Cummingtonite

Chemical formula: &#9744;Mg<sub>2</sub>Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>

Cummingtonite is a silicate from the amphibole group, forming fibrous or radial aggregates ranging in color from gray to brown.

## Characteristics Cummingtonite is a mineral from the amphibole group, belonging to the magnesium-iron series. Typical specimens appear as fibrous, radial, or lamellar aggregates. It rarely forms single, well-developed crystals. Its appearance is often subtle, and aggregates can resemble asbestos, although they do not always take this form. ## Physical Properties This mineral has a hardness ranging from 5-6 on the Mohs scale. The luster is typically vitreous or silky, especially in fibrous varieties. It is translucent to opaque. The density varies from 2.9 to 3.6 g/cm³, depending on the iron content. ## Colors and Varieties Cummingtonite occurs in various shades of gray, green, and brown. Its color is directly related to its iron content – the more iron it contains, the darker and browner the mineral. Amosite is a trade name for an asbestiform, fibrous variety of cummingtonite, which is highly enriched in iron. ## History and Name The mineral's name comes from the town of Cummington, Massachusetts (USA), where it was first described in 1824 by American chemist and mineralogist Chester Dewey. The name refers to its discovery location. ## Uses Due to potential health hazards associated with its fibrous nature (similar to other forms of asbestos), cummingtonite currently has no significant industrial applications. Its importance is mainly limited to scientific and collecting interest.

Properties

Mohs hardness
5-6
Luster
Vitreous
Streak
White
Density
2.9-3.6
Cleavage
Good on {110}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Cummingtonite is most easily identified by its fibrous or radial aggregate form and characteristic silky or vitreous luster. The color, typically in shades of gray, brown, or green, is also a helpful indicator. It occurs in metamorphic rocks. ## Distinguishing from Similar Minerals It can be confused with other amphiboles, such as tremolite or actinolite. Tremolite is usually lighter (white to light gray), while actinolite has a characteristic green color. Definitive differentiation from other members of the cummingtonite-grunerite series often requires chemical analysis to determine the magnesium-to-iron ratio. ## Crystal Forms It most often forms fibrous, radial, lamellar, or granular aggregates. Well-formed, single prismatic crystals are rare.

Geological environment

## Genesis Cummingtonite is a typical mineral of metamorphic rocks that have undergone contact or regional metamorphism under conditions ranging from greenschist to amphibolite facies. It forms as a result of the alteration of magnesium- and iron-rich rocks, such as ultramafic igneous rocks or iron-rich sedimentary formations. ## Mineral Associations It often co-occurs with other amphiboles (such as hornblende, anthophyllite, actinolite), garnets (especially almandine), plagioclase, cordierite, biotite, and magnetite. ## Localities Significant cummingtonite localities include Cummington, Massachusetts (USA) – the type locality. Other important localities are iron formations in the Lake Superior region (USA/Canada), the Broken Hill mine in Australia, as well as deposits in Sweden (e.g., in the Bergslagen region), Norway, Finland, and Greenland.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most attractive specimens are those with well-developed, radial aggregates or rare, visible crystals. Specimens exhibiting a distinct silky luster are also desirable. Contrasting associations with other minerals, such as red garnets, can significantly enhance the aesthetic value of a specimen. ## Popular Localities Specimens from the classic locality in Cummington, Massachusetts, have historical value. High-quality, aesthetic aggregates also come from mines in the Bergslagen region of Sweden and from some localities in Norway.

Care and storage

## Cleaning Cummingtonite specimens, especially those with a fibrous structure, are fragile and should be handled with care. For cleaning, gently wipe with a soft, dry brush or a small brush to remove dust. Avoid washing in water, which can weaken the cohesion of the aggregates. ## What to Avoid It is absolutely essential to avoid inhaling dust or fine fibers that may be released from the specimen, especially from asbestiform varieties (amosite). Do not expose it to strong chemicals or ultrasound. Its fragility makes it susceptible to impacts and falls. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to minimize the risk of mechanical damage and the emission of fibers into the environment. The label should include a warning about the potential harm if dust is inhaled.

External references

Sources

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