Carbonate - Silicate Cycle

The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism.^[1][2] Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxide levels and therefore global temperature.^[2]

However the rate of weathering is sensitive to factors that modulate how much land is exposed. These factors include sea level, topography, lithology, and vegetation changes.^[3] Furthermore, these geomorphic and chemical changes have worked in tandem with solar forcing, whether due to orbital changes or stellar evolution, to determine the global surface temperature. Additionally, the carbonate-silicate cycle has been considered a possible solution to the Faint young Sun paradox.^[1][2]

Contents

• 1Overview of the cycle
  • 1.1Physical and Chemical Processes
  • 1.2Feedbacks
• 2Changes through Earth history
• 3The cycle on other planets
• 4See also
• 5References
• 6External links

Overview of the cycle[edit]

This schematic shows the relationship between the different physical and chemical processes that make up the carbonate-silicate cycle.

The carbonate-silicate cycle is the primary control on carbon dioxide levels over long timescales.^[2] It can be seen a branch of the carbon cycle, which also includes the organic carbon cycle, in which biological processes convert carbon dioxide and water into organic matter and oxygen via photosynthesis.^[4]

Physical and Chemical Processes[edit]

The inorganic cycle begins with the production of carbonic acid (H₂CO₃) from rainwater and gaseous carbon dioxide.^[5] Carbonic acid is a weak acid, but over long timescales, it can dissolve silicate rocks (as well as carbonate rocks). Most of the Earth's crust (and mantle) is composed of silicates.^[6] These substances break down into dissolved ions as a result. For example, calcium silicate CaSiO₃, or wollastonite, reacts with carbon dioxide and water to yield a calcium ion, Ca²⁺, a bicarbonate ion, HCO₃^-, and dissolved silica. This reaction structure is representative of general silicate weathering of calcium silicate minerals.^[7] The chemical pathway is as follows:

${\displaystyle 2CO_{2}+H_{2}O+CaSiO_{3}\rightarrow Ca^{2+}+2HCO_{3}^{-}+SiO_{2}}$

River runoff carries these products to the ocean, where marine calcifying organisms use Ca²⁺ and HCO₃^-to build their shells and skeletons, a process called carbonate precipitation:

${\displaystyle Ca^{2+}+2HCO_{3}^{-}\rightarrow CaCO_{3}+CO_{2}+H_{2}O}$

Two molecules of CO₂ are required for silicate rock weathering; marine calcification releases one molecule back to the atmosphere. The calcium carbonate (CaCO₃) contained in shells and skeletons sinks after the marine organism dies and is deposited on the ocean floor.

The final stage of the process involves the movement of the seafloor. At subduction zones, the carbonate sediments are buried and forced back into the mantle. Some carbonate may be carried deep into the mantle where high pressure and temperature conditions allow it to combine metamorphically with SiO₂ to form CaSiO₃ and CO₂, which is released from the interior into the atmosphere via volcanism, thermal vents in the ocean, or soda springs, which are natural springs that contain carbon dioxide gas or soda water:

${\displaystyle CaCO_{3}+SiO_{2}\rightarrow CaSiO_{3}+CO_{2}}$

This final step returns the second CO₂ molecule to the atmosphere and closes the inorganic carbon budget. 99.6% of all carbon (equating to roughly 10⁸ billion tons of carbon) on Earth is sequestered in the longterm rock reservoir. And essentially all carbon has spent time in the form of carbonate. By contrast, only 0.002% of carbon exists in the biosphere.^[6]

Feedbacks[edit]

Changes to the surface of the planet, such as an absence of volcanoes or higher sea levels, which would reduce the amount of land surface exposed to weathering can change the rates at which different processes in this cycle take place.^[6] Over tens to hundreds of millions of years, carbon dioxide levels in the atmosphere may vary due to natural perturbations in the cycle^[8][9][10] but even more generally, it serves as a critical negative feedback loop between carbon dioxide levels and climate changes.^[5][7] For example, if CO₂ builds up in the atmosphere, the greenhouse effect will serve to increase the surface temperature, which will in turn increase the rate of rainfall and silicate weathering, which will remove carbon from the atmosphere. In this way, over long timescales, the carbonate-silicate cycle has a stabilizing effect on the Earth's climate, which is why it has been called the Earth's thermostat.^[4][11]

Changes through Earth history[edit]

Microscopic shells found in sediment cores may be used to determine past climate conditions including ocean temperatures and aspects of atmospheric chemistry.

