43
Silicon [Si, 14]
43.1 INTRODUCTION
Silicon (Si) is a chemical element of group 14 in the periodic table of elements and is
classified as a metalloid or a nonmetal element. It is the eighth most common element
in the universe, by mass, but very rarely occurs as the pure element in nature. It is the
second most abundant element in the Earth’s crust (about 28% by mass). It is also the
most stable element. However, under specific conditions, its compounds can be dissolved and transported, mainly in colloidal phase. Its content in magmatic rocks is
up to about 35%, more in acid than in alkaline rocks. In sedimentary rocks, especially
in sandstones, its content is over 40%, whereas in calcareous rocks, it is below 30%.
Its content in coal may be up to about 30%. Thus, Si should not be included in the
group of trace elements. However, it is considered as a trace element, with respect to
its biochemical functions.
The oxidation state of Si is +4, but sometimes it may vary from −4 to +2. Its
most common mineral, quartz (SiO 2 ), is also the most common compound in all
terrestrial compartments. It is also the main component of various silicate minerals.
Nonsilicate mineral containing Si is silicon carbide (carborundum), SiC.
Global Si production (rounded) in 2010 was 6900 kt, of which 4600 kt was produced in China, 610 kt in Russia, 330 kt in Norway, and 240 kt in Canada (USGS
2011). Demand for Si comes primarily from the aluminum and chemical industries.
It is used in several different sectors of manufactures of steel and aluminum alloys,
glass and ceramic productions, and various refractory materials. Due to semiconductor properties, Si is broadly used in various electronic devices. It is also applied
in productions of cement, papers, textiles, cosmetics, and pharmaceutics.
43.2 SOILS
Silicon is the most abundant element in soils, averaging about 55%. Its mean contents
in various soil groups differ and vary from 43% to 63% (Table 43.1).
Quartz (SiO 2 ) is the most resistant mineral in soils. It also occurs in a noncrystalline form, opal, which is presumably of a biological origin. Amorphous silicates
contribute to anion adsorption processes and compete with other anions, for example, phosphates, for sites on mineral soil particles. In acidic soils, silicate and phosphate ions form insoluble precipitates that may fix other cations, for example, Fe and
Al. Increased amounts of soluble organic matter in flooded soils induce a higher
Si mobility, apparently due to the reduction of Fe hydrous oxides, which release
adsorbed monosilicic acid.
A part of Si is released from minerals into the soil solution, and this process is
controlled by both soil parameters and climatic factors. Soil pH has an especially
marked impact on Si contents of soil solution, although the mobility of Si in soils
295
Silicon [Si, 14]
43.1 INTRODUCTION
Silicon (Si) is a chemical element of group 14 in the periodic table of elements and is
classified as a metalloid or a nonmetal element. It is the eighth most common element
in the universe, by mass, but very rarely occurs as the pure element in nature. It is the
second most abundant element in the Earth’s crust (about 28% by mass). It is also the
most stable element. However, under specific conditions, its compounds can be dissolved and transported, mainly in colloidal phase. Its content in magmatic rocks is
up to about 35%, more in acid than in alkaline rocks. In sedimentary rocks, especially
in sandstones, its content is over 40%, whereas in calcareous rocks, it is below 30%.
Its content in coal may be up to about 30%. Thus, Si should not be included in the
group of trace elements. However, it is considered as a trace element, with respect to
its biochemical functions.
The oxidation state of Si is +4, but sometimes it may vary from −4 to +2. Its
most common mineral, quartz (SiO 2 ), is also the most common compound in all
terrestrial compartments. It is also the main component of various silicate minerals.
Nonsilicate mineral containing Si is silicon carbide (carborundum), SiC.
Global Si production (rounded) in 2010 was 6900 kt, of which 4600 kt was produced in China, 610 kt in Russia, 330 kt in Norway, and 240 kt in Canada (USGS
2011). Demand for Si comes primarily from the aluminum and chemical industries.
It is used in several different sectors of manufactures of steel and aluminum alloys,
glass and ceramic productions, and various refractory materials. Due to semiconductor properties, Si is broadly used in various electronic devices. It is also applied
in productions of cement, papers, textiles, cosmetics, and pharmaceutics.
43.2 SOILS
Silicon is the most abundant element in soils, averaging about 55%. Its mean contents
in various soil groups differ and vary from 43% to 63% (Table 43.1).
Quartz (SiO 2 ) is the most resistant mineral in soils. It also occurs in a noncrystalline form, opal, which is presumably of a biological origin. Amorphous silicates
contribute to anion adsorption processes and compete with other anions, for example, phosphates, for sites on mineral soil particles. In acidic soils, silicate and phosphate ions form insoluble precipitates that may fix other cations, for example, Fe and
Al. Increased amounts of soluble organic matter in flooded soils induce a higher
Si mobility, apparently due to the reduction of Fe hydrous oxides, which release
adsorbed monosilicic acid.
A part of Si is released from minerals into the soil solution, and this process is
controlled by both soil parameters and climatic factors. Soil pH has an especially
marked impact on Si contents of soil solution, although the mobility of Si in soils
295