Metacalciouranoite

Chemical formula: (Ca,Na,Ba)U<sup>6+</sup><sub>2</sub>O<sub>7</sub>·2H<sub>2</sub>O

An extremely rare, radioactive secondary mineral of calcium and uranium, forming earthy, yellow coatings and crusts.

## Characteristics Metacalciouranoite is a hydrated calcium and uranium oxide, belonging to the uranyl mineral group. It occurs extremely rarely, most often forming earthy or powdery aggregates, as well as fine-grained, crystalline crusts and coatings. Its typical appearance is inconspicuous, resembling a yellow, dusty substance on the surface of the host rock. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2-3. The luster is dull or earthy, which is characteristic of its modes of occurrence. It is translucent to opaque. The calculated density is about 5.41 g/cm³, but due to the nature of its aggregates, measuring it is practically impossible. ## Colors and Varieties Metacalciouranoite ranges in color from yellow to brownish-yellow. No color or commercial varieties are known. ## History and Name The mineral was first described in 1973 by a group of Soviet mineralogists (K. V. Skvortsova, G. A. Sidorenko et al.). Its name refers to the hypothetical, more hydrated mineral "calciouranoite" (the prefix "meta-" indicates lower water content) and to its chemical composition, in which calcium (Latin: *calx*) and uranium dominate. ## Uses Due to its extreme rarity, metacalciouranoite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of uranium minerals.

Properties

Mohs hardness
2-3
Luster
Dull, earthy
Streak
Light yellow
Density
~5.41 (obliczona)
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification The key diagnostic feature of metacalciouranoite is its strong radioactivity, detectable with a Geiger counter. In the field, it is identified by its yellow color, earthy or powdery habit, and occurrence in the oxidation zones of uranium deposits. Definitive identification is only possible through chemical and X-ray diffraction (XRD) analysis. ## Distinguishing from Similar Minerals Metacalciouranoite is visually indistinguishable from many other yellow secondary uranium minerals, such as carnotite, tyuyamunite, or minerals from the zippeite group. Carnotite and tyuyamunite contain vanadium, and the zippeite group contains sulfates, which are absent in metacalciouranoite. Final differentiation requires advanced analytical methods. ## Crystal Forms This mineral primarily forms cryptocrystalline, earthy, and powdery aggregates. Microscopic, poorly formed tabular or bladed crystals are very rarely observed.

Geological environment

## Genesis Metacalciouranoite is a secondary mineral, forming in the oxidation (weathering) zones of uranium deposits, especially those located in sandstones. It forms under arid climate conditions as a result of the alteration of primary uranium minerals, mainly uraninite. ## Mineral Associations It co-occurs with other minerals of the oxidation zone. Its most common associates are uraninite (as a primary mineral), calcite, gypsum, and other secondary uranium oxides and silicates. ## Localities The only confirmed and described locality of metacalciouranoite in the world is the Uchkuduk uranium deposit in the Kyzyl-Kum Desert in Uzbekistan. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria For such a rare mineral, the main criterion of value is the mere presence and identifiability of the specimen. Aesthetics are of secondary importance. The most desirable specimens are those with a clearly visible layer of the mineral on the host rock, preferably with analytically confirmed identity. Scientific value increases when the specimen is accompanied by other rare secondary minerals. ## Popular Localities The only source of specimens is the historical type locality in Uzbekistan. Material from this location is extremely difficult to acquire and appears on the collector's market sporadically, mainly in circulation among specialized collectors.

Care and storage

## Cleaning Metacalciouranoite specimens should absolutely not be cleaned wet or with mechanical tools. It is a very soft, brittle, and dusty mineral. Any attempt at cleaning can lead to its damage and, more importantly, to the release of radioactive dust. If necessary, a stream of compressed air can be used very carefully from a safe distance. ## What to Avoid **The mineral is highly radioactive.** Direct contact, and especially inhalation of dust and ingestion, should be avoided. Hand washing after each contact is mandatory. It should be protected from water, moisture, and chemicals, which can cause its decomposition. It should not be heated or exposed to temperature changes. ## Storage Specimens should be stored in a sealed, transparent container (e.g., a "perky box"), clearly marked with the international radioactivity symbol and the full name of the mineral. The container should be kept away from areas of permanent human presence, and also separate from other minerals that could be damaged (e.g., tarnished) by radiation.

Sources

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