Aspects of the carbonate-silicate cycle have changed through Earth history as a result of biological evolution and tectonic changes. Generally, the formation of carbonates has outpaced that of silicates, effectively removing carbon dioxide from the atmosphere. The advent of carbonate biomineralization near the Precambrian-Cambrian boundary would have allowed more efficient removal of weathering products from the ocean.^[12] Biological processes in soils can significantly increase weathering rates.^[13] Plants produce organic acids that increase weathering. These acids are secreted by root and mycorrhizal fungi, as well as microbial plant decay. Root respiration and oxidation of organic soil matter also produce carbon dioxide, which is converted to carbonic acid, which increases weathering.^[14]

Tectonics can induce changes in the carbonate-silicate cycle. For example, the uplift of major mountain ranges, such as Himalayas and the Andes, is thought to have initiated the Late Cenozoic Ice Age due to increased rates of silicate weathering and draw down of carbon dioxide.^[15] Seafloor weather is linked both to solar luminosity and carbon dioxide concentration.^[16] However, it presented a challenge to modelers who have tried to relate the rate of outgassing and subduction to the related rates of seafloor change. Proper, uncomplicated proxy data is difficult to attain for such questions. For example, sediment cores, from which scientists can deduce past sea levels, are not ideal because sea levels change as a result of more than just seafloor adjustment.^[17] Recent modeling studies have investigated the role of seafloor weathering on the early evolution of life, showing that relatively fast seafloor creation rates worked to drawdown carbon dioxide levels to a moderate extent.^[18]

Observations of so-called deep time indicate that Earth has a relatively insensitive rock weathering feedback, allowing for large temperature swings. With about twice as much carbon dioxide in the atmosphere, paleoclimate records show that global temperatures reached up to 5 to 6 °C higher than current temperatures.^[19] However, other factors such as changes in orbital/solar forcing contribute to global temperature change in the paleo-record.

Human emissions of CO₂ have been steadily increasing, and the consequent concentration of CO₂ in the Earth system has reached unprecedented levels in a very short amount of time.^[20] Excess carbon in the atmosphere that is dissolved in seawater can alter the rates of carbonate-silicate cycle. Dissolved CO₂ may react with water to form bicarbonate ions, HCO₃^-, and hydrogen ions, H⁺. These hydrogen ions quickly react with carbonate, CO₃^2- to produce more bicarbonate ions and reduce the available carbonate ions, which presents an obstacle to the carbon carbonate precipitation process.^[21] Put differently, 30% of excess carbon emitted into the atmosphere is absorbed by the oceans. Higher concentrations of carbon dioxide in the oceans work to push the carbonate precipitation process in the opposite direction (to the left), producing less CaCO₃. This process, which harms shell-building organisms, is called ocean acidification.^[22]

The cycle on other planets[edit]

One should not assume that a carbonate-silicate cycle would appear on all terrestrial planets. To begin, the carbonate-silicate cycle requires the presence of a water cycle. It therefore breaks down at the inner edge of the Solar System's habitable zone. Even if a planet starts out with liquid water on the surface, if it becomes too warm, it will undergo a runaway greenhouse, losing surface water. Without the requisite rainwater, no weathering will occur to produce carbonic acid from gaseous CO₂. Furthermore, at the outer edge, CO₂ may condense, consequently reducing the greenhouse effect and reducing the surface temperature. As a result, the atmosphere would collapse into polar caps.^[4]

Mars is such a planet. Located at the edge of the solar system's habitable zone, its surface is too cold for liquid water to form without a greenhouse effect. With its thin atmosphere, Mars' mean surface temperature is 210 K (−63 °C). In attempting to explain topographical features resembling fluvial channels, despite seemingly insufficient incoming solar radiation, some have suggested that a cycle similar to Earth's carbonate-silicate cycle could have existed – similar to a retreat from Snowball Earth periods.^[23] It has been shown using modeling studies that gaseous CO₂ and H₂O acting as greenhouse gases could not have kept Mars warm during its early history when the sun was fainter because CO₂ would condense out into clouds.^[24] Even though CO₂ clouds do not reflect in the same way that water clouds do on Earth,^[25] which means it could not have had much of a carbonate-silicate cycle in the past.

By contrast, Venus is located at the inner edge of the habitable zone and has a mean surface temperature of 737 K (464 °C). After losing its water by photodissociation and hydrogen escape, Venus stopped removing carbon dioxide from its atmosphere, and began instead to build it up, and experience a runaway greenhouse effect.

On exoplanets, the location of the substellar point will dictate the release of carbon dioxide from the lithosphere.^[26]

 The carbonate-silicate cycle, which plays a key role in stabilizing Earth's climate over long time scales, is shown in Fig. 2. The cycle begins when atmospheric CO₂ dissolves in rainwater, forming carbonic acid, H₂CO₃. Through a process termed "weathering", this weak acid dissolves silicate rocks on the continents, releasing Ca⁺⁺, Mg⁺⁺, HCO₃^- (bicarbonate), and SiO₂ (dissolved silica) into solution. The products of weathering make their way down to the oceans in streams and rivers. There, organisms such as the planktonic foraminifera that live in the surface ocean use them to make shells out of calcium carbonate (CaCO₃). When the organisms die, they fall down into the deep ocean, where most of the shells redissolve. Some of the calcium carbonate survives, however, and is buried in sediments on the seafloor. The seafloor spreads from the midocean ridges and, at some plate margins, is carried down subduction zones. The carbonate minerals recombine with SiO₂, which by this time is the mineral quartz, to reform calcium and magnesium silicates and release gaseous CO₂. This CO₂ is vented into the atmosphere through volcanoes, thereby completing the cycle.

Fig. 2 Diagram illustrating the carbonate-silicate cycle. The term "metamorphosis" should read "metamorphism." (From J. F. Kasting, Science Spectra, 1995, Issue 2, p. 32-36. Adapted from J. F Kasting, 1993.)

