Cummingtonite
A metamorphic amphibole intermediate in the cummingtonite-grunerite series.
Cummingtonite is a metamorphic amphibole mineral with the chemical composition (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2, a magnesium iron silicate hydroxide. As a member of the cummingtonite-grunerite solid solution series, it represents the intermediate composition between magnesium-rich magnesiocummingtonite and iron-rich grunerite.
- 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
It is compositionally similar and polymorphic with orthorhombic anthophyllite, though anthophyllite is more common and metastable. Cummingtonite shares few compositional similarities with alkali amphiboles such as arfvedsonite and glaucophane-riebeckite, with little solubility between them due to differences in crystal habit and substitution limitations.
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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