silica and silicates
/ si-LEE-kuh; SIL-i-kates /
Sand, glass, quartz crystals, granite, clay, and most of the rocks under your feet are built from silicon and oxygen. The Earth's crust is, by mass, mostly oxygen and silicon, and they are almost always found together. Understanding silica and the silicates is, quite literally, understanding what the ground is made of.
Carbon dioxide and silicon dioxide have the same formula type, yet they could not be more different, and the reason is a deep lesson of the group. CO2 is a small discrete molecule with carbon-oxygen double bonds, so it is a gas. Silicon is bigger and forms weak pi bonds, so SiO2 (silica) instead builds an infinite three-dimensional network in which every silicon sits at the center of an SiO4 tetrahedron and every oxygen bridges two silicons; that is why silica is a hard, high-melting solid (quartz). The silicates take this SiO4 tetrahedron as a universal building block and link them by sharing corner oxygens in every conceivable way: isolated tetrahedra (as in olivine), chains and double chains (pyroxenes, amphiboles like asbestos), sheets (micas, clays, talc, which is why they cleave or feel slippery), and full three-dimensional frameworks (feldspars, zeolites) where replacing some Si by Al creates aluminosilicates with charge-balancing cations in the cavities.
Silicates matter because they are the chemistry of the planet and of countless materials: glass and ceramics, cement, the clays that hold soil and make pottery, the zeolite molecular sieves used as catalysts and water softeners, and the gemstones. The contrast with CO2 is the single most important idea in group-14 oxide chemistry: a tiny difference in an element's ability to form pi bonds (carbon yes, silicon largely no) is the difference between a gas you exhale and the rock of the continents.
CO2 is a gas at room temperature; SiO2 melts above 1600 C. Same XO2 formula, but CO2 is small molecules held only by weak forces, while SiO2 is one giant covalent network of corner-sharing SiO4 tetrahedra. The whole difference is carbon's strong pi bonds versus silicon's feeble ones.
Same formula type, gas versus rock: carbon makes pi bonds and discrete molecules; silicon makes networks.
It is wrong to picture solid SiO2 as molecules of 'O=Si=O' stacked together; there are no Si=O double bonds in quartz. Every silicon is single-bonded to four bridging oxygens in a continuous network, which is exactly why silica is a solid and CO2 is a gas.