Fayalite
Fayalite is the iron-rich form of olivine, darker and denser than its magnesium cousin.
Fayalite is the iron end member of the olivine series, with the formula Fe2SiO4. It is darker, heavier and melts at a lower temperature than forsterite, the magnesium end. Most natural olivine sits somewhere between the two.
In meteorites, nearly pure fayalite is rare and interesting. It forms when iron-rich fluids react with rock at fairly low temperatures, so its presence in some CV and CO chondrites is evidence that water once flowed through their parent asteroids. Because fayalite can take up manganese, including the short-lived isotope manganese-53, it has been used to date that watery episode to within a few million years of the Solar System's birth.
The term also gives its name to the fayalite content, or Fa value, which measures how much iron is in the olivine of a chondrite. That single number is one of the quickest ways to sort ordinary chondrites into the H, L and LL groups.
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