Skutterudite
Encyclopedia
Skutterudite is a cobalt
arsenide mineral
that has variable amounts of nickel
and iron
substituting for cobalt with a general formula: (Co,Ni,Fe)As3. Some references give the arsenic a variable formula subscript of 2-3. High nickel varieties are referred to as nickel-skutterudite, previously chloanthite. It is a hydrothermal ore
mineral found in moderate to high temperature veins with other Ni-Co minerals. Associated minerals are arsenopyrite
, native silver
, erythrite, annabergite
, nickeline, cobaltite
, silver sulfosalts, native bismuth
, calcite
, siderite
, barite
and quartz
. It is mined as an ore of cobalt and nickel with a by-product of arsenic
.
The crystal structure
of this mineral has been found to have important technological uses for several compounds
isostructural with the mineral.
The mineral has a bright metallic luster, and is tin white or light steel gray in color with a black streak. The specific gravity
is 6.5 and the hardness is 5.5-6. Its crystal structure
is isometric with cube and octahedron forms similar to that of pyrite
. The arsenic content gives a garlic odor when heated or crushed.
It was discovered in Skuterud Mines, Modum
, Buskerud
, Norway
, in 1845. Smaltite is a synonym for the mineral. Notable occurrences include Cobalt, Ontario
, Skuterud, Norway
, and Franklin, New Jersey
in the United States
. The rare arsenide minerals are classified in Dana's sulfide
mineral group, even though it contains no sulfur.
The crystal structure of the skutterudite mineral was determined in 1928 by Oftedahl to be cubic, belonging to space group
Im-3 (number 204). The unit cell can be considered to consist of eight smaller cubes made up of the Co atoms. Six of these cubes are filled with (almost) square planar rings of As, each of which is oriented parallel to one of the unit cell edges, see image on the right. The As atoms then form octahedra
with Co in the centre.
In crystallographic terms, the Co atoms occupy the 8c sites, while the As atoms occupy the 24g sites. The position of the Co atoms within the unit cell is fixed, while the positions of the As atoms are determined by the parameters x and y. It has been shown that for the As-rings to be fully square, these parameters must satisfy the Oftedahl relation x+y=1/2. Any deviation from this relation yields a rectangular configuration of the As atoms. Indeed, this is the case for all known compounds with this structure, and the As atoms then do not form perfect octahedra.
Together with the unit cell size and the assigned space group, these two parameters fully describe the crystal structure of the material which is often referred to as the skutterudite structure.
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
arsenide mineral
Arsenide mineral
An arsenide mineral is a mineral that contains arsenide as its main anion. Arsenides are grouped with the sulfides in both the Dana and Strunz mineral classification systems.- Examples :* algodonite Cu6As* domeykite Cu3As* löllingite FeAs2* nickeline NiAs...
that has variable amounts of nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
substituting for cobalt with a general formula: (Co,Ni,Fe)As3. Some references give the arsenic a variable formula subscript of 2-3. High nickel varieties are referred to as nickel-skutterudite, previously chloanthite. It is a hydrothermal ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
mineral found in moderate to high temperature veins with other Ni-Co minerals. Associated minerals are arsenopyrite
Arsenopyrite
Arsenopyrite is an iron arsenic sulfide . It is a hard metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it becomes magnetic and gives off toxic fumes...
, native silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, erythrite, annabergite
Annabergite
Annabergite is an arsenate mineral consisting of a hydrous nickelarsenate, Ni32·8H2O, crystallizing in the monoclinic system and isomorphous with vivianite and erythrite. Crystals are minute and capillary and rarely met with, the mineral occurring usually as soft earthy masses and encrustations. A...
, nickeline, cobaltite
Cobaltite
Cobaltite is a sulfosalt mineral composed of cobalt, arsenic and sulfur, CoAsS. It contains up to 10 percent iron and variable amounts of nickel. Structurally it resembles pyrite with one of the sulfur atoms replaced by an arsenic atom....
, silver sulfosalts, native bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, siderite
Siderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
, barite
Barite
Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
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,...
. It is mined as an ore of cobalt and nickel with a by-product of arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
.
The crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
of this mineral has been found to have important technological uses for several compounds
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
isostructural with the mineral.
The mineral has a bright metallic luster, and is tin white or light steel gray in color with a black streak. The specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is 6.5 and the hardness is 5.5-6. Its crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
is isometric with cube and octahedron forms similar to that of 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...
. The arsenic content gives a garlic odor when heated or crushed.
It was discovered in Skuterud Mines, Modum
Modum
Modum is a municipality in Buskerud county, Norway. It is part of the traditional region of Hallingdal. The administrative centre of the municipality is the town of Vikersund. The municipality of Modum was established on 1 January 1838 .The area has a long tradition of skiing with several famous...
, Buskerud
Buskerud
is a county in Norway, bordering Akershus, Oslo, Oppland, Sogn og Fjordane, Hordaland, Telemark, and Vestfold. The county administration is located in Drammen.-Geography:...
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, in 1845. Smaltite is a synonym for the mineral. Notable occurrences include Cobalt, Ontario
Cobalt, Ontario
Cobalt is a town in the district of Timiskaming, province of Ontario, Canada, with a population of 1,223 In 2001 Cobalt was named "Ontario's Most Historic Town" by a panel of judges on the TV Ontario program Studio 2, and in 2002 the area was designated a National Historic Site.-History:Silver was...
, Skuterud, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, and Franklin, New Jersey
Franklin, New Jersey
Franklin is a Borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough population was 5,045.Franklin, known as the "Fluorescent Mineral Capital of the World," is located over a rich ore body containing more than 150 minerals, many of them fluorescent and...
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. The rare arsenide minerals are classified in Dana's sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
mineral group, even though it contains no sulfur.
The skutterudite crystal structure
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
Im-3 (number 204). The unit cell can be considered to consist of eight smaller cubes made up of the Co atoms. Six of these cubes are filled with (almost) square planar rings of As, each of which is oriented parallel to one of the unit cell edges, see image on the right. The As atoms then form octahedra
Octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
with Co in the centre.
In crystallographic terms, the Co atoms occupy the 8c sites, while the As atoms occupy the 24g sites. The position of the Co atoms within the unit cell is fixed, while the positions of the As atoms are determined by the parameters x and y. It has been shown that for the As-rings to be fully square, these parameters must satisfy the Oftedahl relation x+y=1/2. Any deviation from this relation yields a rectangular configuration of the As atoms. Indeed, this is the case for all known compounds with this structure, and the As atoms then do not form perfect octahedra.
Together with the unit cell size and the assigned space group, these two parameters fully describe the crystal structure of the material which is often referred to as the skutterudite structure.