Describing rocks or minerals that are rich in iron and magnesium, typically dark in color and dense, like basalt and olivine.
From 'ferro-' (iron) + 'magnesian' (containing magnesium, from the element magnesium). The term was established in geology in the late 1800s to classify rock types by their mineral composition.
Ferromagnesian rocks make up most of Earth's mantle beneath the crust—they're the building blocks of our planet, and basalt, the dark rock from volcanic eruptions, is a classic ferromagnesian rock!
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