Metavoltine
Chemical formula: K₂Na₆Fe²⁺Fe³⁺₆O₂(S⁶⁺O₄)₁₂·18H₂O
Metavoltine is a rare, secondary iron, sodium, and potassium sulfate, forming characteristic yellow to brown, tabular crystals.
Properties
- Mohs hardness
- 2.5
- Color
- Yellowish brown, orange-brown, greenish brown
- Luster
- Vitreous, Pearly
- Streak
- Pale yellow
- Density
- 2.51
- Cleavage
- Perfect on {0001}.
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Metavoltine is most easily identified by its characteristic yellow color, low hardness (approx. 2.5 on the Mohs scale), and typical form of small, hexagonal, tabular crystals, often grouped into rosettes. An important feature is its genesis – occurrence in the oxidation zones of sulfide deposits, often in mines or mine dumps. ## Distinguishing from Similar Minerals Metavoltine is sometimes confused with other yellow secondary sulfates, such as jarosite or copiapite. Jarosite is harder (2.5-3.5) and often forms more earthy or microcrystalline coatings. Copiapite has similar hardness, but its crystals are usually prismatic or acicular and crystallizes in the triclinic system. Certain differentiation often requires chemical or crystallographic analysis. ## Crystal Forms Metavoltine crystals are typically thin, tabular, with a hexagonal outline. They often occur as radial aggregates, rosettes, scaly clusters, and thin coatings and crusts on host rocks.
Geological environment
## Genesis Metavoltine is a secondary mineral, formed as a result of weathering (oxidation) processes of sulfide minerals, mainly pyrite and marcasite. It forms in sulfate-rich environments, under conditions of low humidity and elevated temperature. It is a typical product of mine dump fires or volcanic exhalation processes (fumaroles). ## Mineral Associations This mineral often co-occurs with other secondary sulfates, such as voltaite, copiapite, rhomboclase, halotrichite, melanterite, as well as gypsum, native sulfur, and alums. ## Localities The most important metavoltine localities worldwide include the fumaroles of Mount Vesuvius and the Phlegraean Fields in Italy (type locality), the Rammelsberg mine in Germany, the Chvaletice mine in the Czech Republic, the Iron Monarch mine in South Australia, and numerous localities in the USA, including the United Verde mine in Arizona and mine dumps in Pennsylvania.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp crystals of intense, canary-yellow color. Particularly sought after are rich groups of crystals forming aesthetic rosettes or carpets on a small rock matrix. Crystal size is also important – specimens with plates exceeding a few millimeters are considered exceptional. The purity of the specimen and the absence of mechanical damage significantly increase its value. ## Popular Localities Classic and highly prized specimens come from fumaroles in Italy (Vesuvius, Pozzuoli). Very good quality crystals have also been found in the Rammelsberg mine in Germany and in Chvaletice in the Czech Republic. In recent years, attractive specimens from mines in Arizona (USA) have also appeared on the market.
Care and storage
## Cleaning Metavoltine specimens are very sensitive to water and moisture. For dust removal, only compressed air or a very soft, completely dry brush should be used. Any contact with water, even distilled, can lead to dissolution or damage to the mineral. ## What to Avoid Contact with water, water vapor, chemical agents, and ultrasonic cleaners must be strictly avoided. The mineral is unstable in humid environments and may dehydrate or decompose. It should be protected from sudden temperature changes and direct sunlight, which can cause fading. ## Storage Metavoltine specimens must be stored in dry conditions. The best solution is a sealed container (a "perky box") with a desiccant, such as silica gel. Storage in an enclosed space also protects brittle crystals from mechanical damage.