This is a special rock.
It has brown parts. It grows in many places. Some rocks change to make it. You can find it in the ground. It is very neat. Do you like rocks?
This is a special kind of rock.
It is made of iron and magnesium. This rock forms when other rocks change. It can be found in many lands. You can find it in Sweden. It is also in South Africa. People found it in a town in Massachusetts. It can grow in brown, thin shapes. Some of these rocks are in volcanic areas. It is a very neat thing to find!
Cummingtonite is a special mineral. It belongs to a group called amphiboles. These are a set of minerals with a specific shape.
This mineral forms through metamorphism. That is a way rocks change under heat and pressure. You can find it in rocks rich in magnesium. It often grows alongside other minerals like talc or hornblende.
Cummingtonite is made of magnesium, iron, silicate, and hydroxide. It is part of a series with another mineral called grunerite. The two minerals change based on how much iron they have. Cummingtonite has between 30 and 70 percent magnesium.
A person named Chester Dewey found it in 1824. He found it in Cummington, Massachusetts. That is how the mineral got its name. You can also find it in Sweden and South Africa. It is also in Scotland and New Zealand. In Sweden, the crystals can look brown and fibrous. Fibrous means they look like many tiny threads.
Caption: Brown, thread-like crystals of cummingtonite from Sweden.
Cummingtonite is a special type of mineral. It belongs to a group called amphiboles. These minerals form in rocks that change under heat and pressure. This change is called metamorphism.
Scientists call it a metamorphic amphibole. It is important because it shows how minerals change. It sits in a group of minerals that share similar parts.
This mineral works through a way called a solid solution series. It is part of the cummingtonite-grunerite series. This means the minerals can change based on their parts. One end is magnesiocummingtonite. The other end is iron-rich grunerite. Cummingtonite is the middle part of this series. It has between 30 and 70 percent magnesium.
Manganese can also swap places with other atoms inside. This happens within the mineral's structure.
People first found this mineral in 1824. It was discovered in Cummington, Massachusetts. A man named Chester Dewey wrote about it. He wrote in the American Journal of Science. He described the geology of western Massachusetts.
The town gave its name to the mineral. This is how many minerals get their names today.
Cummingtonite grows in many different places. You can find it in Sweden and South Africa. It is also in Scotland and New Zealand. In Sweden, it grows in the Dannemora Mine. There, the crystals look brownish and fibrous. Fibrous means they look like tiny threads.
It often grows with minerals like talc or hornblende. It can also be found in dacite rocks.
Think of these minerals like a sliding scale. As more iron is added, the mineral changes. It moves from cummingtonite toward grunerite. Grunerite is common in the Lake Superior region. You can also find it in the Labrador Trough. These minerals tell us about the history of the Earth. They show us how heat and pressure work together.
Cummingtonite is a specific type of metamorphic amphibole. An amphibole is a group of rock-forming minerals. This mineral has a chemical formula of (Mg,Fe)2Si2O6(OH)2. This formula shows it is a magnesium iron silicate hydroxide. It is important because it forms during metamorphism. Metamorphism is the process where rocks change due to heat and pressure. Cummingtonite helps scientists understand these geological changes.
This mineral works through a process called a solid solution series. It belongs to the cummingtonite-grunerite series. This series functions like a sliding scale of chemistry. One end of the scale is magnesiocummingtonite. This end is very high in magnesium. The other end is grunerite. Grunerite is the iron-rich endmember of the series. Cummingtonite is the intermediate member. It contains between 30 and 70 percent magnesium.
Chemical changes happen within the crystal structure of the mineral. Atoms can swap places with one another. For example, manganese can substitute for iron. This happens at the B site atoms within the amphibole structure. Cummingtonite also has specific differences compared to other minerals. It tends toward more calcium substitution than anthophyllite. It also has lower levels of aluminium and ferric iron. These chemical details define exactly what kind of mineral it is.
Cummingtonite has a unique relationship with other minerals. It is a monoclinic mineral. This describes its crystal shape. It is a polymorph of orthorhombic anthophyllite. A polymorph is a mineral with the same composition but a different structure. Anthophyllite is a much more common magnesium-rich amphibole. However, anthophyllite is metastable. This means it is not in its most stable state. Cummingtonite does not mix well with alkali amphiboles. This is because they have different crystal habits. They also cannot substitute alkali elements for ferro-magnesian elements.
History shows us when this mineral was first identified. It was discovered in 1824. The discovery happened in the town of Cummington, Massachusetts. A scientist named Chester Dewey documented it. He published his findings in the American Journal of Science. His work was a sketch of the geology and mineralogy of western Massachusetts. The mineral carries the name of the town where it was first found.
Geologists find cummingtonite in several specific environments. It is commonly found in amphibolites. These are metamorphosed magnesium-rich rocks. It often coexists with other minerals. These include hornblende, actinolite, or talc. It can also be found with serpentine-antigorite minerals. Some versions appear in felsic volcanic rocks like dacites. Manganese-rich versions appear in metamorphosed manganese-rich rock units. The iron-rich grunerite endmember is found in the Lake Superior region. It is also found in the Labrador Trough.
Cummingtonite is found in many parts of the world. You can find it in Sweden, South Africa, Scotland, and New Zealand. In the Dannemora Mine in Sweden, the crystals are brownish and fibrous. Fibrous means they look like tiny threads. There is also a rare variety of grunerite called amosite. Amosite is an asbestiform variety. It was mined in the eastern part of the Transvaal Province in South Africa. The name amosite comes from the acronym for the Asbestos Mines of South Africa.
The mineral also changes as metamorphism progresses. This is called prograde metamorphism. During this process, cummingtonite and grunerite morph into other minerals. They change into members of the olivine and pyroxene series. This shows how minerals evolve as heat and pressure increase. Understanding these transitions helps us map the history of the Earth's crust. It connects the study of single minerals to the study of entire planetary systems.
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