            The stabilizing negative feedback in the carbonate-silicate cycle is produced by the dependence of the silicate weathering rate on temperature. When surface temperatures drop, the weathering rate slows down, and CO₂ accumulates in the planet's atmosphere. Thus, an Earth-sized planet that had such a cycle would be expected to build up a dense CO₂ and a large greenhouse effect if its surface temperature became too low. This suggests that the outer edge of the HZ is relatively far out, perhaps beyond the orbit of Mars (Mischna and Kasting, 2000). It also explains how our own planet escaped from Snowball Earth episodes in the past (Caldeira and Kasting, 1992).

            NASA has plans to explore nearby stars and look for both habitable planets and life. The Terrestrial Planet Finder (TPF) Mission, which is currently under development by NASA, will be a space-based telescope that will look for Earth-sized planets and take spectra of their atmospheres. This mission could be done at either visible or thermal-infrared (IR) wavelengths, or both. In the thermal IR, it would be necessary to do interferometery, i.e., to combine beams from multiple telescopes. One idea is to put these telescopes on separate, free-flying spacecraft (Fig. 3). In the visible/near-IR, one could get by with a single, 3´6-meter mirror equipped with a coronagraph. Both missions are currently being studied. The IR interferometer may eventually be done as a joint mission with the European Space Agency (ESA), where the project goes under the name of Darwin.

Fig. 3 Artist's conception of a free-flying infrared interferometer version of TPF. ESA's Darwin mission is based on the same design. Beams from four separated spacecraft are combined at a fifth spacecraft in order to achieve high spatial resolution. The red lines between the spacecraft represent lasers that are used to accurately measure their positions.

The carbonate-silicate cycle. ( A ) One principal mechanism by which temperatures on the surface of Earth are regulated through the feedback control of the greenhouse gas, CO 2 . The cycle also illustrates the link between plate tectonics (subduction of carbonates) and habitability. ( B ) The carbonate-silicate cycle works by a negative feedback process. 

There are three forms of weathering, constituting physical, chemical and biological processes. Though weathering can be confused with erosion, there are subtle differences. Erosion occurs with the breakdown, transportation and deposition of material, while weathering alters or disintegrates material at its original position. Silicate weathering can help shape the Earth's surface, regulate global and chemical cycles and even determine nutrient supply to ecosystems.

Identification

If you go outside and pick up a rock in your backyard, chances are you are holding a rock that contains silicate minerals. Silicates make up approximately 95 percent of the Earth's crust and mantle and are a major component of igneous rocks--crystalline or glassy rocks formed by the cooling and solidification of magma. Minerals with this combination of silicon and oxygen are also found, though less abundant, in sedimentary rocks (formed by other rock fragments and cemented together) and metamorphic rocks (formed by the heating and pressurization of existing rock).

Makeup

The prime makeup for all silicate minerals is the silicon-oxygen tetrahedron--a solid bounded by polygons with four faces. The composition includes a central silicon cation bonded to four oxygen atoms that are located at the corners of a regular tetrahedron. Approximately 25 percent of all known minerals and 40 percent of the most common ones are silicates. The bonds that tie silicon and oxygen are developed by oppositely charged ions and shared electrons.

The Earth's surface is shaped via weathering, from either physical, chemical or biological factors. These factors can act separately or as a combined force. Physical weathering causes the disintegration of rock material without the presence of decay. Thermal expansion--the alternating process of freezing and thawing as evident in the northern part of the United States and most of Canada--is the primary source for physical weathering. Chemical weathering occurs when the mineral composition of a rock is altered.

The Big Picture

According to Sigurdur R. Gislason, Institute of Earth Sciences (Iceland) and Eric H. Oelkers, Géochimie et Biogéochimie Experimentale (France), "silicate weathering (chemical weathering) is thought to control climate by consuming atmospheric carbon dioxide (CO2)" over a geological time scale. The CO2 is eventually stored as carbonates in the ocean. One third of silicate weathering is the result of weathering on volcanic islands and continents. The atmospheric CO2 consumption flux is due largely in part of the high weathering rate of basalt. For each increase of one degree in temperature, chemical weathering rates increase by approximately 10 percent. But most silicates dissolve inconsistently with weathering as they are attached with other minerals such as clays. These suspended silicates carried to the oceans are highly reactive in ocean waters and thereby dependent on climate.

Impact

•••

Of the rocks exposed at the Earth's surface, approximately 90 percent constitute silicates. Approximately a quarter of that rock is intrusive--for example, granite--a quarter is extrusive--volcanic--and the other half is metamorphic and "Precambrian"--a period of time that extends from about 4 billion years ago (the approximate age of the oldest known rocks) to 542 millions years ago. Being of silicate makeup, volcanic rock weathers the quickest. But it will take over 1 million years for silicate weathering to stabilize atmospheric CO2, even though silicate weathering accelerates CO2 removal. Given this timescale--vegetation suppression and rates of weathering--the CO2 levels will return to above those of pre-industrial times.

