Greenalite

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Greenalite
General
Category Phyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)
(Fe2+,Fe3+)2-3Si2O5OH4
Strunz classification 09.ED.15
Unit cell a = 5.54 Å, b = 9.55 Å, 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
Crystal system Monoclinic
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]

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]

Occurrence[edit]

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[edit]

  1. ^ a b c Handbook of Mineralogy
  2. ^ a b c Greenalite on Mindat.org
  3. ^ Greenalite on Webmineral
  4. ^ 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