Cummingtonite
A metamorphic amphibole intermediate in the cummingtonite-grunerite series.
Cummingtonite is a metamorphic amphibole, a magnesium iron silicate hydroxide with the formula (Mg,Fe²⁺)₂(Mg,Fe²⁺)₅Si₈O₂₂(OH)₂. It is the intermediate member of the cummingtonite-grunerite solid solution series, which spans from the magnesium endmember magnesiocummingtonite to the iron-rich endmember grunerite. Minerals in this series with 30 to 70 percent of the iron-rich formula are called cummingtonite. Manganese can also replace iron and magnesium in the structure, leading to species like tirodite and dannemorite in high-grade metamorphic banded iron formations. Calcium, sodium, and potassium levels are low in cummingtonite, though it tends to accept more calcium than the related mineral anthophyllite. It also contains less ferric iron and aluminum than anthophyllite.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a more common magnesium-rich amphibole and is metastable. Cummingtonite has little compositional overlap with alkali amphiboles such as arfvedsonite or glaucophane-riebeckite, due to differences in crystal habit and the inability of alkali and ferro-magnesian elements to substitute for each other in the amphibole structure.
The mineral was named after Cummington, Massachusetts, where it was discovered in 1824. It also occurs in Sweden, South Africa, Scotland, and New Zealand. A rare asbestiform variety of grunerite, known as amosite, was mined as asbestos in the eastern part of South Africa’s Transvaal Province; the name comes from the acronym of the mining company "Asbestos Mines of South Africa."
Cummingtonite is typically found in metamorphosed magnesium-rich rocks, especially amphibolites, and often coexists with hornblende, actinolite, magnesium clinochlore chlorite, talc, serpentine-antigorite minerals, or metamorphic pyroxene. Magnesium-rich cummingtonite can also occur alongside anthophyllite. It has been identified in some felsic volcanic rocks like dacites, and manganese-rich varieties appear in metamorphosed manganese-rich rock units. The grunerite endmember is characteristic of metamorphosed iron formations in the Lake Superior region and the Labrador Trough. Under prograde metamorphism, cummingtonite and grunerite transform into members of the olivine and pyroxene series.
- type
- Metamorphic amphibole
- chemical_formula
- (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2
- crystal_system
- Monoclinic
- series
- Cummingtonite-grunerite solid solution series
- common_occurrence
- Metamorphosed magnesium-rich rocks, amphibolites
Lore & Background
Cummingtonite is a metamorphic amphibole, a magnesium iron silicate hydroxide. It was first identified in 1824 in Cummington, Massachusetts, which gives the mineral its name. Additional localities include Sweden, South Africa, Scotland, and New Zealand. The mineral is monoclinic and is compositionally similar to orthorhombic anthophyllite, though anthophyllite is more common and metastable. Cummingtonite belongs to a solid solution series with grunerite, ranging from magnesium-rich magnesiocummingtonite to iron-rich grunerite; the term cummingtonite specifically applies to intermediate compositions with 30 to 70 percent of the iron endmember. Manganese can also substitute into the structure, forming a compositional series with tirodite and dannemorite. Calcium, sodium, and potassium concentrations are low, though cummingtonite tends to incorporate more calcium than anthophyllite, and it has lower ferric iron and aluminum. A rare asbestiform variety of grunerite, known as amosite, was mined in the eastern Transvaal of South Africa. Cummingtonite typically occurs in metamorphosed magnesium-rich rocks, such as amphibolites, and often coexists with hornblende, actinolite, chlorite, talc, serpentine, or metamorphic pyroxene. Magnesium-rich varieties can also appear with anthophyllite. It has been found in felsic volcanic rocks like dacites, while manganese-rich species occur in metamorphosed manganese-rich units. The grunerite endmember is characteristic of metamorphosed iron formations in the Lake Superior region and the Labrador Trough. Under prograde metamorphism, cummingtonite and grunerite transform into members of the olivine and pyroxene series.
Reader's Guide
Cummingtonite is significant as an intermediate mineral in the cummingtonite-grunerite solid solution series, providing insight into the compositional variation of amphiboles in metamorphic environments. Its occurrence in metamorphosed magnesium-rich rocks and banded iron formations helps geologists understand the conditions of high-grade metamorphism. The mineral's limited solubility with alkali amphiboles highlights structural constraints in amphibole chemistry. The rare asbestiform variety amosite, mined in South Africa, has historical economic importance, though the article does not discuss health impacts. Cummingtonite's polymorphic relationship with anthophyllite and its transformation to olivine and pyroxene under prograde metamorphism illustrate phase transitions in metamorphic petrology. Its presence in diverse locations—from Massachusetts to Sweden, South Africa, Scotland, and New Zealand—underscores its global distribution in appropriate geological settings.
Did You Know?
- It is the intermediate member of the cummingtonite-grunerite solid solution series, with 30 to 70 percent iron endmember.
- A rare asbestiform variety of grunerite, called amosite, was mined only in the eastern Transvaal Province of South Africa.
- Cummingtonite is polymorphic with orthorhombic anthophyllite, though anthophyllite is more common and metastable.
Frequently Asked Questions
What is Cummingtonite?
Cummingtonite is a metamorphic amphibole mineral best described as a magnesium-iron silicate hydroxide. It crystallizes in the monoclinic system and typically forms within metamorphosed magnesium-rich rocks and amphibolites.
What series does Cummingtonite belong to?
It sits in the middle of the cummingtonite-grunerite solid solution series, bridging the gap between magnesium-rich magnesiocummingtonite on one end and iron-rich grunerite on the other. Its composition is (Mg,Fe²⁺)₂(Mg,Fe²⁺)₅Si₈O₂₂(OH)₂, reflecting that intermediate Mg-to-Fe ratio.
Why is Cummingtonite important to mineral collectors?
Because it represents the intermediate point of a well-known solid solution series, it helps geologists trace the chemistry of metamorphic environments. Collectors also prize it for its role in understanding how amphibole compositions shift with changing pressure and temperature during metamorphism.
Where does Cummingtonite typically occur in nature?
You'll most often find it in metamorphosed magnesium-rich rocks and in amphibolite facies assemblages. It is not a primary igneous phase but rather a product of regional or contact metamorphism acting on mafic protoliths.
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