Afwillite

Chemical formula: Ca<sub>3</sub>(SiO<sub>4</sub>)(SiO<sub>2</sub>(OH)<sub>2</sub>)·2H<sub>2</sub>O

Afwillite is a rare calcium silicate, forming colorless or white, prismatic crystals in contact metamorphic zones.

## Characteristics Afwillite is a hydrated calcium silicate belonging to the nesosilicate group. It most often forms colorless or white, vitreous crystals with a prismatic habit, frequently gathered into radial or fan-shaped aggregates. Individual crystals can be acicular or thick tabular and often show striations on their faces. It also occurs in massive aggregates. It is a brittle mineral, transparent to translucent. ## Physical Properties This mineral is characterized by low hardness, measuring 3 on the Mohs scale, which means it can be scratched by a copper coin. Its density is approximately 2.63 g/cm³. It exhibits perfect cleavage in one direction and good cleavage in another, contributing to its brittleness. The luster is typically vitreous. ## Colors and Varieties Afwillite is usually colorless or white. No colored varieties or trade names are distinguished for it. ## History and Name The mineral was first described in 1925 by John Parry and Frederick E. Wright. The name afwillite commemorates Alpheus Fuller Williams (1874–1953), an American mining engineer and later general manager of the De Beers company in Kimberley, South Africa. The Dutoitspan diamond mine in Kimberley, where the first specimens were found, is considered the type locality. ## Uses Afwillite has no industrial applications. It is solely an object of scientific interest and a prized collector's mineral due to its rarity and attractive crystalline forms.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
White
Density
2.63
Cleavage
Perfect on {101}, good on {001}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of afwillite include its low hardness (3 on the Mohs scale), vitreous luster, and crystal habit – most often prismatic, with a tendency to form radial aggregates. An important indicator is also its occurrence environment – contact metamorphic zones of carbonate rocks. ## Distinguishing from Similar Minerals Afwillite can be confused with other white or colorless minerals found in similar conditions, such as calcite, some zeolites (e.g., natrolite), or portlandite. It differs from calcite by its lack of reaction with hydrochloric acid (unless intergrown with it) and different crystal form. Zeolites, such as natrolite, typically form thinner, acicular crystals. Portlandite rarely occurs in well-formed crystals, more often forming massive aggregates. ## Crystal Forms Afwillite crystals belong to the monoclinic system. They usually have a columnar or thick tabular habit, often with striations on their faces. Twinning is common. The most impressive specimens form radial or stellar aggregates radiating from a single point.

Geological environment

## Genesis Afwillite is a low-temperature hydrothermal mineral. It forms as a result of contact metamorphism of carbonate rocks (limestones, chalk) under the influence of magmatic intrusions. It forms in veins cutting through metamorphosed limestone, as a result of the reaction of silica with calcium carbonate in the presence of water. ## Mineral Associations This mineral often co-occurs with other rare calcium silicates, such as larnite, spurrite, jennite, ettringite, portlandite, as well as with calcite and apophyllite. ## Localities The most important afwillite localities in the world are the Dutoitspan and Bultfontein mines in Kimberley (South Africa), where it was discovered. Classic specimens also come from Scawt Hill in County Antrim (Northern Ireland). Other known localities include Crestmore in California (USA), as well as deposits of the Hatrurim Formation in Israel and Jordan.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued afwillite specimens are those with well-formed, transparent crystals with strong luster. Radial aggregates on a contrasting rock matrix are particularly sought after. The size of individual crystals, their clarity, and the aesthetics of the entire specimen have a key impact on value. Specimens from classic localities, such as Scawt Hill, are historically and collector-wise more valuable. ## Popular Localities Two historical localities are considered the best for this mineral: Scawt Hill in Northern Ireland, from which exemplary, fan-shaped aggregates originate, and the diamond mines in Kimberley, South Africa, which is the type locality. Specimens from Crestmore in California are also prized by collectors.

Care and storage

## Cleaning Afwillite specimens should be cleaned very carefully due to their low hardness and cleavage. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed in distilled water and then gently dried. Ultrasonic cleaners should be avoided, as they can cause crystals to fracture along cleavage planes. ## What to Avoid The mineral is susceptible to mechanical damage – scratches and impacts. It should be protected from contact with harder minerals. It is also susceptible to acids, which cause its decomposition. It does not require special protection from light or humidity and is stable under room conditions. ## Storage It is recommended to store afwillite specimens in separate, padded boxes or display cases to avoid contact with other minerals. Protect from dust and vibrations. Well-formed crystals on a rock matrix are best displayed in enclosed cabinets.

External references

Sources

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