Segnitite

Chemical formula: Pb₂+Fe₃+₃(As₅+O₄)(As₅+O₃OH)(OH)₆

Segnitite is a rare lead-iron arsenate, forming characteristic, lustrous crystals ranging in color from yellowish-brown to greenish-brown.

## Characteristics Segnitite is a hydrated lead and iron arsenate, belonging to the alunite supergroup. This mineral typically forms well-developed, lustrous crystals with a rhombohedral or pseudocubic-octahedral appearance. Crystals often have curved, rhombic faces, giving them a barrel-shaped habit. It also occurs as granular aggregates, crusts, and earthy masses. ## Physical Properties Segnitite crystals are characterized by a Mohs hardness of 4. They have a vitreous to resinous luster, and on some surfaces, it can be dull or earthy. It is a relatively dense mineral, with a specific gravity of approximately 4 g/cm³. It is brittle, and its fracture is conchoidal. ## Colors and Varieties Segnitite most commonly occurs in colors ranging from yellowish-brown, through brown, greenish-brown, to pale yellow. Some specimens can be almost black. No named varieties are distinguished. ## History and Name The mineral was first described in 1991 by P. J. Bridge, M. W. Pryce, and J. E. Just. Its name honors Edgar Ralph Segnit (1923–1999), an Australian mineralogist and petrologist, who first synthesized this compound in 1966, and for his contributions to Australian mineralogy. The type locality is the Broken Hill mine in New South Wales, Australia. ## Uses Segnitite has no industrial applications. It is solely an object of interest for collectors of rare minerals and scientists.

Properties

Mohs hardness: 4
Luster: Vitreous to Resinous
Streak: Pale yellow
Density: 4
Cleavage: None
Fracture: Conchoidal
Transparency: Transparent to Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Segnitite can be identified by its characteristic crystal shape – often barrel-shaped rhombohedra with curved faces. Its brownish-yellow to greenish color, resinous luster, and relatively high density are also important clues. Its occurrence in the oxidation zones of ore deposits, especially in association with other arsenates, is crucial for identification. ## Distinguishing from Similar Minerals Segnitite is sometimes confused with other minerals from the alunite supergroup, such as beudantite or jarosite. Distinguishing it from beudantite, which contains sulfate instead of one of the arsenate groups, is often impossible without advanced chemical analysis (e.g., EDS). It differs from mimetite (often co-occurring) in crystal form and lower hardness. It differs from goethite by its crystalline structure and higher density. ## Crystal Forms Most commonly, it forms single, well-developed trigonal crystals that appear as rhombohedra or pseudocubic-octahedra. Crystal faces are often curved. It also occurs as rosette-like aggregates, spherulitic forms, crusts, and earthy masses.

Geological environment

## Genesis Segnitite is a secondary mineral, forming in the oxidation zones (gossans) of polymetallic ore deposits rich in lead and arsenic. It forms as a result of the weathering of primary sulfides, such as galena, and arsenic minerals, e.g., arsenopyrite, under conditions of low temperature and pressure. ## Mineral Associations This mineral often co-occurs with other secondary arsenate and sulfate minerals. Its most common associations include beudantite, mimetite, duftite, agardite, goethite, hematite, quartz, as well as cerussite and anglesite. ## Localities The most important and well-known occurrences of segnitite worldwide are the Broken Hill mine in New South Wales (Australia), where it was discovered. It also occurs in Tsumeb, Namibia, in mines of the Lavrion region in Greece, in several locations in Germany (e.g., in the Black Forest), and in Arizona and Utah in the USA.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-developed, sharp, and lustrous crystals of intense, honey-yellow or greenish-brown color are most valued by collectors. Crystals set on a contrasting rock matrix are highly prized, especially in association with other rare secondary minerals. Crystal size is crucial – specimens exceeding a few millimeters are already considered significant. ## Popular Localities The world's best segnitite specimens, considered exemplary, come from the type locality – the Broken Hill mine in Australia. Very good quality crystals have also been found in the Tsumeb mine in Namibia, which is famous for its perfectly formed secondary minerals. Specimens from these two locations fetch the highest prices on the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided, as they can damage the brittle crystals. ## What to Avoid As an arsenate, segnitite is toxic. Avoid inhaling dust and wash hands thoroughly after any contact with the mineral. It should not be heated or exposed to acids and other chemicals, which can decompose it and release toxic compounds. Protect it from high temperatures and humidity. ## Storage It is recommended to store segnitite in closed, labeled containers or display cases, away from children and pets. To prevent mechanical damage, it should be isolated from harder minerals. Due to its potential toxicity, it should not be stored in areas where food is prepared.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FSegnitite