Greenalite
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Greenalite is a mineral in the kaolinite-serpentine group with the chemical composition (Fe²⁺, Fe³⁺)₂₋₃Si₂O₅(OH)₄. It is a member of the serpentine group. Wikipedia
Formula (repeating unit): (Fe2+,Fe3+)2-3Si2O5(OH)4
Space group: Cm