Afwillite
| Afwillite | |
|---|---|
Afwillite from Crestmore Quarry, Riverside County, California
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| General | |
| Category | Nesosilicates |
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Formula (repeating unit) |
Ca3(SiO3OH)2·2H2O |
| Strunz classification | 9.AG.75 |
| Crystal system | Monoclinic |
| Crystal class | Domatic (m) (same H-M symbol) |
| Space group | Cc |
| Unit cell | a = 16.278(1), b = 5.6321(4) c = 13.236(1) [Å]; β = 134.9°; Z = 4 |
| Identification | |
| Color | Colorless, white |
| Crystal habit | Prismatic (striated), tabular, radial fibrous, massive |
| Cleavage | Perfect along [101], good along [100] |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 3–4 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.630 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.617 nβ = 1.620 nγ = 1.634 |
| Birefringence | δ = 0.0167 |
| 2V angle | Measured: 50° to 56° |
| Dispersion | r < v |
| Other characteristics | Piezoelectric |
| References | |
Afwillite is a calcium hydroxide nesosilicate mineral with formula Ca3(SiO3OH)2·2H2O. It occurs as glassy, colorless to white prismatic monoclinic crystals. Its Mohs scale hardness is between 3 and 4. It occurs as an alteration mineral in contact metamorphism of limestone. It occurs in association with apophyllite, natrolite, thaumasite, merwinite, spurrite, gehlenite, ettringite, portlandite, hillebrandite, foshagite, brucite and calcite.
It was first described in 1925 for an occurrence in the Dutoitspan Mine, Kimberley, South Africa and was named for Alpheus Fuller Williams (1874–1953), a past official of the De Beers diamond company.
Afwillite is typically found in veins of spurrite and it belongs to the nesosilicate sub-class. It is monoclinic, its space group is P2 and its point group is 2.
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. It appears that the afwillite, as well as the calcite, forms from precipitated fluids. The jennite is actually an alteration of the afwillite, but both formed from calcium silicates through hydration. 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.
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