Afwillite: Formation and Properties

Afwillite is a calcium hydroxide nesosilicate mineral with formula Ca3(SiO3OH)2·2H2O. It occurs as glassy, colorless to white prismatic monoclinic crystals. It is named after Alpheus Fuller Williams (1874-1953), the General Manager of DeBeers Consolidated Mines, South Africa. It occurs as an alteration mineral in contact metamorphism of limestone. The color of afwillite varies from colorless to white. It is brittle by nature and exhibits a vitreous lustre. It occurs in association with apophyllite, natrolite, thaumasite, merwinite, spurrite, gehlenite, ettringite, portlandite, hillebrandite, foshagite, brucite, and calcite. Its hardness varies from 3 to 4

It was discovered in the year 1925 at the Dutoitspan Mines, South Africa and was named for Alpheus Fuller Williams (1874–1953), a past official of the De Beers diamond company.

General Information:

  • Category: Nesosilicates
  • Formula: Ca3(SiO3OH)22H2O
  • Strunz classification: 9.AG.75
  • Crystal system: Monoclinic.

Identification

  • Color: Colorless, white
  • Crystal habit: Prismatic (striated), tabular, radial fibrous, massive
  • Fracture: Conchoidal
  • Tenacity: Brittle
  • Mohs scale hardness: 3-4
  • Diaphaneity: Transparent to translucent
  • Specific gravity: 2.630

Global Distribution

Afwillite is widely distributed in the following locations: South Africa, Ireland, Germany, Italy, Israel, USA, and Japan.

Occurrence and Useful Mineral Association

Afwillite is a product formed during the metamorphism of limestones. It is often associated with minerals such as natrolite, merwinite, ettringite, brucite, calcite, foshagite, portlandite and hillebrandite.

Formation of afwillite

It is suggested that afwillite forms in fractured veins of the mineral spurrite. Jennite, afwillite, oyelite, and calcite are all minerals that form in layers within spurrite veins. Laboratory studies determined that afwillite forms at a temperature below 200 °C (392 °F), usually around 100 °C. Afwillite and spurrite are formed through contact metamorphism of limestone. Contact metamorphism is caused by the interaction of rock with heat and/or fluids from a nearby crystallizing silicate magma.

Physical Properties of Afwillite

  • Cleavage:{001} Perfect, {???} Distinct
  • Color: Colorless, White.
  • Density: 2.62
  • Diaphaneity: Transparent to Subtransparent to translucent
  • Habit: Massive – Uniformly indistinguishable crystals forming large masses.
  • Habit: Prismatic – Crystals Shaped like Slender Prisms (e.g. tourmaline).
  • Hardness: 3 – Calcite
  • Luster: Vitreous (Glassy)
  • Streak: white

 

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