Carbonate–silicate cycle

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This figure describes the geological aspects and processes of the carbonate silicate cycle, within the long-term carbon cycle.

The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism.^[1][2] Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxidelevels and therefore global temperature.^[2]

However the rate of weathering is sensitive to factors that modulate how much land is exposed. These factors include sea level, topography, lithology, and vegetation changes.^[3] Furthermore, these geomorphic and chemical changes have worked in tandem with solar forcing, whether due to orbital changes or stellar evolution, to determine the global surface temperature. Additionally, the carbonate-silicate cycle has been considered a possible solution to the Faint young Sun paradox.^[1][2]

Contents

Overview of the cycleEdit

This schematic shows the relationship between the different physical and chemical processes that make up the carbonate-silicate cycle.

The carbonate-silicate cycle is the primary control on carbon dioxide levels over long timescales.^[2] It can be seen a branch of the carbon cycle, which also includes the organic carbon cycle, in which biological processes convert carbon dioxide and water into organic matter and oxygen via photosynthesis.^[4]

Physical and Chemical ProcessesEdit

The inorganic cycle begins with the production of carbonic acid (H₂CO₃) from rainwater and gaseous carbon dioxide.^[5] Carbonic acid is a weak acid, but over long timescales, it can dissolve silicate rocks (as well as carbonate rocks). Most of the Earth's crust (and mantle) is composed of silicates.^[6]These substances break down into dissolved ions as a result. For example, calcium silicate CaSiO₃, or wollastonite, reacts with carbon dioxide and water to yield a calcium ion, Ca²⁺, a bicarbonate ion, HCO₃^-, and dissolved silica. This reaction structure is representative of general silicate weathering of calcium silicate minerals.^[7] The chemical pathway is as follows:

${\displaystyle 2CO_{2}+H_{2}O+CaSiO_{3}\rightarrow Ca^{2+}+2HCO_{3}^{-}+SiO_{2}}$

River runoff carries these products to the ocean, where marine calcifying organisms use Ca²⁺ and HCO₃^- to build their shells and skeletons, a process called carbonate precipitation:

${\displaystyle Ca^{2+}+2HCO_{3}^{-}\rightarrow CaCO_{3}+CO_{2}+H_{2}O}$

Two molecules of CO₂ are required for silicate rock weathering; marine calcification releases one molecule back to the atmosphere. The calcium carbonate (CaCO₃) contained in shells and skeletons sinks after the marine organism dies and is deposited on the ocean floor.

The final stage of the process involves the movement of the seafloor. At subduction zones, the carbonate sediments are buried and forced back into the mantle. Some carbonate may be carried deep into the mantle where high pressure and temperature conditions allow it to combine metamorphically with SiO₂ to form CaSiO₃ and CO₂, which is released from the interior into the atmosphere via volcanism, thermal vents in the ocean, or soda springs, which are natural springs that contain carbon dioxide gas or soda water:

${\displaystyle CaCO_{3}+SiO_{2}\rightarrow CaSiO_{3}+CO_{2}}$

This final step returns the second CO₂ molecule to the atmosphere and closes the inorganic carbon budget. 99.6% of all carbon (equating to roughly 10⁸ billion tons of carbon) on Earth is sequestered in the longterm rock reservoir. And essentially all carbon has spent time in the form of carbonate. By contrast, only 0.002% of carbon exists in the biosphere.^[6]

FeedbacksEdit

Changes to the surface of the planet, such as an absence of volcanoes or higher sea levels, which would reduce the amount of land surface exposed to weathering can change the rates at which different processes in this cycle take place.^[6] Over tens to hundreds of millions of years, carbon dioxide levels in the atmosphere may vary due to natural perturbations in the cycle^[8][9][10] but even more generally, it serves as a critical negative feedback loop between carbon dioxide levels and climate changes.^[5][7] For example, if CO₂ builds up in the atmosphere, the greenhouse effect will serve to increase the surface temperature, which will in turn increase the rate of rainfall and silicate weathering, which will remove carbon from the atmosphere. In this way, over long timescales, the carbonate-silicate cycle has a stabilizing effect on the Earth's climate, which is why it has been called the Earth's thermostat.^[4][11]

Changes through Earth historyEdit

Microscopic shells found in sediment cores may be used to determine past climate conditions including ocean temperatures and aspects of atmospheric chemistry.

Aspects of the carbonate-silicate cycle have changed through Earth history as a result of biological evolution and tectonic changes. Generally, the formation of carbonates has outpaced that of silicates, effectively removing carbon dioxide from the atmosphere. The advent of carbonate biomineralization near the Precambrian-Cambrian boundary would have allowed more efficient removal of weathering products from the ocean.^[12] Biological processes in soils can significantly increase weathering rates.^[13] Plants produce organic acids that increase weathering. These acids are secreted by root and mycorrhizal fungi, as well as microbial plant decay. Root respiration and oxidation of organic soil matter also produce carbon dioxide, which is converted to carbonic acid, which increases weathering.^[14]

