Coffinite
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| Coffinite | |
|---|---|
 Pitchblende and coffinite in a sample from a Czech mine | |
| General | |
| Category | Nesosilicate |
| Formula | U(SiO4)1−x(OH)4x |
| IMA symbol | Cof[1] |
| 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
72.63% (U) |
| References | [2][3][4][5][6][7][8][9][10][11][12][13][14] |
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,[5] and named for American geologist Reuben Clare Coffin (1886–1972).[3] 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.[3] It occurs in association with uraninite, thorite, pyrite, marcasite, roscoelite, clay minerals and amorphous organic matter.[3]
Coffinite's chemical formula is U(SiO4)1−x(OH)4x.[6][15] X-ray powder patterns from samples of coffinite allowed geologists to classify it as a new mineral in 1955.[6] A comparison to the x-ray powder pattern of zircon (ZrSiO4) and thorite (ThSiO4) was the basis for this classification.[7] Preliminary chemical analysis indicated that the uranous silicate exhibited hydroxyl substitution.[7] 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.[8] The hydroxyl substitution occurs as (OH)44− for (SiO4)4−.[8] The hydroxyl constituent in coffinite later proved to be nonessential in the formation of a stable synthetic mineral.[9] 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.[9]
Crystal structure
Coffinite is isostructural with the orthosilicates zircon (ZrSiO4) and thorite (ThSiO4).[16] Stieff et al. analyzed coffinite using the x-ray powder diffraction technique and determined that it has a tetragonal structure.[8] Occurring naturally with U4+ cations, the UO8 triangular dodecahedra coordinate with edge-sharing, alternating SiO4 tetrahedra in chains along the c-axis.[12] The central uranium site of coffinite is surrounded by eight SiO4 tetrahedra. The lattice dimensions of naturally occurring and synthetic coffinite are similar, with a naturally occurring sample from Arrowhead Mine, Mesa County, Colorado having a=6.93kx, c=6.30kx, and a sample synthesized by Hoekstra and Fuchs having a=6.977kx and c=6.307kx.[15][13]
Physical properties
Initial examination of coffinite by Stieff et al. described the mineral as black in color with an adamantine luster, indistinguishable from uraninite (UO2).[8] Additionally, the discoverers reported that although no cleavage is seen in coffinite, it does exhibit subconchoidal fracturing and is very fine grained. Initial samples showed a brittle texture and a hardness between 5 and 6, with a specific gravity of 5.1.[8] Later samples from Woodrow Mine in New Mexico collected by Moench showed fibrous internal structure and exceptional crystallization.[10] A polished thin section of coffinite has a brown color and shows anisotropic transmission of light.[10] Optical analysis yielded a refractive index of about 1.74.[10]
