Wollastonite-2<i>M</i>

Chemical formula: CaSiO<sub>₃</sub>

Wollastonite-2M is a monoclinic polymorph of wollastonite, a calcium silicate, most often forming fibrous or acicular aggregates of white color.

## Characteristics Wollastonite-2M is a monoclinic polymorph of wollastonite, belonging to the pyroxenoid group. It forms tabular or prismatic crystals, often elongated and acicular. Most commonly, it occurs as fibrous, felted, radial, or granular aggregates. It is a brittle mineral, and its aggregates can resemble mineral wool in appearance. ## Physical Properties The mineral is characterized by a hardness of 4.5-5 on the Mohs scale and a density of approximately 2.91 g/cm³. Wollastonite's luster is typically vitreous, and pearly on cleavage surfaces. In fibrous aggregates, it may exhibit a silky luster. It is translucent to opaque. Some specimens may show fluorescence under ultraviolet light, most often yellow, orange, or pink. ## Colors and Varieties Wollastonite-2M is most commonly white, grayish-white, or colorless. Impurities can give it yellowish, pinkish, reddish, or brownish hues. No named varieties are distinguished; classification is based on structural polymorphism (e.g., wollastonite-1T, wollastonite-2M). ## History and Name The name "wollastonite" honors the English chemist and mineralogist William Hyde Wollaston (1766–1828). It was given in 1818 by Jöns Jakob Berzelius. The suffix "-2M" refers to its specific monoclinic crystal structure with two layers in the unit cell. ## Applications Wollastonite is an important industrial raw material. Due to its properties (low moisture absorption, thermal resistance, chemical inertness), it is used as a filler in the production of plastics, ceramics (especially tiles and sanitary ware), paints, and friction materials (brake pads). It also serves as an asbestos substitute in insulation and building materials.

Properties

Mohs hardness: 4.5-5
Luster: Vitreous, pearly, silky
Streak: White
Density: 2.91
Cleavage: Perfect on {100}, good on {001} and {102}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features of wollastonite include its white color, fibrous or acicular appearance of aggregates, and excellent cleavage in two directions. The luster is sometimes vitreous, but on cleavage planes, it becomes pearly, which is helpful for identification. Reaction with hydrochloric acid (decomposition without effervescence) is also a diagnostic feature. ## Distinguishing from Similar Minerals Wollastonite is sometimes confused with tremolite, pectolite, or natrolite. Tremolite is harder (5-6) and does not decompose in hydrochloric acid. Pectolite is very similar but often has a more silky luster and decomposes in hydrochloric acid much faster, sometimes with effervescence. Natrolite and other zeolites usually have lower density and form crystals with a different cross-section. ## Crystal Forms Wollastonite-2M crystallizes as tabular or prismatic crystals, often flattened. However, it is most commonly found in the form of fibrous, radial, felted (similar to felt), or coarse granular aggregates with a lamellar structure.

Geological environment

## Genesis Wollastonite is a typical mineral of contact metamorphism. It forms as a result of reactions between carbonates (limestones, marbles) and silica under high-temperature conditions during magma intrusion. It occurs in skarns, hornfelses, and contact marbles. Less commonly, it is found in igneous rocks, such as carbonatites and some nepheline syenites, as well as in carbonate xenoliths in volcanic lavas. ## Mineral Associations This mineral often co-occurs with other skarn minerals, such as diopside, grossular (garnet), vesuvianite, epidote, tremolite, and calcite. Depending on the location, it may also be accompanied by scheelite, andradite, or monticellite. ## Localities The most economically important wollastonite deposits are found in China, India, the United States (especially in Willsboro and Lewis, New York, and in California), Mexico, and Finland. Well-formed crystals come from localities such as: Franklin Mine in New Jersey (USA), the vicinity of Mount Vesuvius (Italy), Chiapas (Mexico), Pargas (Finland), and from many skarns in Romania and Japan.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are well-formed, single crystals, which are relatively rare. Transparent specimens with sharp edges and a clean, white color are highly valued. Aesthetically pleasing, radial aggregates of acicular crystals on a rock matrix are also attractive. Specimens exhibiting strong fluorescence are an additional asset. ## Popular Localities Classic collector specimens, including large, well-formed crystals, come from the Franklin and Sterling Hill mines in New Jersey, USA. Beautiful, radial aggregates were found in mines in Romania (e.g., Dognecea, Băița Bihor). The state of Chiapas in Mexico supplied large, tabular crystals. Italian localities near Vesuvius are known for microcrystals of wollastonite in volcanic bombs.

Care and storage

## Cleaning Wollastonite specimens should be cleaned carefully, using a soft, dry brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided, especially for fragile, fibrous aggregates. Allow the specimen to dry completely at room temperature. ## What to Avoid Wollastonite is sensitive to acids, which cause its decomposition with the release of gelling silica gel. Contact with household chemicals and acids should be avoided. The mineral is relatively soft, so it must be protected from scratching by harder minerals (e.g., quartz). ## Storage Fragile, acicular, and fibrous wollastonite specimens are very delicate and susceptible to mechanical damage. They should be stored in separate, padded boxes, away from vibrations and the risk of falling. Other specimens should not be placed on top of them.