Umohoite

Chemical formula: (U⁶⁺O₂)Mo⁶⁺O₄·₂H₂O

Umohoite is a rare uranium and molybdenum mineral, forming characteristic blue-black, platy crystals and aggregates.

## Characteristics Umohoite is a hydrated uranyl molybdate, belonging to the group of rare minerals. Its name, pronounced "you-mo-ho-ite", refers to its chemical composition – uranium (U), molybdenum (Mo), and hydrogen (H). It forms very small, thin, platy crystals with a hexagonal outline, which usually occur as radial or spherulitic aggregates. Due to its chemical composition, it is a highly radioactive mineral. ## Physical Properties Umohoite crystals are soft and brittle, with a Mohs hardness of approximately 2. The luster is metallic to submetallic on fresh surfaces, but it can dull over time. It is an opaque mineral with a density ranging from 4.55-4.96 g/cm³. ## Colors and Varieties It is characterized by an intense, blue-black color. The streak is black. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1953 by Gerald P. Brophy and Paul F. Kerr. Its name is derived from the symbols of the elements comprising it: U (uranium), Mo (molybdenum), and H (hydrogen), with the addition of the standard "-ite" suffix. The type locality is the Freedom No. 2 mine in Marysvale, Piute County, Utah, USA. ## Uses Due to its rarity and small accumulations, umohoite has no industrial application. It is solely an object of interest for collectors specializing in uranium minerals and for scientific institutions.

Properties

Mohs hardness: 2
Color: Black, blue-black, dark green
Luster: Metallic to submetallic
Streak: Black
Density: 4.53
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Umohoite is recognizable by its characteristic blue-black color, metallic luster, and typical forms of occurrence as spherulitic or radial aggregates of small, platy crystals. Strong radioactivity is a key diagnostic feature, easily verifiable with a Geiger counter. ## Distinguishing from Similar Minerals It can be confused with other dark, secondary uranium minerals, such as uraninite (pitchblende). However, uraninite is usually harder, occurs in massive or cubic forms, and has a pitch-black or brownish-black color, without a blue tint. Reaction to UV radiation can be helpful – unlike many uranyl minerals, umohoite does not fluoresce. ## Crystal Forms Crystals are very small, thin, platy, with a hexagonal outline. Most often, they form radial, rosette-like, or spherulitic (spherical) aggregates, as well as coatings and efflorescences on the host rock.

Geological environment

## Genesis Umohoite is a secondary uranium and molybdenum mineral. It forms in reducing zones of hydrothermal uranium deposits, often in veins and fractures of sedimentary rocks (mainly sandstones) or volcanic rocks. It forms under anoxic conditions, below the groundwater table, as a result of the alteration of earlier uranium minerals, such as uraninite, in the presence of molybdenum-rich solutions. ## Mineral Associations It often co-occurs with other uranium and molybdenum minerals. Typical associated minerals include uraninite, jordisite (amorphous molybdenum sulfide), ilsemannite, as well as pyrite, fluorite, and quartz. ## Localities The most important and classic localities for this mineral are in the USA, particularly in the Marysvale district in Utah (type locality). It is also known from several other locations in Wyoming, Colorado, and Arizona. Outside the USA, its occurrences have been reported in Mexico (Nopal I deposit in Chihuahua) and the Czech Republic.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, spherulitic or radial aggregates of intense, blue-black color. Contrast with the light host rock (matrix) on which the crystals are deposited is important. Due to the microscopic size of the crystals, the size and richness of the aggregates on a specimen significantly increase its value. ## Popular Localities By far, the most sought-after specimens come from the type locality in Marysvale, Utah, USA. These are considered classic and exemplary for this mineral. Specimens from other American localities, as well as from Mexico, are also found in collections, but those from Marysvale are most highly regarded.

Care and storage

## Cleaning Umohoite specimens should generally not be wet cleaned. Dust can be removed very carefully with a soft brush or a stream of compressed air from a safe distance. Contact with water should be avoided, as it can lead to its decomposition. ## What to Avoid Umohoite is an unstable mineral under oxidizing conditions and in contact with water. All chemicals, acids, and detergents must be strictly avoided. It should not be heated or exposed to prolonged moisture. As a radioactive mineral, it requires careful handling – hands should be washed after each contact, and dust inhalation should be avoided. ## Storage Specimens should be stored in a dry, stable environment, preferably in a tightly sealed, labeled container (e.g., a "perky box"). This type of storage protects delicate crystals from mechanical damage and limits radiation exposure. It should be kept away from other minerals that could be damaged by radiation.