Söhngeite

Chemical formula: Ga(OH)<sub>3</sub>

Gallium hydroxide, a rare secondary mineral forming characteristic pseudocubic crystals, known primarily from the Tsumeb mine in Namibia.

## Characteristics Söhngeite is a hydrated gallium hydroxide. It forms small, usually microscopic crystals with a characteristic pseudocubic or pseudo-octahedral appearance, which is a result of specific twinning. Crystals are often transparent to translucent, colorless, white, or grayish. ## Physical Properties Its Mohs hardness is 3-4. The mineral has a vitreous luster, becoming pearly on cleavage surfaces. It is relatively dense for a hydroxyl mineral, with a calculated density of 3.85 g/cm³. ## Colors and Varieties This mineral has no distinct color varieties or commercial forms. It occurs as colorless, white, or grayish. ## History and Name Söhngeite was described in 1965 by Hugo Strunz. The name honors Paul Gerhard Söhnge (1913–2006), a German-South African geologist who was the chief geologist of the Tsumeb Corporation and made significant contributions to the study of this deposit. ## Uses Due to its extreme rarity and microscopic crystal sizes, söhngeite has no industrial applications. It is solely an object of interest for specialized collectors and scientists.

Properties

Mohs hardness
3-4
Luster
Vitreous to Pearly
Streak
White
Density
3.85
Cleavage
Perfect on {010}, good on {100} and {001}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification The characteristic feature is the pseudocubic or pseudo-octahedral shape of the crystals. High density and the specific occurrence environment in paragenesis with other gallium and germanium minerals also facilitate identification. ## Distinguishing from Similar Minerals It can be confused with other colorless or white secondary minerals from the Tsumeb deposit. Certain differentiation from similar minerals, such as gallite or tsumgallite, requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS), which is standard for such rare minerals. ## Crystal Forms Although belonging to the orthorhombic system, crystals exhibit pseudocubic symmetry. They take forms similar to cubes or octahedra. They usually occur as single, intergrown crystals or small, loose groups.

Geological environment

## Genesis It is a rare secondary mineral, formed in the oxidation zone of polymetallic ore deposits rich in gallium and germanium. It forms as a result of hydrothermal alteration or weathering of primary sulfides, mainly germanite. ## Mineral Associations It most commonly co-occurs with germanite, gallite, tennantite, galena, sphalerite, and chalcocite. ## Localities The main and practically only locality for well-formed söhngeite crystals is the historic Tsumeb mine in the Oshikoto Region of Namibia. This is its type locality.

Rarity

Very rare

For collectors

## Quality Criteria The most important criterion for collector value is the size, sharpness of edges, and transparency of the pseudocubic crystals. Specimens where well-formed söhngeite crystals are embedded on a contrasting rock matrix, preferably in association with other rare Tsumeb minerals, are highly valued. ## Popular Localities The only source of collector specimens is the Tsumeb mine in Namibia, which closed in 1996. Material available on the market comes exclusively from old collections.

Care and storage

## Cleaning Only use compressed air or a very soft brush for cleaning. Contact with water, especially an ultrasonic cleaner, is risky due to its perfect cleavage and can lead to specimen damage. ## What to Avoid Avoid contact with chemicals, especially acids and bases. The mineral is soft and brittle, so it must be protected from shocks, falls, and scratches. ## Storage Söhngeite specimens, usually in the form of micromounts, should be stored in sealed, padded boxes to protect them from dust and mechanical damage.

Sources

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