Greenalite

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

Greenalite
Greenalite (green) with galena and siderite, from Spain
General
CategoryPhyllosilicates
Kaolinite-serpentine group
Formula
(repeating unit)		(Fe2+,Fe3+)2-3Si2O5(OH)4
IMA symbolGre[1]
Strunz classification9.ED.15
Crystal systemMonoclinic
Crystal classDomatic (m)
(same H-M symbol)
Space groupCm
Unit cella = 5.54, b = 9.55
c = 7.44 [Å]; β = 104.2°; Z = 2
Identification
ColorGreen, light yellow-green
Crystal habitRare minute crystals, rounded grains common; as porphyroblasts, oolites
CleavageNone
Mohs scale hardness2.5
LusterDull, earthy
StreakGreenish-gray
DiaphaneityTranslucent to subopaque
Specific gravity2.85 - 3.15
Optical propertiesBiaxial (+)
Refractive indexnα = 1.650 - 1.675 nβ = 1.674 nγ = 1.674
Birefringenceδ = 0.024
PleochroismX = pale yellow, Y and Z = green
Other characteristicsMagnetic
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 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.[2]

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Handbook of Mineralogy
  3. Greenalite on Mindat.org
  4. Greenalite on Webmineral
  5. 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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