Wollastonite

Chemical formula: CaSiO<sub>3</sub>

Wollastonite is a calcium silicate, valued in industry for its physical properties, and known among collectors for its acicular and fibrous crystals.

## Characteristics Wollastonite is a calcium silicate belonging to the pyroxenoid group. It most often forms aggregates with a fibrous, acicular, or radial structure. Well-formed crystals are rare and have a tabular or prismatic habit. It usually occurs as compact, felted masses, which in appearance may resemble some zeolites or tremolite. ## Physical Properties This mineral is characterized by a hardness of 4.5-5 on the Mohs scale. It has a vitreous, pearly, or silky luster, especially visible on cleavage surfaces and in fibrous aggregates. It is a relatively brittle mineral. Its density is approximately 2.9 g/cm³. ## Colors and Varieties Wollastonite is most often white, but can also be gray, cream, pale pink, or light brown, depending on impurities. It does not form distinct color varieties, and its name is applied uniformly to the mineral with a defined chemical composition. ## History and Name The mineral's name was given in 1818 by J. Léman in honor of the English chemist and mineralogist William Hyde Wollaston (1766–1828), who discovered palladium and rhodium and invented the refractometer. However, wollastonite was known and described earlier under other names. ## Applications Due to its fibrous structure, low thermal and electrical conductivity, and high melting point, wollastonite is an important industrial raw material. It is used in the production of ceramics, paints, plastics, friction materials (brake pads), and as an asbestos substitute in insulation and building materials.

Properties

Mohs hardness
4.5-5
Luster
Vitreous, Pearly on cleavage surfaces
Streak
White
Density
2.86-3.09
Cleavage
Perfect on {100}, Good on {001} and {102}
Fracture
Uneven
Transparency
Translucent to transparent
Crystal system
Triclinic

Diagnostic features

## Identification The most important diagnostic feature of wollastonite is its mode of occurrence – most often as white, silky or pearly fibrous and acicular aggregates. Characteristic is also its perfect cleavage in two directions, visible in better-formed crystals. It reacts with hydrochloric acid. ## Distinguishing from Similar Minerals Wollastonite is sometimes confused with tremolite, which also forms fibrous aggregates. However, tremolite is harder (5-6 on the Mohs scale) and does not react with acids in the same way. It can also be mistaken for some zeolites (e.g., natrolite), which often have a similar form, but usually occur in gas vesicles in volcanic rocks, whereas wollastonite is typical of contact metamorphic rocks. ## Crystal Forms Crystals are rare, most often tabular or short prismatic. Dominant forms are fibrous, acicular, radial, felted, or granular aggregates.

Geological environment

## Genesis Wollastonite is a typical mineral of contact metamorphism. It forms in contact zones of magmatic intrusions (mainly granitoid) with calcium carbonate-rich sedimentary rocks, such as limestones and marls. This process occurs under conditions of high temperature and relatively low pressure. It is less commonly found in regional metamorphic rocks and some skarns. ## Mineral Associations Minerals commonly associated with wollastonite include calcite, diopside, grossular (garnet), andradite (garnet), vesuvianite, epidote, tremolite, and quartz. ## Localities Significant deposits and classic localities of wollastonite are found in Willsboro, New York (USA), where it is mined on a large scale. Beautiful collector's specimens come from the Santa Rita mine in New Mexico (USA), from Franklin, New Jersey (USA), from the Chiapas region in Mexico, and from Pargas, Finland. It also occurs in Romania (Ciclova Română), Italy (in the Vesuvius volcanic complex), and many other places worldwide.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with well-formed, large, and often transparent crystals, which are, however, very rare. Aesthetic, radial aggregates of white, acicular crystals on a contrasting matrix, for example, in association with green diopside or brown vesuvianite, are also highly valued. Purity (lack of dirt) and absence of damage to delicate fibers significantly increase the value of a specimen. ## Popular Localities The most prized specimens by collectors come from classic contact metamorphic localities, such as Franklin, New Jersey (USA), where wollastonite occurs in association with fluorescent minerals. Specimens from Mexico (e.g., from the Pilares mine in Sonora) are renowned for their large, well-formed crystals. European localities, such as Pargas, Finland, have also provided historical, important specimens.

Care and storage

## Cleaning Wollastonite specimens can be cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water may be used. Ultrasonic cleaners should be avoided, as they can damage delicate, fibrous aggregates. ## What to Avoid Wollastonite is sensitive to acids, which cause its decomposition with the release of silica gel. It should be protected from contact with household and laboratory chemicals. It is not particularly sensitive to light or temperature changes under household conditions. ## Storage Specimens, especially those with a delicate, acicular structure, should be stored in closed boxes or display cases to protect them from dust and mechanical damage. Avoid placing heavier minerals on fibrous wollastonite aggregates.

External references

Sources

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