Coffinite
| Coffinite | |
|---|---|
Pitchblende and coffinite in a sample from a Czech mine
|
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| General | |
| Category | Nesosilicate |
|
Formula (repeating unit) |
U(SiO4)1−x(OH)4x |
| Strunz classification | 9.AD.30 |
| Crystal system | Tetragonal |
| Crystal class | Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m) |
| Space group | I41/amd |
| Unit cell | a = 6.97 Å, c = 6.25 Å; Z = 4 |
| Identification | |
| Color | Black (from organic inclusions; pale to dark brown in thin section |
| Crystal habit | Rarely as crystals, commonly as colloform to botryoidal incrustations, fibrous, pulverulent masses |
| Fracture | Irregular to subconchoidal |
| Tenacity | Brittle to friable |
| Mohs scale hardness | 5-6 |
| Luster | Dull to adamantine |
| Streak | Grayish black |
| Diaphaneity | Opaque, transparent on thin edges |
| Specific gravity | 5.1 |
| Optical properties | Uniaxial (+/-) |
| Refractive index | nα = 1.730 - 1.750 nβ = 1.730 - 1.750 |
| Birefringence | δ = 1.730 |
| Pleochroism | Moderate; pale yellow-brown parallel to and medium brown perpendicular to long axis |
| Alters to | Metamict |
| Other characteristics | Radioactive |
| References | |
Coffinite is a uranium-bearing silicate mineral with formula: U(SiO4)1−x(OH)4x.
It occurs as black incrustations, dark to pale-brown in thin section. It has a grayish-black streak. It has a brittle to conchoidal fracture. The hardness of coffinite is between 5 and 6.
It was first described in 1954 for an occurrence at the La Sal No. 2 Mine, Beaver Mesa, Mesa County, Colorado, US, and named for American geologist Reuben Clare Coffin (1886–1972). It has widespread global occurrence in Colorado Plateau-type uranium ore deposits of uranium and vanadium. It replaces organic matter in sandstone and in hydrothermal vein type deposits. It occurs in association with uraninite, thorite, pyrite, marcasite, roscoelite, clay minerals and amorphous organic matter.
Coffinite's chemical formula is U(SiO4)1−x(OH)4x. X-ray powder patterns from samples of coffinite allowed geologists to classify it as a new mineral in 1955. A comparison to the x-ray powder pattern of zircon (ZrSiO4) and thorite (ThSiO4) was the basis for this classification. Preliminary chemical analysis indicated that the uranous silicate exhibited hydroxyl substitution. The results of Sherwood’s preliminary chemical analysis were based on samples from three locations. Hydroxyl bonds and silicon-oxygen bonds also proved to exist after infrared absorption spectral analyses were performed. The hydroxyl substitution occurs as (OH)44− for (SiO4)4−. The hydroxyl constituent in coffinite later proved to be nonessential in the formation of a stable synthetic mineral. Recent electron microprobe analysis of the submicroscopic crystals uncovered an abundance of calcium, yttrium, phosphorus, and minimal lead substitutions along with traces of other rare earth elements.
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