Here's why:
* Iron's color: Iron in its mineral form often appears dark green, brown, or black.
* Abundance in these minerals: Olivine, pyroxene, amphibole, and biotite all contain significant amounts of iron in their chemical formulas.
* Light absorption: Iron ions in these minerals absorb light in the red and yellow portions of the visible spectrum, which is why we perceive them as dark green, brown, or black.
While other elements like magnesium (Mg) are also present in these minerals, they tend to contribute more to lighter shades of green. Iron is the dominant element responsible for the darker hues.
