Greenalite

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Greenalite

Greenalite is a green mineral in the kaolinite-serpentine group with the formula (Fe2+,Fe3+)2-3Si2O5(OH)4. It usually forms as tiny green grains or rare small crystals and is commonly found in oolitic textures. The color ranges from green to pale yellow-green, and its streak is greenish-gray. It has a dull, earthy luster, no cleavage, and a hardness of about 2.5 on the Mohs scale. It is translucent to somewhat opaque.

This mineral is typically associated with banded iron formations (BIFs) and occurs with quartz, siderite, chamosite, pyrite, and minnesotaite. Greenalite was first described in 1903 from the Mesabi Range near Biwabik, Minnesota, and is named for its distinctive green color. It is common in very old (Archean) rocks.

Formation in ancient seas may have involved rapid growth of greenalite in seawater, a process that could have altered the water’s metal content by removing elements like zinc, copper, and vanadium, while leaving more manganese, molybdenum, and cadmium. These changes in early ocean chemistry are thought to have been relevant to life at the time.