Greenalite
Greenalite is a mineral in the kaolinite-serpentine group with the chemical composition (Fe2+,Fe3+)2-3Si2O5OH4.[1][4] It is a member of the serpentine group.[2]
| Greenalite | |
|---|---|
 Greenalite (green) with galena and siderite, from Spain | |
| General | |
| Category | Phyllosilicates Kaolinite-serpentine group |
| Formula (repeating unit) | (Fe2+,Fe3+)2-3Si2O5OH4 |
| Strunz classification | 9.ED.15 |
| Crystal system | Monoclinic |
| Crystal class | Domatic (m) (same H-M symbol) |
| Space group | Cm |
| Unit cell | a = 5.54, b = 9.55 c = 7.44 [Å]; β = 104.2°; Z = 2 |
| Identification | |
| Color | Green, light yellow-green |
| Crystal habit | Rare minute crystals, rounded grains common; as porphyroblasts, oolites |
| Cleavage | None |
| Mohs scale hardness | 2.5 |
| Luster | Dull, earthy |
| Streak | Greenish-gray |
| Diaphaneity | Translucent to subopaque |
| Specific gravity | 2.85 - 3.15 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.650 - 1.675 nβ = 1.674 nγ = 1.674 |
| Birefringence | δ = 0.024 |
| Pleochroism | X = pale yellow, Y and Z = green |
| Other characteristics | Magnetic |
| References | [1][2][3] |
Occurrence
Greenalite was first described in 1903 for an occurrence in the Mesabi Range near Biwabik, St. Louis County, Minnesota and named for its green color.[2]
Greenalite occurs as a primary phase in banded iron formations. Rocks which contain greenalite are usually bright green, pale green or pale brown. Greenalite occurs with quartz, stilpnomelane, siderite, chamosite, pyrite and minnesotaite. It is commonly oolitic.[1]
References
- Handbook of Mineralogy
- Greenalite on Mindat.org
- Greenalite on Webmineral
- Sleep N.H., Bird D.K. (2007): Niches of the pre-photosynthetic biosphere and geologic preservation of Earth’s earliest ecology. Geobiology 5, 101-117
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