Tectonics can induce changes in the carbonate-silicate cycle. For example, the uplift of major mountain ranges, such as Himalayas and the Andes, is thought to have initiated the Late Cenozoic Ice Age due to increased rates of silicate weathering and draw down of carbon dioxide.^[15] Seafloor weather is linked both to solar luminosity and carbon dioxide concentration.^[16] However, it presented a challenge to modelers who have tried to relate the rate of outgassing and subduction to the related rates of seafloor change. Proper, uncomplicated proxy data is difficult to attain for such questions. For example, sediment cores, from which scientists can deduce past sea levels, are not ideal because sea levels change as a result of more than just seafloor adjustment.^[17]Recent modeling studies have investigated the role of seafloor weathering on the early evolution of life, showing that relatively fast seafloor creation rates worked to drawdown carbon dioxide levels to a moderate extent.^[18]

Observations of so-called deep time indicate that Earth has a relatively insensitive rock weathering feedback, allowing for large temperature swings. With about twice as much carbon dioxide in the atmosphere, paleoclimate records show that global temperatures reached up to 5 to 6 °C higher than current temperatures.^[19] However, other factors such as changes in orbital/solar forcing contribute to global temperature change in the paleo-record.

Human emissions of CO₂ have been steadily increasing, and the consequent concentration of CO₂ in the Earth system has reached unprecedented levels in a very short amount of time.^[20] Excess carbon in the atmosphere that is dissolved in seawater can alter the rates of carbonate-silicate cycle. Dissolved CO₂ may react with water to form bicarbonate ions, HCO₃^-, and hydrogen ions, H⁺. These hydrogen ions quickly react with carbonate, CO₃^2- to produce more bicarbonate ions and reduce the available carbonate ions, which presents an obstacle to the carbon carbonate precipitation process.^[21] Put differently, 30% of excess carbon emitted into the atmosphere is absorbed by the oceans. Higher concentrations of carbon dioxide in the oceans work to push the carbonate precipitation process in the opposite direction (to the left), producing less CaCO₃. This process, which harms shell-building organisms, is called ocean acidification.^[22]

The cycle on other planetsEdit

One should not assume that a carbonate-silicate cycle would appear on all terrestrial planets. To begin, the carbonate-silicate cycle requires the presence of a water cycle. It therefore breaks down at the inner edge of the Solar System's habitable zone. Even if a planet starts out with liquid water on the surface, if it becomes too warm, it will undergo a runaway greenhouse, losing surface water. Without the requisite rainwater, no weathering will occur to produce carbonic acid from gaseous CO₂. Furthermore, at the outer edge, CO₂ may condense, consequently reducing the greenhouse effect and reducing the surface temperature. As a result, the atmosphere would collapse into polar caps.^[4]

Mars is such a planet. Located at the edge of the solar system's habitable zone, its surface is too cold for liquid water to form without a greenhouse effect. With its thin atmosphere, Mars' mean surface temperature is 210 K (−63 °C). In attempting to explain topographical features resembling fluvial channels, despite seemingly insufficient incoming solar radiation, some have suggested that a cycle similar to Earth's carbonate-silicate cycle could have existed – similar to a retreat from Snowball Earth periods.^[23] It has been shown using modeling studies that gaseous CO₂ and H₂O acting as greenhouse gases could not have kept Mars warm during its early history when the sun was fainter because CO₂ would condense out into clouds.^[24] Even though CO₂ clouds do not reflect in the same way that water clouds do on Earth,^[25] which means it could not have had much of a carbonate-silicate cycle in the past.

By contrast, Venus is located at the inner edge of the habitable zone and has a mean surface temperature of 737 K (464 °C). After losing its water by photodissociation and hydrogen escape, Venus stopped removing carbon dioxide from its atmosphere, and began instead to build it up, and experience a runaway greenhouse effect.

On exoplanets, the location of the substellar point will dictate the release of carbon dioxide from the lithosphere.^[26]

Sodium silicate

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"E550" redirects here. For the Italian locomotive, see FS Class E550.

Learn more It has been suggested that this article be merged with Sodium metasilicate. (Discuss)

Sodium silicate is a generic name for chemical compounds with the formula Na
2xSiO
2+x or (Na
2O)
x·SiO
2, such as sodium metasilicateNa
2SiO
3, sodium orthosilicate Na
4SiO
4, and sodium pyrosilicate Na
6Si
2O
7. The anions are often polymeric. These compounds are generally colorless transparent solids or white powders, and soluble in water in various amounts.

Sodium silicate is also the technical and common name for a mixture of such compounds, chiefly the metasilicate, also called waterglass, water glass, or liquid glass. The product has a wide variety of uses, including the formulation of cements, passive fire protection, textile and lumber processing, manufacture of refractory ceramics, as adhesives, and in the production of silica gel. The commercial product, available in water solution or in solid form, is often greenish or blue owing to the presence of iron-containing impurities.

In industry, the various grades of sodium silicate are characterized by their SiO₂:Na₂O weight ratio (which can be converted to molar ratio by multiplication with 1.032). The ratio can vary between 2:1 and 3.75:1.^[1] Grades with ratio below 2.85:1 are termed alkaline. Those with a higher SiO₂:Na₂O ratio are described as neutral.

