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”) |