Pentlandite
Encyclopedia
Pentlandite is an iron
-nickel
sulfide, (Fe
,Ni
)9S
8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt
.
Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness
of 3.5 - 4 and specific gravity
of 4.6 - 5.0 and is non-magnetic. It has a yellowish bronze color.
scientist Joseph Barclay Pentland
(1797-1873), who first noted the mineral.
Nickel, being a chalcophile element, prefers to inhabit a sulfide phase versus a silicate or oxide phase within most terrestrial lithochemical systems (a few exceptions exist in unusual compositions). This behaviour is seen only when the particular rock is molten and sulfur saturated.
In sulfur undersaturated melts, nickel will substitute for other transition metal
s within ferromagnesian minerals, the most usual being olivine
, although nickeliferous varieties of amphibole
, biotite
, pyroxene
and spinel
s are known. Ni substitutes most readily for Fe2+ and Mg2+.
In sulfur saturated melts, nickel behaves as a chalcophile element and partitions
strongly into the sulfide phase. Because most nickel exists in ultramafic rocks and behaves as a compatible element in igneous differentiation
processes, the formation of nickel-bearing sulfides is essentially restricted to sulfur saturated mafic and ultramafic melts.
The sulfide melt, being at or above 1000°C, is in the form of monosulfide solid solution (MSS), an amalgam of compositional "mineral" components of pentlandite, pyrite
and pyrrhotite
, and usually containing a small percentage of chalcopyrite
(Cu
being chalcophile), all of which are in an amorphous form. It is only upon cooling past ~550°C (dependent on composition) that the MSS undergoes exsolution into its component sulfide phases.
These phases are typically formed in an aphanitic equigranular granoblastic
massive sulfide phase, or as matrix ore or disseminated sulfides held within the overlying silicate rock matrix. Intact magmatic massive sulfide is rarely preserved as, aside from the Norilsk
deposit, most deposits of nickeliferous sulfide have been metamorphosed.
Metamorphism, especially if it is of at least middle greenschist
facies, will cause the solid massive sulfide to revert to MSS. During deformation the MSS will act in a ductile fashion, and it is often considered to have the consistency of toothpaste, able to travel great distances into the country rock and along structures. Upon cessation of metamorphism, the MSS solution reverts again to the component sulfides, but it usually inherits a foliated
or sheared texture, and typically sees growth of bright, equigranular to globular aggregates of porphyroblast
ic pentlandite crystals known colloquially as "fish scales".
Metamorphism may also reconstitute the MSS and sulfide composition, which may alter the concentration of Ni and the Ni:Fe ratio and Ni:S ratio of the sulfides (see sulfide tenor). In this case, pentlandite may be replaced by millerite
, and rarely heazlewoodite
. Metamorphism may also see the introduction of aggressive metasomatism
, and it is particularly common for arsenic
to enter the MSS, producing nickeline, gersdorffite
and other Ni-Co arsenides.
, South Africa
, the Voiseys Bay troctolite
intrusive complex in Canada
, the Duluth gabbro, in North America, and various other localities throughout the world. In these locations it forms an important nickel ore.
Pentlandite is also the principal ore mineral won from Kambalda type komatiitic nickel ore deposits
, the type examples of which are in the Yilgarn Craton
of Western Australia
. Similar deposits exist at Nkomati, Namibia
, in the Thompson Belt
, Canada, and a few examples from Brazil.
Pentlandite, but primarily chalcopyrite and PGEs
, are won from the supergiant Norilsk
nickel deposit, in trans-Siberian Russia.
The Sudbury
deposit in Ontario
, Canada
, is associated with a meteorite
impact crater. Pentlandite-pyrite-pyrrhotite ore in this location was formed from an extensive melt sheet formed by melting of rock post impact, which became sulfur saturated and formed extensive sheetlike ore deposits.
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...
-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...
sulfide, (Fe
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...
,Ni
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...
)9S
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
8. Pentlandite usually has a Ni:Fe ratio of close to 1:1. It also contains minor cobalt
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....
.
Pentlandite forms isometric crystals, but is normally found in massive granular aggregates. It is brittle with a hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
of 3.5 - 4 and 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...
of 4.6 - 5.0 and is non-magnetic. It has a yellowish bronze color.
Name and discovery
It is named after the IrishRepublic of Ireland
Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
scientist Joseph Barclay Pentland
Joseph Barclay Pentland
Joseph Barclay Pentland was an Irish geographer, natural scientist, and traveller. Born in Ireland, Pentland was educated at Armagh. He also studied in Paris, and worked with Georges Cuvier....
(1797-1873), who first noted the mineral.
Paragenesis
Pentlandite is the most common terrestrial nickel sulfide mineral formed from immiscible sulfide-silicate melts under normal mantle and crustal conditions.Nickel, being a chalcophile element, prefers to inhabit a sulfide phase versus a silicate or oxide phase within most terrestrial lithochemical systems (a few exceptions exist in unusual compositions). This behaviour is seen only when the particular rock is molten and sulfur saturated.
In sulfur undersaturated melts, nickel will substitute for other transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s within ferromagnesian minerals, the most usual being olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, although nickeliferous varieties of amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
and spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
s are known. Ni substitutes most readily for Fe2+ and Mg2+.
In sulfur saturated melts, nickel behaves as a chalcophile element and partitions
Partition coefficient
In chemistry and the pharmaceutical sciences, a partition- or distribution coefficient is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium. The terms "gas/liquid partition coefficient" and "air/water partition coefficient" are...
strongly into the sulfide phase. Because most nickel exists in ultramafic rocks and behaves as a compatible element in igneous differentiation
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
processes, the formation of nickel-bearing sulfides is essentially restricted to sulfur saturated mafic and ultramafic melts.
