Ferrohortonolite is a mineral in the olivine series composed of 70% to 90% fayalite and 30% to 10% forsterite. Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ... Fayalite (Fe2SiO4) is the iron rich end-member of the olivine solid-solution series. ... Forsterite (Mg2SiO4) is the magnesium rich end-member of the olivine solid-solution series. ...