Meta-alunogen

Chemical formula: Al₂(S⁶⁺O₄)₃·₁₄H₂O

Meta-alunogen is a rare, hydrated aluminum sulfate, forming tiny, fibrous crystals or efflorescences, found in mines and on mine dumps.

## Characteristics Meta-alunogen is a hydrated aluminum sulfate, closely related to alunogen. It occurs as very small, acicular or fibrous crystals, which often form aggregates, coatings, and efflorescences. Due to its delicate nature and small crystal size, individual forms are rarely visible to the naked eye. It is usually observed as white or pale yellow, silky masses. ## Physical Properties It is a very soft and light mineral. Its crystals are flexible. The luster is most often silky, and the mineral is translucent. It is soluble in water. ## Colors and Varieties Meta-alunogen is typically white or pale yellow. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition and its relation to alunogen. The prefix "meta-" refers to a lower degree of hydration compared to alunogen (which has 17 H₂O molecules). It was first described in 1942 by Samuel George Gordon based on material from Chile. ## Uses Meta-alunogen has no industrial application. It is solely of interest to collectors specializing in rare or secondary minerals.

Properties

Mohs hardness: 1-2
Luster: Silky
Streak: White
Density: 0
Cleavage: {010}
Fracture: Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Meta-alunogen is identified by its typical forms of occurrence – silky, fibrous coatings and aggregates in volcanic or mining environments. Key characteristics include its solubility in water and very low hardness. It reacts to moisture, becoming sticky to the touch. ## Distinguishing from Similar Minerals It can be confused with other secondary sulfates, such as alunogen, epsomite, or halotrichite. Alunogen has a very similar appearance but a different water content, which is impossible to verify without laboratory analysis. Epsomite and halotrichite also form acicular crystals, but often have a bitter or astringent taste, which is a diagnostic feature (although taste testing is not recommended). ## Crystal Forms Crystals are very fine, acicular or hair-like, often grouped into bundles, fibrous aggregates, crusts, and efflorescences. They rarely form shapes observable without magnification.

Geological environment

## Genesis Meta-alunogen is a secondary mineral. It forms as a result of the weathering (oxidation) of sulfides, such as pyrite, in aluminum-rich rocks. It is also a product of volcanic fumarole activity, where it crystallizes from hot gases. It is often found as efflorescences on mine walls and on mine dumps. ## Mineral Associations It often co-occurs with other sulfates, such as alunogen, melanterite, epsomite, gypsum, and also with native sulfur in fumarolic environments. ## Localities The type locality (locus typicus) is Oficina María Elena in the Antofagasta region of Chile. Other known localities include the area around Mount Vesuvius in Italy, mine dumps in the Czech Republic (e.g., Lomnice), and some sites in the USA.

Rarity

Rare

For collectors

## Quality Criteria The collector's appeal of a specimen depends on the abundance and purity of the fibrous aggregates. Specimens with well-formed, silky aggregates, contrasting with the color of the host rock, are most valued. Due to its rarity, even small but well-identified samples are valuable to systematic collectors. ## Popular Localities Specimens from classic localities, such as Vesuvius or the type locality in Chile, are more sought after by collectors.

Care and storage

## Cleaning The mineral is soluble in water, so it must absolutely not be wet cleaned. Specimens can only be cleaned very gently, using a soft brush or compressed air from a safe distance, to remove dust. ## What to Avoid Avoid contact with water and any liquids that could dissolve it. It is very sensitive to airborne moisture, which can lead to its slow degradation. It should be protected from high temperatures, which can cause further dehydration and transformation into another mineral. ## Storage Meta-alunogen specimens must be stored in dry conditions. The best solution is a sealed container (e.g., a "perky box") with a desiccant (e.g., silica gel). It should be kept away from heat sources and direct sunlight.

External references

https://www.mindat.org/min-2660.html