Ettringite
Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.
| Ettringite | |
|---|---|
Ettringite, Kalahari manganese fields, Northern Cape Province, South Africa | |
| General | |
| Category | Sulfate minerals |
| Formula (repeating unit) | Ca6Al2(SO4)3(OH)12·26H2O |
| IMA symbol | Ett |
| Strunz classification | 7.DG.15 |
| Crystal system | Trigonal |
| Crystal class | Ditrigonal pyramidal (3m) H-M symbol: (3m) |
| Space group | P31c |
| Unit cell | a = 11.23, c = 21.44 [Å]; Z = 2 |
| Identification | |
| Color | Colorless, pale yellow, milky white |
| Crystal habit | Acicular growth, striated prismatic crystals; fibrous to cottonlike |
| Cleavage | Perfect on {1010} |
| Mohs scale hardness | 2–2.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 1.77 |
| Optical properties | Uniaxial (−) |
| Refractive index | nω = 1.491, nε = 1.470 |
| Birefringence | δ = 0.021 |
| Ultraviolet fluorescence | Non-fluorescent |
| Solubility | Partially soluble in water |
| Alters to | Partially dehydration on atmospheric exposure, becomes opaque |
| References | |
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