Contents

HistoryEdit

Soluble silicates of alkali metals (sodium or potassium) were observed by European alchemists already in the 1500s. Giambattista della Portaobserved in 1567 that tartari salis (cream of tartar, potassium hydrogen tartrate) caused powdered crystallum (quartz) to melt at a lower temperature.^[2] Other possible early references to alkali silicates were made by Basil Valentine in 1520,^[3] and by Agricola in 1550. Around 1640, Jean Baptist van Helmont reported the formation of alkali silicates as a soluble substance made by melting sand with excess alkali, and observed that the silica could be precipitated quantitatively by adding acid to the solution.^[4]

In 1646, Glauber made potassium silicate, that he termed liquor silicum by melting potassium carbonate (obtained by calcinating cream of tartar) and sand in a crucible, and keeping it molten until it ceased to bubble (due to the release of carbon dioxide). The mixture was allowed to cool and then was ground to a fine powder. When the powder was exposed to moist air, it gradually formed a viscous liquid, which Glauber called "Oleum oder Liquor Silicum, Arenæ, vel Crystallorum" (i.e., oil or solution of silica, sand or quartz crystal).^[5][6]

However, it was later claimed that the substances prepared by those alchemists were not waterglass as it is understood today.^[7][8][9] That would have been prepared in 1818 by Johann Nepomuk von Fuchs, by treating silicic acid with an alkali; the result being soluble in water, "but not affected by atmospheric changes".^[10] [11]

The terms "water glass" and "soluble glass" were used by Leopold Wolff in 1846,^[12], by Émile Kopp in 1857,^[13] and by Hermann Krätzer in 1887.^[14]

In 1892, Rudolf Von Wagner distinguished soda, potash, double (soda and potash), and fixing (i.e., stabilizing) as types of water glass. The fixing type was "a mixture of silica well saturated with potash water glass and a sodium silicate" used to stabilize inorganic water color pigments on cement work for outdoor signs and murals.^{[15][16][17] [18]}

PropertiesEdit

Sodium silicates are colorless glassy or crystalline solids, or white powders. Except for the most silicon-rich ones, they are readily soluble in water, producing alkaline solutions.

Sodium silicates are stable in neutral and alkaline solutions. In acidic solutions, the silicate ions react with hydrogen ions to form silicic acids, which tend to decompose into hydrated silicon^[19] dioxide gel. Heated to drive off the water, the result is a hard translucent substance called silica gel, widely used as a desiccant. It can withstand temperatures up to 1100°C^[20]

ProductionEdit

Solutions of sodium silicates can be produced by treating a mixture of silica (usually as quartz sand), caustic soda, and water, with hot steam in a reactor. The overall reaction is

2x NaOH + SiO
2 → (Na
2O)
x·SiO
2 + x H
2O

Sodium silicates can also be obtained by dissolving silica SiO
2 (whose melting point is 1713 °C) in molten sodium carbonate (that melts with decomposition at 851 °C):^[21]

x Na
2CO
3 + SiO
2 → (Na
2O)
x·SiO
2 + CO
2

The material can be obtained also from sodium sulfate (melting point 884 °C) with carbon as a reducing agent:

2x Na
2SO
4 + C + 2 SiO
2 → 2 (Na
2O)
x·SiO
2 + 2 SO
2 + CO
2

In 1990, 4 million tons of alkali metal silicates were produced.^[1]

UsesEdit

The main applications of sodium silicates are in detergents, paper, water treatment, and construction materials.^[1]

EngineeringEdit

AdhesiveEdit

The largest application of sodium silicate solutions is a cement for producing cardboard.^[1] When used as a paper cement, the tendency is for the sodium silicate joint eventually to crack within a few years, at which point it no longer holds the paper surfaces cemented together.

Drilling fluidsEdit

Sodium silicate is frequently used in drilling fluids to stabilize borehole walls and to avoid the collapse of bore walls. It is particularly useful when drill holes pass through argillaceous formations containing swelling clay minerals such as smectite or montmorillonite.

Concrete and general masonry treatmentEdit

Concrete treated with a sodium silicate solution helps to reduce porosity in most masonry products such as concrete, stucco, and plasters. This effect aids in reducing water penetration, but has no known effect on reducing water vapor transmission and emission^[22]. A chemical reaction occurs with the excess Ca(OH)₂ (portlandite) present in the concrete that permanently binds the silicates with the surface, making them far more durable and water repellent. This treatment generally is applied only after the initial cure has taken place (7 days or so depending on conditions). These coatings are known as silicate mineral paint.

Detergent auxiliariesEdit

It is used in detergent auxiliaries such as complex sodium disilicate and modified sodium disilicate. The detergent granules gain their ruggedness from a coating of silicates.^[1]

Water treatmentEdit

Sodium silicate is used as an alum coagulant and an iron flocculant in wastewater treatment plants. Sodium silicate binds to colloidalmolecules, creating larger aggregates that sink to the bottom of the water column. The microscopic negatively charged particles suspended in water interact with sodium silicate. Their electrical double layer collapses due to the increase of ionic strength caused by the addition of sodium silicate (doubly negatively charged anion accompanied by two sodium cations) and they subsequently aggregate. This process is called coagulation.^[1]

Refractory useEdit

Water glass is a useful binder of solids, such as vermiculite and perlite. When blended with the aforementioned lightweight aggregates, water glass can be used to make hard, high-temperature insulation boards used for refractories, passive fire protection and high temperature insulations, such as moulded pipe insulation applications. When mixed with finely divided mineral powders, such as vermiculite dust (which is common scrap from the exfoliation process), one can produce high temperature adhesives. The intumescence disappears in the presence of finely divided mineral dust, whereby the waterglass becomes a mere matrix. Waterglass is inexpensive and abundantly available, which makes its use popular in many refractory applications.

