Mineral Information and Data:

U[SiO4,(OH)4] [CNMNC approved formula]

Named for Reuben Clare Coffin (1886-1972), Tulsa, Oklahoma, USA, pioneer geologist in the study of uranium deposits of the Colorado Plateau.

IMA status : Valid species (Pre-IMA 1955) Grandfathered
(8th edition) :  
  (51) Nesosilicate Insular SiO4 Groups Only
  (51.05) with cations in >[6] coordination
  (51.05.02) Zircon group
(10th edition) :
  (09) Silicates
  (09.A) Nesosilicates (Isolated tetrahedron) structures
  (09.AD) Nesosilicates without additional anions; cations in [6] and/or greater coordination
Crystal system: Tetragonal System
Point group (H-M): 4/mmm (or 4/m 2/m 2/m) — ditetragonal-dipyramidal
Space group: I41/amd
Unit cell: a = 6.979Å, c = 6.252Å, Z = 4
Crystal Habit: As crystals, rarely > 20 μm, and in colloform to botryoidal incrustations, radially fibrous. Commonly pulverulent, in aggregates of extremely fine crystallites, or massive.
Color: Black from included organic matter; pale brown to dark brown in thin section
Diaphaneity: Opaque, transparent on very thin edges
Luster: Dull to adamantine
Hardness (Mohs): 5 - 6
Measured Density: 5.1 g/cm3
Tenacity: Brittle or friable
Fracture: Irregular to subconchoidal
Geologic Setting: In Colorado Plateau-type black unoxidized U{V deposits, replacing organic material in sandstone; in other sedimentary and hydrothermal vein uranium deposits
Mineral Association: Uraninite, thorite, pyrite, marcasite, roscoelite, clay minerals, amorphous organic
Ore deposits: A chief primary ore mineral of uranium.
Other Properties: Radioactive.


Massive adamantine brown brannerite and black coffinite (often in thin veinlets) in quartz-rich matrix from a uraniferous lignite deposit
Origin: Dylen, Western Bohemia, Czech Republic
Sample size: 3.5 x 3.5 cm
Photo: Diederik Visser