes in the isometric system, forming distinct, isometric crystals. Dominant forms are the octahedron {111} and the rhombic dodecahedron {110}, often occurring in combinations. Crystals can reach several centimeters in size.

Geological environment

## Genesis Voltaite is a secondary mineral. It forms as a result of volcanic fumarolic activity, where it crystallizes from hot, sulfur-rich gases. It also forms in the oxidation zones (gossans) of sulfide ore deposits, especially pyrite and chalcopyrite, under dry and desert climatic conditions. It is a product of the evaporation of acidic mine waters. ## Mineral Associations This mineral often co-occurs with other sulfates, such as melanterite, copiapite, halotrichite, pickeringite, as well as with pyrite, goethite, and native sulfur. ## Localities Key voltaite localities worldwide include the areas around Vesuvius and Vulcano volcano in Italy (type locality); the Rammelsberg mine in the Harz Mountains, Germany; the Iron Monarch mine in South Australia; Jerome in Arizona, Bisbee in Arizona, and Calico Hills in California (USA); and the Atacama Desert in Chile.

Rarity

Rare

For collectors

## Quality Criteria Specimens with sharp, well-formed, large crystals with distinct luster are most valued by collectors. Combinations of crystal forms (e.g., octahedron with dodecahedron) are particularly sought after. A contrasting matrix with other associated minerals, such as bright yellow copiapite, significantly enhances the specimen's attractiveness. A dark green, olive color is considered more desirable than black. ## Popular Localities Classic, historical specimens come from the fumaroles of Vesuvius in Italy. Large and well-formed crystals were found in Jerome, Arizona (USA). The Rammelsberg mine in Germany also provided many high-quality collector specimens.

Care and storage

## Cleaning Voltaite is water-soluble, so it must never be wet cleaned. To remove dust, use only a soft brush or compressed air from a safe distance. ## What to Avoid Avoid any contact with water, water vapor, and high humidity, which can cause chemical decomposition of the mineral. It should also be protected from high temperatures and direct sunlight, which can accelerate dehydration and oxidation processes. Avoid contact with chemicals. ## Storage Voltaite specimens require storage in dry conditions. It is best to place them in an airtight, sealed box (such as a "membrane box" or a box with a desiccant, e.g., silica gel). Display should be in a closed display case, away from sources of moisture and heat.

Sources

IMA Mineral List (CNMNC)