Dye auxiliaryEdit

Sodium silicate solution is used as a fixative for hand dyeing with reactive dyes that require a high pH to react with the textile fiber. After the dye is applied to a cellulose-based fabric, such as cotton or rayon, or onto silk, it is allowed to dry, after which the sodium silicate is painted on to the dyed fabric, covered with plastic to retain moisture, and left to react for an hour at room temperature.^[23]

Passive fire protectionEdit

Expantrol proprietary sodium silicate suspended in about a 6.5-mm-thick layer of red rubber, type 3M FS195, inserted into a metal pipe, then heated, to demonstrate hard charintumescence, strong enough to shut a melting plastic pipe

Palusol-based intumescent plastic pipe device used for commercial firestopping

Sodium silicates are inherently intumescent. They come in prill (solid beads) form, as well as the liquid, water glass. The solid sheet form (Palusol) must be waterproofed to ensure long-term passive fire protection (PFP).

Standard, solid, bead-form sodium silicates have been used as aggregate within silicone rubber to manufacture plastic pipe firestop devices. The silicone rubber was insufficient waterproofing to preserve the intumescing function and the products had to be recalled, which is problematic for firestops concealed behind drywall in buildings.

Pastes for caulking purposes are similarly unstable. This, too, has resulted in recalls and even litigation. Only 3M's "Expantrol" version, which has an external heat treatment that helps to seal the outer surface, as part of its process standard, has achieved sufficient longevity to qualify for DIBt approvals in the US for use in firestopping.

Not unlike other intumescents, sodium silicate, both in bead form and in liquid form, are inherently endothermic, due to liquid water in the water glass and hydrates in the prill form. The absence in the US of mandatory aging tests, whereby PFP systems are made to undergo system performance tests after the aging and humidity exposures, are at the root of the continued availability, in North America, of PFP products that can become inoperable within weeks of installation. Indiscriminate use of sodium silicates without proper waterproofing measures are contributors to the problems and risk. When sodium silicates are adequately protected, they function extremely well and reliably for long periods. Evidence of this can be seen in the many DIBt approvals for plastic pipe firestop devices using Palusol (a product of BASF), which use waterproofed sodium silicate sheets.

Metal repairEdit

Sodium silicate is used, along with magnesium silicate, in muffler repair and fitting paste. When dissolved in water, both sodium silicate and magnesium silicate form a thick paste that is easy to apply. When the exhaust system of an internal combustion engine heats up to its operating temperature, the heat drives out all of the excess water from the paste. The silicate compounds that are left over have glass-like properties, making a temporary, brittle repair.

Automotive repairEdit

Sodium silicate is also used currently as an exhaust system joint and crack sealer for repairing mufflers, resonators, tailpipes, and other exhaust components, with and without fiberglass reinforcing tapes. In this application, the sodium silicate (60–70%) is typically mixed with kaolin (40-30%), an aluminium silicate mineral, to make the sodium silicate "glued" joint opaque. The sodium silicate, however, is the high-temperature adhesive; the kaolin serves simply as a compatible high-temperature coloring agent. Some of these repair compounds also contain glass fibres to enhance their gap-filling abilities and reduce brittleness.

Sodium silicate can be used to fill gaps within the head gasket. Commonly used on aluminum alloy cylinder heads, which are sensitive to thermally induced surface deflection. This can be caused by many things including head-bolt stretching, deficient coolant delivery, high cylinder head pressure, overheating, etc.

"Liquid glass" (sodium silicate) is added to the system through the radiator, and allowed to circulate. Sodium silicate is suspended in the coolant until it reaches the cylinder head. At 100–105°C (212-221°F), sodium silicate loses water molecules to form a glass seal with a remelt temperature above 810°C (1,490°F).

A sodium silicate repair can last two years or longer. The repair occurs rapidly, and symptoms disappear instantly. This repair works only when the sodium silicate reaches its "conversion" temperature at 100–105°C. Contamination of engine oil is a serious possibility in situations in which a coolant-to-oil leak is present. Sodium silicate (glass particulate) contamination of lubricants is detrimental to their function.

Sodium silicate solution is used to inexpensively, quickly, and permanently disable automobile engines. Running an engine with about 2 liters of a sodium silicate solution instead of motor oil causes the solution to precipitate, catastrophically damaging the engine's bearings and pistons within a few minutes.^[24] In the United States, this procedure was used to comply with requirements of the Car Allowance Rebate System (CARS) program.^[24][25]

Safe constructionEdit

A mixture of sodium silicate and sawdust has been used in between the double skin of certain safes. This not only makes them more fire resistant, but also makes cutting them open with an oxyacetylene torch extremely difficult due to the smoke emitted.

