Pseudowollastonite

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

Calcium silicate (CaSiO₃), a high-temperature polymorph of wollastonite, forming characteristic tabular, pseudohexagonal crystals.

## Characteristics Pseudowollastonite is a calcium silicate and a high-temperature polymorph of wollastonite, meaning it has the same chemical composition (CaSiO₃) but a different crystal structure. It is stable at temperatures above 1120°C. Typical specimens consist of small, tabular or bladed crystals, which often form radial or rosette aggregates. Although the crystals crystallize in the triclinic system, they exhibit a distinct pseudohexagonal outline, which is one of their distinguishing features. ## Physical Properties The Mohs hardness of pseudowollastonite ranges from 4.5 to 5, and its density is approximately 2.91 g/cm³. This mineral has a vitreous luster, and on cleavage surfaces, it can be pearly. It is transparent to translucent. It exhibits good cleavage in two directions. ## Colors and Varieties Pseudowollastonite is most commonly colorless, white, or grayish. It does not form colored varieties or possess distinct trade names. Its value is based on its structure and rarity, rather than its color. ## History and Name The mineral's name refers to its close relationship with wollastonite. The Greek prefix "pseudo" means "false," indicating its different, though related, nature. It was accurately described as a distinct mineral in the first half of the 20th century, as X-ray diffraction techniques developed, allowing for the differentiation of polymorphs. ## Applications As a natural mineral, pseudowollastonite is primarily of scientific and collector's interest. However, it is an important component of some industrial products, such as specialized cements and metallurgical slags, where it forms as a product of high-temperature synthesis.

Properties

Mohs hardness
4.5-5
Luster
Vitreous, Pearly
Streak
White
Density
2.91
Cleavage
Good on {100} and {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification A key diagnostic clue is the environment of occurrence – very high-temperature contact metamorphic zones (pyrometamorphism). The crystal habit is characteristic: small, thin tablets with a pseudohexagonal outline, often gathered in radial aggregates. ## Distinguishing from Similar Minerals Visual differentiation of pseudowollastonite from wollastonite is extremely difficult and usually requires X-ray diffraction (XRD) analysis. It can be confused with other white silicates, such as tremolite or minerals from the melilite group (gehlenite, åkermanite). It differs from tremolite in habit (lack of fibrous habit), and from melilite often in hardness and crystal form. ## Crystal Forms It forms thin, tabular or bladed crystals. Despite belonging to the triclinic system, these crystals often combine in such a way that they form pseudohexagonal symmetry. Aggregates usually have a radial, rosette, or irregular habit.

Geological environment

## Genesis Pseudowollastonite is a high-temperature mineral, forming under conditions of pyrometamorphism, i.e., at the highest temperatures of contact metamorphism (above 1120°C) at low pressure. It forms in skarns created at the contact of magmatic intrusions with carbonate sedimentary rocks (limestones, marls). It also occurs in carbonate xenoliths entrained by volcanic lava and in some slags and Portland cements. ## Mineral Associations This mineral co-occurs with other high-temperature calcium silicates, such as larnite, spurrite, tilleyite, as well as with minerals from the melilite group (gehlenite, åkermanite), diopside, vesuvianite, and grossular. ## Localities The most important global localities include sites associated with contact metamorphism. The classic type locality is Scawt Hill in Northern Ireland. Other known occurrences include the Monte Somma-Vesuvius volcanic complex in Italy, the Crestmore quarry in California (USA), as well as the Eifel and Mayen volcanic regions in Germany, and some volcanoes in Japan.

Rarity

Rare

For collectors

## Quality Criteria For collectors, the most valuable specimens are those with well-formed, sharp, and transparent crystals, preferably embedded in a contrasting rock matrix. Rich aggregates with a radial structure are also highly prized. Association with other rare pyrometamorphic minerals is crucial, as it enhances the scientific and aesthetic value of the specimen. ## Popular Localities The most valued specimens by collectors come from classic, historical localities. These include Scawt Hill in Northern Ireland and Crestmore Quarry in California. Specimens from these locations are sought after by specialists and advanced collectors of rare minerals.

Care and storage

## Cleaning Pseudowollastonite specimens should be cleaned carefully, using a soft brush and distilled water. Due to its moderate hardness and brittleness, strong scrubbing should be avoided. Ultrasonic cleaners are not recommended, as they can damage delicate aggregates. ## What to Avoid The mineral is sensitive to strong acids. It should be protected from impacts and contact with harder minerals that could scratch it. It does not require special protection from light or room temperature. ## Storage It is best to store specimens in separate, padded boxes or display cases to prevent mechanical damage. Display in a closed cabinet will protect fragile crystals from dust and accidental bumps.

External references

Sources

Read more