The compound silica (SiO2) is formed from silicon and oxygen atoms. Because oxygen is the most abundant element in the Earth's crust and silicon is the second most abundant, the formation of silica is quite common in nature.

]The silica sand, just mentioned as the substance used to derive pure silicon, is made of quartz, which is the most common form of silica found in nature.

Silica can also be biological in origin, produced by tiny organisms. The most significant of these are diatoms (plants) and radiolarians (animals), both of which extract silica from the water around them to form their structures or shells. For both organisms, silica is a nutrient they must have to survive. In nature, they use the dissolved silica that originates from sedimentary rocks at the bottom of a lake, river, or ocean. When diatoms grown in the laboratory extract all the available silica from the aquarium water, they attach themselves to the walls of the aquarium and use the small amounts of dissolved silica etched from the glass itself. In nature, when diatoms and radiolarians die, they sink to the bottom of the water and accumulate into sediment, which can become hardened into diatomite and radiolarite. Diatomite is a commercially useful rock. It's highly porous and, thus, is effective for filtering as well as for use as filler and as a mild abrasive. Thus, silica can be found in more than one state—amorphous as in the remains from a diatom and crystalline as in a quartz crystal, as we shall explain later. Both are SiO2 , but they are quite different physically. What's more, silica in its crystalline state is found in more than one form. This phenomenon is called polymorphism (literally “many forms”)



    •  Forms
    •  Natural Occurrence