Cummingtonite is a metamorphic
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
with the chemical composition (
MgMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
,
FeIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
)
7SiSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
8OOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
22(
OHIn chemistry, hydroxyl is composed of molecules consisting of an oxygen atom and a hydrogen atom connected by a covalent bond . The neutral form is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a charge of negative one...
)
2, magnesium iron silicate hydroxide.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic
anthophylliteAnthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
, which is a much more common form of
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
-rich amphibole, the latter being metastable.
Cummingtonite shares few compositional similarities with
alkaliIn chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Alkalis are best known for being bases that dissolve in water. Bases are compounds with a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base, especially for...
amphiboles such as
arfvedsoniteArfvedsonite is a sodium amphibole mineral with composition: Na34Fe3+Si8O222. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms...
,
glaucophaneGlaucophane is a mineral belonging to the amphibole group, chemical formula Na23Al2Si8O222. It is named from its typical blue color . Glaucophane crystallizes in the monoclinic system.The blue color is very diagnostic for this species...
-
riebeckiteRiebeckite is a sodium-rich member of the amphibole group of minerals, chemical formula Na25Si8O222. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous,...
.
Cummingtonite is a metamorphic
amphiboleAmphibole defines an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain SiO
4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Amphiboles crystallize into two crystal systems,...
with the chemical composition (
MgMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
,
FeIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
)
7SiSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
8OOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
22(
OHIn chemistry, hydroxyl is composed of molecules consisting of an oxygen atom and a hydrogen atom connected by a covalent bond . The neutral form is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a charge of negative one...
)
2, magnesium iron silicate hydroxide.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic
anthophylliteAnthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
, which is a much more common form of
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
-rich amphibole, the latter being metastable.
Cummingtonite shares few compositional similarities with
alkaliIn chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Alkalis are best known for being bases that dissolve in water. Bases are compounds with a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base, especially for...
amphiboles such as
arfvedsoniteArfvedsonite is a sodium amphibole mineral with composition: Na34Fe3+Si8O222. It crystallizes in the monoclinic prismatic crystal system and typically occurs as greenish black to bluish grey fibrous to radiating or stellate prisms...
,
glaucophaneGlaucophane is a mineral belonging to the amphibole group, chemical formula Na23Al2Si8O222. It is named from its typical blue color . Glaucophane crystallizes in the monoclinic system.The blue color is very diagnostic for this species...
-
riebeckiteRiebeckite is a sodium-rich member of the amphibole group of minerals, chemical formula Na25Si8O222. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous,...
. There is little solubility between these minerals due to different
crystal habitIn mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystalsThe many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a...
and inability of substitution between alkali elements and ferro-magnesian elements within the amphibole structure.
Cummingtonite was first described in 1824 for an occurrence near
Cummington, MassachusettsCummington is a town in Hampshire County, Massachusetts, United States. The population was 978 at the 2000 census. It is part of the Springfield, Massachusetts Metropolitan Statistical Area.- History :...
, hence the name. It is also found in
SwedenSweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe...
,
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
, and
ScotlandScotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
.
Chemistry
Cummingtonite is a member of the cummingtonite-
gruneriteGrunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O222. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The...
solid solutionA solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase.This...
series which ranges from Fe
2Mg
5Si
8O
22(OH)
2 for cummingtonite to the iron rich grunerite endmember Fe
7Si
8O
22(OH)
2. Cummingtonite is used to describe minerals of this formula with between 30 and 70 per cent Fe
7Si
8O
22(OH)
2.
ManganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
also substitutes for (Fe,Mg) within cummingtonite amphibole, replacing B site atoms. These minerals are found in high-grade metamorphic
banded iron formationBanded iron formations are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
and form a compositional series between
MnManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
2Mg
5Si
8O
22(OH)
2 (tirodite) and Mn
2Fe
5Si
8O
22(OH)
2 (dannemorite).
CalciumCalcium 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 grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
,
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
and
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
concentrations in cummingtonite are low. Cummingtonite tends toward more
CaCalcium 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 grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
substitution than related
anthophylliteAnthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
. Similarly, cummingtonite has lower Fe
3+ and Al than anthophyllite.
Amosite is a rare
asbestiformAsbestos is a naturally occurring silicate mineral with long, thin fibrous crystals. The word asbestos is a borrowed Greek adjective meaning inextinguishable. The Greeks termed asbestos the miracle mineral because of its soft and pliant properties, as well as its ability to withstand...
variety of grunerite that is mined as asbestos only in the eastern part of the
TransvaalThe Transvaal is the name of an area of northern South Africa. Originally the bulk of the independent Boer South African Republic, after the Anglo-Boer War of 1899-1902 it became the Transvaal Colony, and one of the founding provinces of the Union of South Africa, with its regional capital in...
Province of
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
. The origin of the name is
Amosa, the acronym for the mining company "Asbestos Mines of South Africa."
Occurrence
Cummingtonite is commonly found in metamorphosed magnesium-rich rocks and occurs in amphibolites. Usually it coexists with
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
or
actinoliteActinolite is an amphibole silicate mineral with the chemical formula .-Mineralogy:Actinolite is an intermediate member in a solid-solution series between magnesium-rich tremolite, , and iron-rich ferro-actinolite, . Mg and Fe ions can be freely exchanged in the crystal structure...
, magnesium clinochlore
chloriteThe chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
,
talcTalc is a mineral composed of hydrated magnesium silicate with the chemical formula H
2Mg
34 or Mg
3Si
4O
102. In loose form, it is the widely used substance known as talcum powder...
,
serpentineThe serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate minerals; they may contain minor amounts of other elements including chromium, manganese, cobalt and nickel. In mineralogy and gemology, serpentine may refer to any of 20 varieties belonging to the...
-antigorite minerals or metamorphic
pyroxeneThe pyroxenes are a group of important rock-forming silicate 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...
. Magnesium-rich cummingtonite can also coexist with
anthophylliteAnthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
.
Cummingtonite has also been found in some
felsicFelsic is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. The term combines the words "feldspar" and "silica." Felsic minerals are usually light in color and have specific...
igneous rocks such as
daciteDacite is an igneous, volcanic rock. It is intermediate in compositions between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
s.
ManganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
rich species can be found in metamorphosed Mn-rich rock units. The
gruneriteGrunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O222. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The...
end member is characteristic of the metamorphosed
iron formationsBanded iron formations are a distinctive type of rock often found in primordial sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...
of the
Lake SuperiorLake Superior is the largest of the five Great Lakes of North America. It is bounded to the north by Ontario, Canada and Minnesota, United States, and to the south by the U.S. states of Wisconsin and Michigan...
region and the
Labrador TroughThe Labrador Trough is a long and wide geologic belt in Canada, extending southeast from Ungava Bay through Quebec and Labrador. It is one of the largest iron ore belts on Earth and has had mining operations since 1954....
. With prograde metamorphism cummingtonite and grunerite morph to members of the
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
and pyroxene series.