Greenalite

(repeating unit)(Fe²⁺,Fe³⁺)_2-3Si₂O₅(OH)₄IMA symbolGre^[1]Strunz classification9.ED.15Crystal systemMonoclinicCrystal classDomatic (m)
(same H-M symbol)Space groupCmUnit cella = 5.54, b = 9.55
c = 7.44 [Å]; β = 104.2°; Z = 2IdentificationColorGreen, light yellow-greenCrystal habitRare minute crystals, rounded grains common; as porphyroblasts, oolitesCleavageNoneMohs scale hardness2.5LusterDull, earthyStreakGreenish-grayDiaphaneityTranslucent to subopaqueSpecific gravity2.85 - 3.15Optical propertiesBiaxial (+)Refractive indexn_α = 1.650 - 1.675 n_β = 1.674 n_γ = 1.674Birefringenceδ = 0.024PleochroismX = pale yellow, Y and Z = greenOther characteristicsMagneticReferences^[2][3][4]

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]

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]

References