COFFINITE

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Tetragonal
    Chemistry : U(SiO4)(OH)4
    Rarity : Quite common

Coffinite is a hydrated uranium silicate, which appears isostructural with zircon. It was named in honor of the American geologist Reuben Coffin who was one of the first to study sedimentary uranium deposits. This important uranium ore has long gone unnoticed, due to its expression in tiny beaches and unattractive black coatings. The macroscopic crystals (a few millimeters), chunky to elongated, are exceptional. Coffinite usually occurs in very fine crystals (less than 20 µm), in collomorphic botryoidal or fibroradiated encrustations, and in powdery aggregates, always black in color. Coffinite is subject to metamictization and frequently decomposed into uraninite and quartz in Precambrian deposits.

Main photo : Coffinite from Morbihan, Brittany, France © Roberto Bosi

Coffinite in the World

It is a primary uranium mineral particularly abundant in the large sedimentary "roll" uranium deposits of Colorado, where the ore permeates detrital layers rich in organic matter, uraninite, pyrite, and often vanadium minerals. These Permian to Eocene age deposits are scattered across four states, and are particularly important in the Grants region (New Mexico). Coffinite is also found in Saxon polymetallic uranium veins (Marienberg, Johanngeorgestadt, Schneeberg). The extremely rare millimeter crystals are known from the uranium deposits of Kovary (Poland) and Pribram (Czech Republic).

Coffinite in France

In France, coffinite is abundant in pitchblende veins, where it forms original radiated aggregates around nickel arsenide cores at La Rabasse and Rabejac (Hérault). It is also found in St-Priest-la-Prugne (Loire) as well as in many other localities.

Twinning

No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.


Hardness : 5 to 6
Density : 5.1
Fracture : Irregular to sub-conchoidal
Streak : Black to dark brown

TP : Translucent to transparent
RI : 1.730 to 1.750
Birefringence : 1.730
Optical character : Uniaxial (+/-)
Pleochroism : Visible
Fluorescence : None

Solubility : Acids


Magnetism : None
Radioactivity : Very strong