Magadiite
Magadiite is a hydrous sodium silicate mineral (NaSi7O13(OH)3·4(H2O)) which precipitates from alkali brines as an evaporite phase. It forms as soft (Mohs hardness of 2) white powdery monoclinic crystal masses.[2][3] The mineral is unstable and decomposes during diagenesis leaving a distinctive variety of chert (Magadi-type chert).[6]
| Magadiite | |
|---|---|
 Magadiite. Collected in 1990 from Lake Magadi, Kajiado County, Kenya. | |
| General | |
| Category | Phyllosilicate |
| Formula (repeating unit) | NaSi7O13(OH)3·4(H2O) |
| Strunz classification | 9.EA.20 |
| Crystal system | Monoclinic Unknown space group |
| Space group | C2/m (no. 12) |
| Unit cell | a = 7.22 Å, b = 15.70 Å, c = 6.91 Å; β = 97.27°; Z=1 |
| Identification | |
| Color | White |
| Crystal habit | Minute platy crystals; spherulitic aggregates; powdery |
| Tenacity | Puttylike |
| Mohs scale hardness | 2 |
| Luster | Vitreous - dull |
| Streak | White |
| Diaphaneity | Translucent to opaque |
| Specific gravity | 2.25 calculated |
| Optical properties | Biaxial |
| Refractive index | nα = 1.470 |
| Ultraviolet fluorescence | Yellow-white under both long and short wave |
| References | [1][2][3][4][5] |
The mineral was first described by Hans P. Eugster in 1967 for an occurrence in Lake Magadi, Kenya, and is also found at Olduvai Gorge, Tanzania.[3][6] It is also reported from alkalic intrusive syenites as in Mont Saint-Hilaire, Canada.[2]
References
- Mineralienatlas
- Mineral Handbook
- Webmineral
- Mindat.org
- Garcés, Juan M. (1988). "Hypothetical Structures of Magadiite and Sodium Octosilicate and Structural Relationships Between the Layered Alkali Metal Silicates and the Mordenite- and Pentasil-Group Zeolites1". Clays and Clay Minerals. 36 (5): 409–418. doi:10.1346/CCMN.1988.0360505.
- Encyclopedia of Sediments & Sedimentary Rocks, Springer, 2003, p. 417, ISBN 1-4020-0872-4
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