The most simple type of silicates are the “nesosilicates”, those with isolated tetrahedrons SiO44- (left). Each tetrahedron can bond iron or magnesium, both bivalent, forming the minerals known as Fayalite Fe2SiO4 and Forsterite Mg2SiO4, respectively. As the Fe and Mg percentage in the crystal lattices is variable, the minerals are referred to as Olivine “solid solution”, also know as Peridot. The mineralogical formula is (Fe,Mg)2SiO4, with variable content of the two minerals.
By adding one silica molecule to Olivines we obtain the silicates known as Pyroxenes, where tetrahedrons are spatially organized in chains, so that they’re called “inosilicates” (right). Bonded cations may be various: (Fe,Mg,Ca,Na,Al)Si2O6. For example, by adding silica SiO2 to the magnesian Olivine “Forsterite” we have Mg2SiO4 + SiO2 = Mg2Si2O6, which is a Pyroxen known as Enstatite. Doing the same thing to the ferrous Olivine we would obtain Ferrosilite. In nature, we can find some calcium atoms in place of one iron or magnesium atoms. This would lead to Hedembergite or the more common Diopside. CaMgSi2O6.
Going on like that we could ideally obtain the double-chain inosilicates called “Amphiboles” (left), whose more complex formula contains the hydroxylic ion Si8O22(OH)27- obtained by adding silica and water to a Pyroxene in high temperature and pressure environments typical of metamorphism.