Moctezumite

Chemical formula: Pb<sup>2+</sup>(U<sup>6+</sup>O<sub>2</sub>)(Te<sup>4+</sup>O<sub>3</sub>)<sub>2</sub>

A rare lead uranyl tellurite, forming tiny, orange-red crystals with an adamantine luster.

## Characteristics Moctezumite is a very rare mineral from the tellurite group, chemically a lead uranyl tellurite. It occurs as small, prismatic or acicular crystals, rarely exceeding 1 mm in length. The crystals are usually well-formed and exhibit longitudinal striations. Its intense, orange-red to red color and strong luster make it distinctive, despite its small size. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of approximately 3. The crystals are brittle. It is characterized by an adamantine luster, which can transition to resinous. It is a heavy mineral due to its lead and uranium content, with a density of about 6.86 g/cm³. It is strongly radioactive. ## Colors and Varieties Moctezumite occurs in characteristic shades from orange-red, through red, to brownish-red. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name comes from the Moctezuma mine in Mexico, which is its type locality (the place of its first discovery). It was described in 1965 by Richard V. Gaines, who named it after the discovery location. The mine, in turn, is named after Moctezuma II, the last Aztec ruler. ## Applications Due to its extreme rarity, small crystal size, and radioactivity, moctezumite has no industrial applications. It is solely an object of scientific interest and a collector's item for advanced collectors of rare minerals.

Properties

Mohs hardness
3
Luster
Adamantine
Streak
Light red
Density
6.86
Cleavage
Good on {001}
Fracture
Conchoidal
Transparency
Transparent to Translucent
Crystal system
Tetragonal

Diagnostic features

## Identification Moctezumite can be identified by its characteristic orange-red color, adamantine luster, and form of small, prismatic crystals. A key diagnostic feature is its strong radioactivity, which can be measured with a Geiger counter. It occurs in the oxidation zones of hydrothermal deposits rich in tellurium. ## Distinguishing from Similar Minerals It can be confused with other rare, secondary tellurium minerals of similar color, such as kuranakhite or some tellurates. However, the combination of crystal habit, intense color, adamantine luster, and very high radioactivity is unique to moctezumite. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis. ## Crystal Forms It forms elongated, prismatic crystals, often acicular in habit, reaching up to 1 mm in length. Crystals are usually striated along the principal axis. It occurs as single crystals or small, radial aggregates on the host rock.

Geological environment

## Genesis Moctezumite is a secondary mineral formed in the oxidation (weathering) zone of hydrothermal deposits rich in tellurium. It forms as a result of reactions between uranium-bearing solutions and tellurium minerals, in the presence of lead. ## Mineral Associations It most commonly co-occurs with other tellurium minerals and tellurites, such as native tellurium, emmonsite, mackayite, as well as with quartz, barite, galena, and pyrite. At the type locality, it was found in association with schmitterite and paratellurite. ## Localities This is an extremely rare mineral, known from only a few locations worldwide. The most important and type locality is the Moctezuma (Bambolla) mine in Sonora, Mexico. Outside of Mexico, its occurrence has been reported in the Otto Mountain mine in the Baker area of California, USA.

Rarity

Very rare

For collectors

## Quality Criteria The most prized moctezumite specimens are those with well-formed, undamaged crystals of intense, orange-red color and strong luster. Due to their small size, crystal size is crucial – the larger, the more valuable. The attractiveness of a specimen is enhanced by a contrasting, light host rock (matrix) on which the crystals are embedded. Purity and lack of damage are also important, though difficult to achieve due to the mineral's brittleness. ## Popular Localities By far the most sought-after and classic specimens come from the type locality – the Moctezuma mine in Mexico. They serve as the benchmark for this mineral and are most highly valued by collectors specializing in rare species or minerals from specific localities.

Care and storage

## Cleaning Due to the brittleness and small size of the crystals, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and chemicals. ## What to Avoid The mineral is strongly radioactive – limit exposure and avoid direct skin contact. It should not be stored near radiation-sensitive materials. It is brittle and sensitive to shocks and temperature changes. Protect it from acids and other chemical reagents. ## Storage Moctezumite specimens must be stored in specialized, lead-lined containers designed for radioactive minerals, away from workplaces and living areas. The container should be clearly marked with a radioactivity symbol. Display should only occur in enclosed, ventilated showcases, with all precautions observed.

External references

Sources

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