Metakirchheimerite

Chemical formula: Co₂+(U₆+O₂)₂(As₅+O₄)₂·₈H₂O

Metakirchheimerite is a rare, radioactive secondary mineral of the meta-autunite group, forming characteristic yellow, tabular crystals in the oxidation zones of uranium deposits.

## Characteristics Metakirchheimerite is a hydrated uranyl cobalt arsenate, belonging to the meta-autunite group. This mineral most often forms thin coatings, crusts, and earthy aggregates. Well-formed crystals are rare, taking the form of very small, thin, square tablets, often arranged in fan-like groups or rosettes. Its color is typically straw-yellow to greenish-yellow. ## Physical Properties Metakirchheimerite is a very soft mineral, with a Mohs hardness of 2-2.5. It is characterized by a vitreous luster, waxy on some surfaces, and dull in the case of earthy aggregates. It is transparent to translucent. It possesses perfect cleavage in one direction, parallel to the base of the tabular crystals, which is typical for minerals of its group. Its density is approximately 3.13 g/cm³. ## Colors and Varieties This mineral occurs in shades of yellow, from straw-yellow to yellowish-green. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name honors Franz Kirchheimer (1911–1984), a German geologist and paleontologist, director of the Geological Survey of Baden-Württemberg. The prefix "meta-" indicates a lower degree of hydration compared to the hypothetical, unconfirmed in nature mineral "kirchheimerite". Metakirchheimerite was described as a new mineral by Kurt Walenta in 1964 based on specimens found in Germany. ## Applications Due to its rarity, small crystal size, and radioactivity, metakirchheimerite has no industrial applications. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness: 2-2.5
Luster: Vitreous to waxy, dull
Streak: Light yellow
Density: 3.13
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Characteristic features of metakirchheimerite include its straw-yellow color, habit of thin, square tabular crystals, and perfect cleavage in one direction. A key diagnostic feature is intense, green fluorescence under ultraviolet light (both short-wave and long-wave). Occurrence in paragenesis with other secondary uranium minerals is also an important clue. ## Distinguishing from Similar Minerals Metakirchheimerite is visually almost impossible to distinguish from other minerals of the meta-autunite group without advanced studies. Metazeunerite (with copper instead of cobalt) usually has a more greenish hue. Meta-autunite (with calcium) and metatorbernite (with copper) are very similar. Certain identification requires chemical analysis (e.g., EDS) to confirm the presence of cobalt and the absence of other dominant cations. ## Crystal Forms Crystals form thin, square tablets with a perfectly flat basal surface. They often coalesce into stacks, fan-like groups, rosettes, or form scaly coatings and crusts on the host rock.

Geological environment

## Genesis Metakirchheimerite is a secondary mineral, forming in the oxidation zones of hydrothermal uranium-cobalt-arsenic deposits. It forms as a result of weathering and alteration of primary uranium minerals, mainly uraninite, in an environment rich in arsenic and cobalt. ## Mineral Associations This mineral co-occurs with other secondary uranyl arsenates, such as metazeunerite, kahlerite, nováčekite, as well as with erythrite (cobalt bloom), uraninite, and other ore minerals. ## Localities The type locality is the Sophia mine in Wittichen (Black Forest, Germany). Other known occurrences include the Schneeberg area in Saxony (Germany) and Jáchymov in the Czech Republic. These are classic localities for many rare, secondary uranium minerals.

Rarity

Rare

For collectors

## Quality Criteria The most prized metakirchheimerite specimens are those with well-formed, sharp, tabular crystals forming aesthetic groups or rosettes. The intensity and purity of the yellow color are important. Due to the small size of the crystals, specimens are usually micromounts, and their value increases when they are on a contrasting rock matrix or in association with other rare minerals. ## Popular Localities The best quality, classic specimens come from historical localities in Germany, especially from the Sophia mine in Wittichen and the Schneeberg area. These localities have provided reference specimens that serve as a benchmark for collectors.

Care and storage

## Cleaning Metakirchheimerite specimens are extremely delicate and sensitive to water. They should only be dry-cleaned, using a soft brush or a gentle stream of compressed air to remove dust. Contact with water can lead to changes in hydration and damage to the mineral. ## What to Avoid Contact with water and all chemicals must be strictly avoided. The mineral is sensitive to elevated temperatures, which can cause dehydration. As a uranium mineral, it is radioactive – prolonged exposure and direct skin contact should be avoided. Hands should be washed after each contact. It should not be stored near light-sensitive materials or other minerals that may be damaged by radiation. ## Storage It is recommended to store specimens in sealed, transparent containers (e.g., micromount type), which protect them from mechanical damage and dust, and also limit potential dust contamination. The container must be clearly labeled as radioactive material. Specimens should be kept away from areas of permanent human presence, especially bedrooms and living rooms.

External references

https://es.wikipedia.org/wiki/https%3A%2F%2Fes.wikipedia.org%2Fwiki%2FMetakirchheimerita