Weilerite

Chemical formula: BaAl₃(S₆+O₄)(As₅+O₄)(OH)₆

Weilerite is a discredited mineral name, historically describing a barium-, arsenic-, and sulfate-rich mineral from the alunite supergroup.

## Characteristics Weilerite is not currently recognized as a distinct mineral species by the International Mineralogical Association (IMA). This name refers to a material that is an isomorphic series member within the beudantite group, representing an intermediate link rich in barium, arsenic, and sulfate groups. It typically occurs as very fine pseudorhombohedral or pseudocubic crystals, forming small druses, crusts, or granular aggregates. Due to its status, specimens referred to as weilerite are in fact other, formally recognized minerals from this group, such as arsenian svanbergite or barium-rich beudantite. ## Physical Properties This mineral is relatively soft and brittle. Its luster ranges from vitreous to resinous, and in the case of earthy aggregates, it can be dull. The density is relatively high due to the presence of barium in the structure. ## Colors and Varieties Specimens historically described as weilerite exhibit various colors, most commonly yellowish, greenish, grayish-white, or brown. The color depends on the precise chemical composition and the presence of impurities. Since it is not a formal mineral, no varieties are distinguished; the name "weilerite" itself functions as a designation for a specific chemical variety within the group. ## History and Name The name originates from the primary discovery location – the Michael mine in Weiler (near Lahr, Black Forest, Germany). The mineral was described in 1966 by Kurt Walenta. It was later discredited when more precise studies showed that it was merely a member of a complex solid solution series, and not a unique end-member mineral. ## Uses Due to its rarity and unclear taxonomic status, weilerite has no industrial applications. It is solely an object of interest for collectors specializing in mineral systematics and scientists studying the alunite group.

Properties

Mohs hardness: 4.5
Luster: Vitreous, Resinous
Streak: White
Density: 3.97
Cleavage: Good on {0001}
Fracture: Uneven, Conchoidal
Transparency: Translucent to opaque
Crystal system: Trigonal

Diagnostic features

## Identification Visual identification is practically impossible. The small size of crystals and occurrence as crusts make observation difficult. Reliable identification requires advanced chemical analyses (e.g., EDS) to determine the proportions of barium, lead, calcium, strontium, and arsenate, sulfate, and phosphate groups. ## Distinguishing from Similar Minerals It is impossible to distinguish it with the naked eye from other members of the alunite supergroup, such as beudantite (lead-dominant), corkite (lead and phosphate), svanbergite (strontium and phosphate), hinsdalite (lead and phosphate), or gorceixite (barium and phosphate). The differences lie solely in the dominant cation and anion in the crystal structure. ## Crystal Forms It forms small, trigonal crystals that often appear cubic or rhombohedral. Most commonly found as microcrystalline crusts, earthy aggregates, or spheroidal aggregates.

Geological environment

## Genesis It is a secondary mineral, formed in the oxidation zones (iron caps) of ore deposits, mainly polymetallic. Its formation requires the simultaneous presence of a source of barium (most often from the weathering of barite), arsenic (from the oxidation of arsenopyrite or other arsenic sulfides), and sulfur (from pyrite or other sulfides). ## Mineral Associations It co-occurs with other secondary minerals of the oxidation zone, such as barite, goethite, quartz, and especially with other members of the alunite supergroup (beudantite, corkite, hinsdalite). Sometimes mimetite and pyromorphite are also associated with it. ## Localities The historical, type locality is the Michael mine in Weiler, Lahr, Black Forest, Germany. Materials with similar chemical compositions are found in oxidation zones worldwide, but they are classified under correct, recognized mineral names.

Rarity

Very rare

For collectors

## Quality Criteria For specialized collectors, the value of a specimen lies in its historical significance and origin from the type locality. The most desirable samples are those with visible, even microscopic, well-formed crystals on a contrasting matrix. The color and aesthetics of the crust also play a role. ## Popular Localities The only significant locality from a historical and collector's point of view is the Michael mine in the Black Forest (Germany), from which the original specimens described as weilerite originate.

Care and storage

## Cleaning Specimens should only be cleaned with lukewarm distilled water and a very soft brush. Ultrasonic cleaners should be avoided, as they can damage fine crystals and brittle aggregates. ## What to Avoid The mineral is sensitive to acids, which can damage it. As a relatively soft and brittle mineral, it is susceptible to mechanical damage – impacts and scratches should be avoided. Due to its arsenic content, it is recommended to wash hands after contact with the specimen. ## Storage Store in stable conditions, away from dust, moisture, and direct sunlight. It is best to keep it in a closed display box to protect delicate crystals.

Sources

IMA Mineral List (CNMNC)