Greenalite

Greenalite is a mineral in the kaolinite-serpentine group^[3] with the chemical composition (Fe²⁺,Fe³⁺)_2-3Si₂O₅(OH)₄.^[2][5]

Greenalite
Greenalite (green) with galena and siderite, from Spain
General
Category	Phyllosilicates Kaolinite-serpentine group
Formula	(Fe2+,Fe3+)2-3Si2O5(OH)4
IMA symbol	Gre[1]
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	[2][3][4]

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.^[3]

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.^[2]

Effect on early life

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]