The sulfide melt, being at or above 1000°C, is in the form of monosulfide solid solution (MSS), an amalgam of compositional "mineral" components of pentlandite, 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...
and pyrrhotite
Pyrrhotite
Pyrrhotite is an unusual iron sulfide mineral with a variable iron content: FeS . The FeS endmember is known as troilite. Pyrrhotite is also called magnetic pyrite because the color is similar to pyrite and it is weakly magnetic...
, and usually containing a small percentage of chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
(Cu
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
being chalcophile), all of which are in an amorphous form. It is only upon cooling past ~550°C (dependent on composition) that the MSS undergoes exsolution into its component sulfide phases.
These phases are typically formed in an aphanitic equigranular granoblastic
Granoblastic
Granoblastic is an anhedral phaneritic equi-granular metamorphic rock texture. Granoblastic texture is typical of quartzite, marble and other non-foliated metamorphic rocks without porphyroblasts. Characteristics defining granoblastic texture include: grains visible to the unaided eye, sutured...
massive sulfide phase, or as matrix ore or disseminated sulfides held within the overlying silicate rock matrix. Intact magmatic massive sulfide is rarely preserved as, aside from the Norilsk
Norilsk
Norilsk is an industrial city in Krasnoyarsk Krai, Russia, located between the Yenisei River and the Taymyr Peninsula. Population: It was granted city status in 1953. It is the northernmost city in Siberia and the world's second largest city north of the Arctic Circle...
deposit, most deposits of nickeliferous sulfide have been metamorphosed.
Metamorphism, especially if it is of at least middle greenschist
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies, will cause the solid massive sulfide to revert to MSS. During deformation the MSS will act in a ductile fashion, and it is often considered to have the consistency of toothpaste, able to travel great distances into the country rock and along structures. Upon cessation of metamorphism, the MSS solution reverts again to the component sulfides, but it usually inherits a foliated
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
or sheared texture, and typically sees growth of bright, equigranular to globular aggregates of porphyroblast
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
ic pentlandite crystals known colloquially as "fish scales".
Metamorphism may also reconstitute the MSS and sulfide composition, which may alter the concentration of Ni and the Ni:Fe ratio and Ni:S ratio of the sulfides (see sulfide tenor). In this case, pentlandite may be replaced by millerite
Millerite
Millerite is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates...
, and rarely heazlewoodite
Heazlewoodite
Heazlewoodite, Ni3S2, is a rare sulfur-poor nickel sulfide mineral found in serpentinitized dunite. It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallize in the trigonal crystal system. It has a hardness of 4, a specific gravity of 5.82,...
. Metamorphism may also see the introduction of aggressive metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
, and it is particularly common for 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...
to enter the MSS, producing nickeline, gersdorffite
Gersdorffite
Gersdorffite is a nickel arsenic sulfide mineral with formula NiAsS. It crystallizes in the isometric system showing diploidal symmetry. It occurs as euhedral to massive opaque, metallic grey-black to silver white forms. Gersdorffite belongs to a solid solution series with cobaltite, CoAsS....
and other Ni-Co arsenides.
Occurrence
Pentlandite is found within the lower margins of mineralised layered intrusions, the prime example being the Bushveld igneous complexBushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, the Voiseys Bay troctolite
Troctolite
Troctolite is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with variable minor pyroxene. It is an olivine-rich, pyroxene-depleted relative of gabbro. However, unlike gabbro, no troctolite corresponds in composition to a...
intrusive complex in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, the Duluth gabbro, in North America, and various other localities throughout the world. In these locations it forms an important nickel ore.
Pentlandite is also the principal ore mineral won from Kambalda type komatiitic nickel ore deposits
Kambalda type komatiitic nickel ore deposits
Kambalda type nickel ore deposits are a class of magmatic nickel-copper ore deposit in which the physical processes of komatiite volcanology serve to enrich, concentrate and deposit nickel-bearing sulfide within the lava flow environment of an erupting komatiite volcano.-Classification:The...
, the type examples of which are in the Yilgarn Craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
. Similar deposits exist at Nkomati, Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
, in the Thompson Belt
Thompson Belt
The Thompson Belt, also referred to as the Thompson Nickel Belt, is an Archean and early Proterozoic geologic feature in Manitoba, Canada. It contains gneiss related to deformation of the Trans-Hudson orogeny....
, Canada, and a few examples from Brazil.
Pentlandite, but primarily chalcopyrite and PGEs
Platinum group
The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...
, are won from the supergiant Norilsk
Norilsk
Norilsk is an industrial city in Krasnoyarsk Krai, Russia, located between the Yenisei River and the Taymyr Peninsula. Population: It was granted city status in 1953. It is the northernmost city in Siberia and the world's second largest city north of the Arctic Circle...
nickel deposit, in trans-Siberian Russia.
The Sudbury
Sudbury Basin
The Sudbury Basin, also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geologic structure in Ontario, Canada. It is the second-largest known impact crater or astrobleme on Earth, as well as one of the oldest....
deposit in Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, is associated with a meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
impact crater. Pentlandite-pyrite-pyrrhotite ore in this location was formed from an extensive melt sheet formed by melting of rock post impact, which became sulfur saturated and formed extensive sheetlike ore deposits.
See also
- Ore genesisOre genesisThe various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore genesis theories are very dependent on the mineral or commodity....
- Igneous differentiationIgneous differentiationIn geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
- Rock microstructureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
- Ultramafic rocks
- KomatiiteKomatiiteKomatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...