Coffinite - Encyclopedia
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
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