Coffinite

Chemical formula: U⁴⁺(SiO₄)·nH₂O

Coffinite is a highly radioactive uranium mineral from the silicate group, an important ore of this element.

## Characteristics Coffinite is a uranium silicate that often occurs in earthy, powdery, or massive aggregates of black color. It rarely forms small, tetragonal crystals, which usually do not exceed fractions of a millimeter. Its appearance is inconspicuous, often resembling soot or coal, and due to its strong radioactivity and tendency to become metamict (destruction of crystal structure by its own radiation), well-formed crystals are extremely rare. It is usually opaque and has a dull or pitchy luster. ## Physical Properties Coffinite is relatively soft, with a Mohs hardness of 5 to 6. It is a brittle mineral, with an uneven or conchoidal fracture. Its density is high, ranging from 2.2 to 6.1 g/cm³, depending on the degree of hydration and internal structural damage. This mineral is highly radioactive. ## Colors and Varieties The dominant color of coffinite is black or dark brown. No color or commercial varieties are distinguished. ## History and Name Coffinite was first described in 1955. Its name comes from the American geologist Reuben Clare

Properties

Mohs hardness: 5-6
Color: Black
Luster: Pitchy to dull
Streak: Black to grayish brown
Density: 5.1
Cleavage: None
Fracture: Irregular/Uneven,Sub-Conchoidal
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification The most important diagnostic feature of coffinite is its strong radioactivity, which can be easily detected with a Geiger counter. Its black color, pitchy or dull luster, and occurrence in massive or earthy aggregates are also characteristic. It often forms pseudomorphs after uraninite or wood. ## Distinguishing from Similar Minerals Coffinite is sometimes confused with uraninite, with which it often co-occurs. However, uraninite is harder and significantly denser. From other black, massive ore minerals (e.g., manganese oxides), it is primarily distinguished by its strong radioactivity. In the form of crusts, it may resemble pitchblende, which is a mixture of various uranium oxides. ## Crystal Forms Well-formed crystals are extremely rare and appear as small, tetragonal prisms or needles. Most commonly found are compact, massive, earthy, reniform aggregates, or as coatings and crusts. It also forms fibrous and radial aggregates.

Geological environment

## Genesis Coffinite is a low-temperature mineral, forming in hydrothermal processes. It most often forms in "roll-front" type uranium deposits in sedimentary rocks, such as sandstones and conglomerates. It forms in the reducing zone, where uranium-rich solutions precipitate it in the presence of organic matter (e.g., carbonized wood) or sulfides. ## Mineral Associations Coffinite often co-occurs with uraninite (which it can replace), pyrite, marcasite, quartz, calcite, fluorite, and clay minerals. In sandstone deposits, it is often accompanied by carbonized organic matter. ## Locations As an important uranium ore, coffinite occurs in many deposits worldwide. Major locations include the Colorado Plateau region in the USA (states of Utah, Colorado, Arizona, New Mexico), where it was discovered. Significant deposits are also found in Canada (Athabasca Lake region), Australia, Gabon, Niger, and Kazakhstan.

Rarity

Not very common

For collectors

## Quality Criteria From a collector's perspective, coffinite is valued primarily for the rarity of well-formed specimens. Specimens with visible, even microscopic, crystals are most desirable. Pseudomorphs of coffinite after organic matter, such as wood, where its original structure is preserved, are also attractive. Due to radiological hazards, it is a mineral mainly for specialized collectors. ## Popular Localities The most famous specimens, including those with well-formed microcrystals, come from mines in the Grants region of New Mexico and from La Sal in Utah, USA.

Care and storage

## Cleaning Due to its brittleness and potential dusting, cleaning coffinite should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove loose particles. Water and any chemical agents should be avoided. ## What to Avoid Coffinite is a very sensitive mineral. Contact with water, acids, and other chemicals must be strictly avoided. It should not be heated or subjected to ultrasound. As a radioactive mineral, it requires special precautions – avoid inhaling dust and direct, prolonged skin contact. Always wash hands after handling specimens. ## Storage Coffinite specimens must be stored in accordance with radiological protection principles. They should be kept in sealed, lead or thick glass containers, away from other minerals that could be damaged by radiation. Store in a dry place, away from people, especially children.