Sewardite

Chemical formula: CaFe³⁺₂(As⁵⁺O₄)₂(OH)₂

Sewardite is a rare calcium and iron arsenate, forming acicular crystals ranging in color from dark red to orange.

## Characteristics Sewardite is an arsenate belonging to the carminite group. It occurs as very small, acicular or bladed crystals, which often form radial or tangled aggregates. Individual crystals rarely exceed 1 mm in length, making them primarily micromount specimens. Its appearance is distinctive due to its intense color and vitreous luster. ## Physical Properties This mineral has a hardness of 3.5 on the Mohs scale. It exhibits a vitreous luster and is transparent to translucent. The streak left on a plate is reddish-brown. The density has not been precisely measured, but the calculated value is approximately 4.15 g/cm³. ## Colors and Varieties Sewardite ranges in color from dark red, through reddish-brown, to bright orange. No named varieties are distinguished. ## History and Name The mineral is named in honor of Terry M. Seward (born 1940), a geochemist from ETH Zurich, Switzerland, for his contributions to research on metal transport in hydrothermal solutions. It was described as a new mineral in 1993 by John Innes, A.C. Roberts, and J.A. Grice, based on material from the Tsumeb mine in Namibia. ## Uses Sewardite has no industrial application. It is solely a mineral of scientific and collector's interest, sought after by specialists collecting rare minerals and micromounts.

Properties

Mohs hardness: 3.5
Luster: Vitreous
Streak: Reddish-brown
Density: 0
Cleavage: Imperfect on {100} and {011}
Fracture: Irregular/Uneven,Splintery
Transparency: Transparent,Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Sewardite can be identified by its characteristic form – small, acicular crystals forming radial aggregates. The intense reddish-orange color, vitreous luster, and reddish-brown streak are key identifying features. Its occurrence in the oxidation zones of polymetallic deposits is also an important clue. ## Distinguishing from Similar Minerals It can be confused with other arsenates from the carminite group, such as carminite itself (PbFe₂(AsO₄)₂(OH)₂), which has a similar color and form but contains lead. Differentiation requires advanced chemical analysis (EDS). It differs from arseniosiderite by the presence of calcium and its crystal habit. ## Crystal Forms Sewardite crystals are elongated, acicular or bladed, extended along the crystallographic axis [001]. They usually form fan-shaped, radial, or haphazardly tangled aggregates. They rarely occur as single, isolated crystals.

Geological environment

## Genesis Sewardite is a secondary mineral formed in the oxidation zones (gossans) of hydrothermal polymetallic deposits, rich in arsenic. It forms as a result of the weathering of primary ore minerals, mainly arsenopyrite and tennantite, in the presence of calcium- and iron-rich solutions. ## Mineral Associations At its type locality in Tsumeb, it co-occurs with quartz, tennantite, arseniosiderite, prosopite, hematite, and anglesite. In other locations, it has been found in association with beudantite, olivenite, agardite, and pharmacosiderite. ## Localities The most important and type locality for sewardite is the Tsumeb mine in Namibia. It is also known from several other places worldwide, including the Clara mine in the Black Forest (Germany) and the Greek region of Lavrion (Sounion and Agios Konstantinos mines).

Rarity

Very rare

For collectors

## Quality Criteria The most prized sewardite specimens are those with well-formed, undamaged, radial aggregates of intense red or orange color. Due to the microscopic size of the crystals, the aesthetics of the entire aggregate and its contrast with the rock matrix, most often quartz, are crucial. The purity of the specimen and the absence of mechanical damage significantly increase its value. ## Popular Localities Specimens from the type locality – the Tsumeb mine in Namibia – are by far the most sought after by collectors. They are considered classic and exemplary for this mineral.

Care and storage

## Cleaning Sewardite specimens should be cleaned with the utmost care, preferably using compressed air to remove dust. Due to its fragility and potential reactivity, contact with water and any chemical agents should be avoided. If mechanical cleaning is necessary, a very soft, dry brush can be used. ## What to Avoid Absolutely avoid contact with water, acids, and other chemicals that could damage or dissolve the mineral. Sewardite is brittle, so it must be protected from impacts, vibrations, and sudden temperature changes. As an arsenate, mineral dust is toxic – avoid inhaling it. ## Storage It is recommended to store specimens in closed, stable containers, preferably in "micromount" boxes, which protect them from dust, moisture, and mechanical damage. It should be stored away from minerals that might react with it.