Orthowalpurgite

Chemical formula: (U₆+O₂)Bi₃+₄O₄(As₅+O₄)₂·₂H₂O

Orthowalpurite is a very rare, radioactive uranyl bismuth arsenate, forming small, tabular, yellow crystals.

## Characteristics Orthowalpurite is a rare secondary mineral from the arsenate group, chemically classified as a hydrated uranyl bismuth arsenate. It occurs as very small, thin, tabular or bladed crystals, rarely exceeding 1-2 mm in length. These crystals often form small, radial aggregates, fans, or disordered clusters on the surface of the host rock. Its appearance is typical for secondary uranium minerals, characterized by a vivid, waxy-yellow to yellowish-brown color. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 3. It is characterized by a very high density, approximately 6.45 g/cm³, which is a result of the presence of heavy elements – bismuth and uranium. The luster is described as greasy to almost adamantine. It is transparent to translucent. Due to its uranium content, orthowalpurite is a strongly radioactive mineral. ## Colors and Varieties Orthowalpurite occurs in shades of yellow – from waxy-yellow, through golden-yellow, to yellowish-brown. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1993 by Kurt Walenta. Its name refers to the fact that it is an orthorhombic dimorph of trigonal walpurite, with which it is structurally related. The name of the parent mineral, walpurite, comes from the Walpurgis vein in the Weißer Hirsch mine in Schneeberg (Germany), where both minerals were discovered. ## Applications Due to its extreme rarity and occurrence as microscopic crystals, orthowalpurite has no industrial application. It is solely an object of scientific interest and a prized acquisition for specialized collectors of rare minerals.

Properties

Mohs hardness: 3
Luster: Greasy to Adamantine
Streak: Light yellow
Density: 6.45
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Characteristic features of orthowalpurite include its waxy-yellow color, small, tabular crystals, very high density, and strong radioactivity. Its specific occurrence environment – oxidation zones of U-Bi-As deposits and co-occurrence with other rare bismuth and uranium arsenates – is also crucial for identification. ## Distinguishing from Similar Minerals Orthowalpurite is visually indistinguishable from walpurite. They are dimorphs, meaning they have the same chemical composition but different crystal structures. Certain differentiation requires advanced analytical methods, such as X-ray diffraction (XRD). It is distinguished from other yellow secondary uranium minerals (e.g., autunite, uranophane) by its significantly higher density and paragenesis with bismuth minerals. ## Crystal Forms It forms thin, tabular crystals with a rectangular or bladed outline. These crystals often arrange themselves into fan-like or rosette aggregates, as well as disordered clusters on the rock surface.

Geological environment

## Genesis Orthowalpurite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal ore deposits rich in uranium, bismuth, and arsenic. It forms as a result of complex chemical processes occurring under the influence of groundwater and atmospheric oxygen, which interact with primary ores of uraninite, native bismuth, and arsenopyrite. ## Mineral Associations It most often co-occurs with its dimorph – walpurite. It is also accompanied by other rare secondary minerals, such as atelestite, preisingerite, zeunerite, torbernite, uranospinite, as well as remnants of primary minerals, e.g., native bismuth or quartz. ## Localities This is an extremely rare mineral, known mainly from its type locality – the historic Weißer Hirsch mine in Neustädtel, in the Schneeberg area in Saxony (Germany). This is a classic locality for many rare uranyl and bismuth arsenates. Beyond this locality, its occurrences are very few and uncertain.

Rarity

Very rare

For collectors

## Quality Criteria The most valued orthowalpurite specimens are those with sharply formed, distinct crystals, even if they are microscopic in size. Rich clusters of crystals forming fans or rosettes on a small rock matrix are highly prized. The contrasting color of associated minerals (e.g., white quartz) enhances the attractiveness of the specimen. Due to its rarity, every well-documented specimen is valuable. ## Popular Localities The only source for collecting orthowalpurite specimens is its type locality – the Schneeberg area in Germany. Specimens from the historic dumps and old workings of this mine are extremely sought after by specialized collectors.

Care and storage

## Cleaning Orthowalpurite specimens are extremely delicate and sensitive. Cleaning should be kept to an absolute minimum. Only careful dust removal with a soft brush or blowing it off with a stream of compressed air from a safe distance is permissible. Any contact with water and chemicals should be avoided. ## What to Avoid Orthowalpurite is a strongly radioactive and toxic mineral due to its uranium and arsenic content. Inhaling dust and direct skin contact must be strictly avoided. Hand washing after each contact is mandatory. The mineral is sensitive to heat and acids. It should not be heated or exposed to prolonged strong light. ## Storage Specimens should be stored in tightly sealed, clearly labeled containers (e.g., "perky box" type), with a warning label about radioactivity and toxicity. They should be kept away from other minerals that could be damaged by radiation. Store in a dry, cool, and well-ventilated place, away from areas of permanent human habitation, especially bedrooms and living rooms.

Sources

IMA Mineral List (CNMNC)