Coffinite
Encyclopedia
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
, USA, 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
.
(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)2-. 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
, phosphorous, and minimal lead substitutions along with traces of other rare earth elements.
with the orthosilicates zircon (ZrSiO4) and thorite (ThSiO4). Stieff et al. analyzed coffinite using the x-ray powder diffraction technique and determined that it has a tetragonal structure. Occurring naturally with U4+ cations, the UO8 triangular dodecahedra coordinate with edge-sharing, alternating SiO4 tetrahedra in chains along the c-axis. 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.
(UO2). 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. Later samples from Woodrow Mine in New Mexico collected by Moench showed fibrous internal structure and exceptional crystallization. A polished thin section of coffinite has a brown color and shows anisotropic transmission of light. Optical analysis yielded a refractive index of about 1.74.
minerals, uraninite and finely dispersed black organic material. Other materials associated with later finds from the same region were clay
and quartz
. In vein deposits of the Copper King Mine in Colorado, coffinite was also found to occur with uraninite
and pitchblende. Formation of coffinite requires a uranium
source, and may happen in reducing conditions
, as evidenced by the associated presence of low-valence vanadium minerals. Silica-rich solution provides such a reducing condition in cases where coffinite results as an alteration product of uraninite
. Hansley and Fitzpatrick also noted that the brownish color of their coffinite samples was caused by organic material, leading them to conclude that coffinite can also form in low temperature conditions if organic carbon is present. This finding is consistent with the coffinite samples of the Colorado Plateau, which included fossilized wood. In China, coffinite can be found in granite
in addition to sandstone
. Hansley and Fitzpatrick concluded that coarse-grained coffinite most likely forms in high temperature environments. Coffinite and uraninite precipitate inside brecciated and fractured regions of altered granite at pressures between 500 to 800 bars and temperatures at 126 to 178 °C.
, the United States Geological Survey
(USGS), and several other institutions attempted unsuccessfully to synthesize coffinite in the mid-1950s after its initial discovery. In 1956, Hoekstra and Fuchs managed to create stable samples of synthetic coffinite. All of this research was conducted for the United States Atomic Energy Commission
.
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
-bearing silicate mineral with formula: U(SiO4)1-x(OH)4x.
It occurs as black incrustations, dark to pale-brown in thin section
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
. It has a grayish black streak. It has a brittle to conchoidal fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
. 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
Mesa County, Colorado
Mesa County is the fourth most extensive and the eleventh most populous of the 64 counties of the state of Colorado in the United States. The county was named for the many large mesas in the area, including Grand Mesa. The county population was 146,723 at the 2010 United States Census. The...
, USA, and named for American geologist Reuben Clare Coffin (1886-1972).
It has widespread global occurrence in Colorado Plateau
Colorado Plateau
The Colorado Plateau, also called the Colorado Plateau Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 337,000 km2 within western Colorado, northwestern New Mexico,...
-type uranium ore deposits of uranium and vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
. It replaces organic matter in sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
and in hydrothermal vein type deposits. It occurs in association with uraninite
Uraninite
Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...
, thorite
Thorite
Thorite, SiO4, is a rare nesosilicate of thorium that crystallizes in the tetragonal system and is isomorphous with zircon and hafnon. It is the most common mineral of thorium and is nearly always strongly radioactive. It was named in 1829 to reflect its thorium content...
, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, marcasite
Marcasite
The mineral marcasite, sometimes called white iron pyrite, is iron sulfide with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide...
, roscoelite
Roscoelite
Roscoelite is a green mineral from the mica group that contains vanadium.The chemical formula is K2AlSi3O102.Crystals of roscoelite take on the monoclinic form, and are from the 2/m point group....
, clay minerals and amorphous organic matter
Organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
.
Composition
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 zirconZircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
(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)2-. The hydroxyl constituent in coffinite later proved to be nonessential in the formation of a stable synthetic mineral. Recent electron microprobe
Electron microprobe
An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...
analysis of the submicroscopic crystals uncovered an abundance of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, yttrium
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
, phosphorous, and minimal lead substitutions along with traces of other rare earth elements.
Crystal structure
Coffinite is isostructuralIsostructural
The term isostructural is used for chemical compounds that have similar chemical structures, and also with the meaning of isomorphous when used in relation to crystal structures...
with the orthosilicates zircon (ZrSiO4) and thorite (ThSiO4). Stieff et al. analyzed coffinite using the x-ray powder diffraction technique and determined that it has a tetragonal structure. Occurring naturally with U4+ cations, the UO8 triangular dodecahedra coordinate with edge-sharing, alternating SiO4 tetrahedra in chains along the c-axis. 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.
Physical properties
Initial examination of coffinite by Stieff et al. described the mineral as black in color with an adamantine luster, indistinguishable from uraniniteUraninite
Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...
(UO2). 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. Later samples from Woodrow Mine in New Mexico collected by Moench showed fibrous internal structure and exceptional crystallization. A polished thin section of coffinite has a brown color and shows anisotropic transmission of light. Optical analysis yielded a refractive index of about 1.74.
Geological occurrence
Coffinite was first discovered in sedimentary uranium deposits in the Colorado Plateau region, but has also been discovered in sedimentary uranium deposits and hydrothermal veins in many other locations. Samples of coffinite from the Colorado Plateau were found with black fine-grained low-valence vanadiumVanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
minerals, uraninite and finely dispersed black organic material. Other materials associated with later finds from the same region were clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
. In vein deposits of the Copper King Mine in Colorado, coffinite was also found to occur with uraninite
Uraninite
Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...
and pitchblende. Formation of coffinite requires a uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
source, and may happen in reducing conditions
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, as evidenced by the associated presence of low-valence vanadium minerals. Silica-rich solution provides such a reducing condition in cases where coffinite results as an alteration product of uraninite
Uraninite
Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...
. Hansley and Fitzpatrick also noted that the brownish color of their coffinite samples was caused by organic material, leading them to conclude that coffinite can also form in low temperature conditions if organic carbon is present. This finding is consistent with the coffinite samples of the Colorado Plateau, which included fossilized wood. In China, coffinite can be found in granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
in addition to sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
. Hansley and Fitzpatrick concluded that coarse-grained coffinite most likely forms in high temperature environments. Coffinite and uraninite precipitate inside brecciated and fractured regions of altered granite at pressures between 500 to 800 bars and temperatures at 126 to 178 °C.
Special characteristics
A large percentage of the Earth’s uranium supply is contained in coffinite deposits, which is significant because of uranium’s uses in nuclear energy and weaponry applications. Sedimentary deposits contain the most radioactive samples, as evidenced by the intensely radioactive coffinite found in the Colorado Plateau. Researchers at Harvard UniversityHarvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
, the United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
(USGS), and several other institutions attempted unsuccessfully to synthesize coffinite in the mid-1950s after its initial discovery. In 1956, Hoekstra and Fuchs managed to create stable samples of synthetic coffinite. All of this research was conducted for the United States Atomic Energy Commission
United States Atomic Energy Commission
The United States Atomic Energy Commission was an agency of the United States government established after World War II by Congress to foster and control the peace time development of atomic science and technology. President Harry S...
.