| Greenalite | |
| Category: | Phyllosilicates Kaolinite-serpentine group |
| Formula: | (Fe2+,Fe3+)2-3Si2O5(OH)4 |
| Imasymbol: | Gre[1] |
| Strunz: | 9.ED.15 |
| System: | Monoclinic |
| Class: | Domatic (m) (same H-M symbol) |
| Symmetry: | Cm |
| Unit Cell: | a = 5.54, b = 9.55 c = 7.44 [Å]; β = 104.2°; Z = 2 |
| Color: | Green, light yellow-green |
| Habit: | Rare minute crystals, rounded grains common; as porphyroblasts, oolites |
| Cleavage: | None |
| Mohs: | 2.5 |
| Luster: | Dull, earthy |
| Streak: | Greenish-gray |
| Diaphaneity: | Translucent to subopaque |
| Gravity: | 2.85 - 3.15 |
| Opticalprop: | Biaxial (+) |
| Refractive: | nα = 1.650 - 1.675 nβ = 1.674 nγ = 1.674 |
| Birefringence: | δ = 0.024 |
| Pleochroism: | X = pale yellow, Y and Z = green |
| Other: | Magnetic |
| References: | [2] [3] |
Greenalite is a mineral in the kaolinite-serpentine group with the chemical composition (Fe2+,Fe3+)2-3Si2O5(OH)4.[4] [5] It is a member of the serpentine group.[2]
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 mineral 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.[4]
Greenalite, which is common in Archean rocks, formed rapidly in Archean seawater removing zinc, copper and vanadium in the process. This left the seawater rich in manganese, molybdenum, and cadmium, which are metals favoured by lifeforms at that time. Experiments have shown that the removed metals would have been removed permanently, having a significant effect on early seawater.[6]