Crystal gardensEdit

When crystals of a number of metallic salts are dropped into a solution of water glass, simple or branching stalagmites of coloured metal silicates are formed. This phenomenon has been used by manufacturers of toys and chemistry sets to provide instructive enjoyment to many generations of children from the early 20th century until the present. An early mention of crystals of metallic salts forming a "chemical garden" in sodium silicate is found in the 1946 Modern Mechanix magazine.^[26] Metal salts used included the sulfates and/or chlorides of copper, cobalt, iron, nickel, and manganese.

PotteryEdit

Sodium silicate is used as a deflocculant in casting slips helping reduce viscosity and the need for large amounts of water to liquidize the clay body. It is also used to create a crackle effect in pottery, usually wheel-thrown. A vase or bottle is thrown on the wheel, fairly narrow and with thick walls. Sodium silicate is brushed on a section of the piece. After 5 minutes, the wall of the piece is stretched outward with a rib or hand. The result is a wrinkled or cracked look.

It is also the main agent in "magic water", which is used when joining clay pieces, especially if the moisture level of the two differs.^[27]

Sealing of leaking water-containing structuresEdit

Sodium silicate with additives was injected into the ground to harden it and thereby to prevent further leakage of highly radioactive water from the Fukushima Daiichi nuclear power plant in Japan in April, 2011.^[28] The residual heat carried by the water used for cooling the damaged reactors accelerated the setting of the injected mixture.

On June 3, 1958, the USS Nautilus, the world's first nuclear submarine, visited Everett and Seattle. In Seattle, crewmen dressed in civilian clothing were sent in to secretly buy 140 quarts of an automotive product containing sodium silicate (originally identified as Stop Leak) to repair a leaking condenser system. The Nautilus was en route to the North Pole on a top secret mission to cross the North Pole submerged.^[29]

Firearm cartridgesEdit

A historical use of the adhesive properties of sodium silicates is the production of paper cartridges for black powder revolvers produced by Colt's Manufacturing Company during the period from 1851 until 1873, especially during the American Civil War. Sodium silicate was used to seal combustible nitrated paper together to form a conical paper cartridge to hold the black powder, as well as to cement the lead ball or conical bullet into the open end of the paper cartridge. Such sodium silicate cemented paper cartridges were inserted into the cylinders of revolvers, thereby speeding the reloading of cap-and-ball black powder revolvers. This use largely ended with the introduction of Colt revolvers employing brass-cased cartridges starting in 1873.^[30][31] Similarly, sodium silicate was also used to cement the top wad into brass shotgun shells, thereby eliminating any need for a crimp at the top of the brass shotgun shell to hold a shotgun shell together. Reloading brass shotgun shells was widely practiced by self-reliant American farmers during the 1870s, using the same waterglass material that was also used to preserve eggs. The cementing of the top wad on a shotgun shell consisted of applying from three to five drops of waterglass on the top wad to secure it to the brass hull. Brass hulls for shotgun shells were superseded by paper hulls starting around 1877. The newer paper-hulled shotgun shells used a roll crimp in place of a waterglass-cemented joint to hold the top wad in the shell. However, whereas brass shotshells with top wads cemented with waterglass could be reloaded nearly indefinitely (given powder, wad, and shot, of course), the paper hulls that replaced the brass hulls could be reloaded only a few times.

Food and medicineEdit

While not actually a medical use, sodium silicate, and other silicates, are the primary components in "instant" wrinkle remover creams, which temporarily tighten the skin to minimize the appearance of wrinkles & under-eye bags. These creams, when applied as a thin film and allowed to dry for a few minutes, can present dramatic results. Unfortunately, the results are not permanent. But, the effect can last for 8-12 hours, or until the dried film is washed off.

Food preservationEdit

World War I poster suggesting the use of waterglass to preserve eggs (lower right).

Waterglass has been used as an egg preservative with large success, primarily when refrigeration is not available. Fresh-laid eggs are immersed in a solution of sodium silicate (waterglass). After being immersed in the solution they were removed and allowed to dry. A permanent air tight coating remains on the eggs. If they are then stored in appropriate environment, the majority of bacteria which would otherwise cause them to spoil are kept out and their moisture is kept in. According to the cited source, treated eggs can be kept fresh using this method for up to five months. When boiling eggs so preserved, the shell is no longer accessible to water, and the egg will tend to crack unless a hole in the shell is made (e.g. with a pin) in order to allow steam to escape.^[32]

HomebrewingEdit

Sodium silicate flocculant properties are also used to clarify wine and beer by precipitating colloidal particles. As a clearing agent, though, sodium silicate is sometimes confused with isinglass which is prepared from collagen extracted from the dried swim bladders of sturgeon and other fishes. Eggs preserved in a bucket of waterglass gel, and their shells are sometimes also used (baked and crushed) to clear wine.^[33]

AquacultureEdit

Sodium silicate gel is also used as a substrate for algal growth in aquaculture hatcheries.^[34]