Cummingtonite
A metamorphic amphibole intermediate in the cummingtonite-grunerite series.
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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."
Occurrence
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.
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.
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. 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.
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.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Cummingtonite (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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