Otjisumeite

Chemical formula: Pb₂+Ge₄O₉

Otjisumeite is an extremely rare lead germanate, forming colorless, hexagonal crystals, known exclusively from the Tsumeb mine in Namibia.

## Characteristics Otjisumeite is a rare secondary mineral from the germanate group. It forms characteristic, well-developed, hexagonal crystals with a tabular or short-prismatic habit. Crystals are usually small and often occur as attractive, rosette-like or fan-shaped aggregates, as well as in subparallel intergrowths. It is colorless to white, with a strong, adamantine luster, which gives it a unique appearance, especially on a micro scale. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It is also extremely dense for a non-metallic mineral – its density is 6.45 g/cm³, which is a result of its high lead and germanium content. It has perfect cleavage in one plane, making it very brittle and delicate. It is transparent to translucent. ## Colors and Varieties Otjisumeite occurs exclusively in colorless or white forms. No colored varieties or trade names are known. ## History and Name The mineral's name comes from its discovery location – the Otjisume shaft in the famous Tsumeb mine in Namibia. It was first described in 1981 by a team of mineralogists: Paul Keller, Heinz Hess, and Pete J. Dunn, as a new mineral species. ## Applications Due to its extreme rarity, otjisumeite has no industrial applications. It is solely an object of scientific interest and is a highly valued specimen in advanced systematic and regional (Tsumeb) mineral collections.

Properties

Mohs hardness: 2
Luster: Adamantine
Streak: White
Density: 6.45
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Key diagnostic features of otjisumeite include its characteristic crystal form (hexagonal tablets, often in rosettes), adamantine luster, very high density, and extreme rarity and origin from the only known locality. The tactile response (softness) is also a clue, but scratching attempts are not recommended due to the specimen's delicacy. ## Differentiation from Similar Minerals In the Tsumeb mine, otjisumeite can be confused with other colorless secondary minerals. Schaurteite forms acicular crystals, not tabular. Fleischerite also crystallizes in the hexagonal system, but contains sulfates and has a slightly different habit. Certain differentiation most often requires advanced analytical methods, such as EDS spectroscopy or X-ray diffraction (XRD). ## Crystal Forms Otjisumeite forms tabular or short-prismatic hexagonal crystals. The most typical and desired are aggregates in the form of rosettes, fans, and subparallel intergrowths of small crystals.

Geological environment

## Genesis Otjisumeite is a secondary mineral that formed in the oxidation zone of a polymetallic deposit hosted in dolomites. It formed as a result of weathering and alteration of primary germanium-rich sulfides, such as germanite and renieryite, under hydrothermal conditions. ## Mineral Associations This mineral occurs in association with other, often rare, minerals from the oxidation zone of the Tsumeb deposit. It is most commonly found with germanite, renieryite, galena, tennantite, anglesite, and another rare germanate – schaurteite. ## Localities The only confirmed locality of otjisumeite in the world is its type locality – the Tsumeb mine (specifically the Otjisume shaft area) in the Otjikoto region of Namibia. All known specimens originate from this single location.

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly valued otjisumeite specimens are those that exhibit sharp, well-formed crystals with strong luster, forming aesthetic rosettes or fan-shaped aggregates. The matrix is also important – ideally, it should contrast with the colorless crystals. Purity (lack of damage) and associations with other rare Tsumeb minerals significantly increase its collector's value. ## Popular Localities As this mineral is known from only one place in the world, all collector specimens come from the Tsumeb mine in Namibia. It is a classic, highly sought-after micromineral from this locality.

Care and storage

## Cleaning Otjisumeite specimens should only be dry-cleaned, using a very soft brush to remove dust. A photographic blower can be helpful, but should be used from a distance and with caution. Absolutely avoid washing with water, ultrasonic cleaners, and any chemical agents. ## What to Avoid The mineral is extremely delicate due to its low hardness and perfect cleavage – avoid any impacts or shocks. As a lead mineral, it is toxic – avoid inhaling dust and wash hands after any contact. It should not be heated or exposed to acids. ## Storage It is recommended to store specimens only in sealed "micromount" boxes to protect them from dust, mechanical damage, and direct contact. It should be kept in stable conditions, away from sources of vibration.

Sources

IMA Mineral List (CNMNC)