When it does not form quartz, silicon, which has 4 available bonds, binds with 4 oxygen atoms, each of which allows one bond of the two it has available to be free. The 4 oxygen atoms orient themselves around the silicon atom so that they are as far apart from each other as possible, ending up arranged according to the direction of the 4 vertices of a tetrahedron, a solid with 4 faces in the shape of an equilateral triangle. In practice, the 4 bonds thus oriented in space resemble a tripod with 4 identical arms. In jargon, however, geologists call it a “silica tetrahedron.” When one of these tetrahedra bonds with iron and/or magnesium it forms minerals known as Olivines. Adding a molecule of SiO₂ to an Olivine results in a Pyroxene, which is formed by chains of tetrahedra with metals or alkalis in the spaces. Adding SiO₂ and water again we get Amphiboles, which instead have a double chain of tetrahedra.

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The most simple type of silicates are the “nesosilicates”, those with isolated tetrahedrons SiO₄^4- (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)₂^7- obtained by adding silica and water to a Pyroxene in high temperature and pressure environments typical of metamorphism.

Nesosilicates		|—————	Inosilicates	—————|
OLIVINES	+SiO2 ->	PIROXENES	+SiO2+H2O ->	AMPHIBOLES
		(rombic)		(rombic)	Ferromagnesian
Mg2SiO4	+SiO2 ->	Mg2Si2O6	+SiO2+H2O ->	Mg7Si8O22(OH)2
Forsterite		Enstatite		Antophyllite
Fe2SiO4	+SiO2 ->	Fe2Si2O6	+SiO2+H2O ->	Fe7Si8O22(OH)2
Fayalite		Ferrosilite		Grunerite
		(monocline)		(monocline)	Calcic
		CaMgSi2O6	+SiO2+H2O ->	Ca2Mg5Si8O22(OH)2
		Diopside		Tremolite
		CaFeSi2O6	+SiO2+H2O ->	Ca2Fe5Si8O22(OH)2
		Hedembergite		Actinote
					Sodic
		Na,(Al,Fe)Si2O6	+SiO2+H2O ->	Na2(Mg3Al2)Si8O22(OH)2
		Jadeite		Glaucophane
		Na,FeSi2O6		Na2Fe2+3Fe3+2Si8O22(OH)2
		Aegirine		Riebeckite
					Lithium-aluminic
		LiAlSi2O6
		Spodumene
		(complex)		(complex)
		(Ca,Na)(Mg,Fe2+,Al)Si2O6	+SiO2+H2O ->	NaCa2(Mg,Fe)5Si8O22(OH)2
		Omphacite (diopside+jadeite)		Horneblenda (simplified)
		(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
		Augite