Which solvent can dissolve silicon dioxide or SiO?

Silicon and silicon dioxide

quartz



silicon

Silicon is a chemical element with the symbol Si and the atomic number 14. It is in the 4th main group (carbon group), or the 14th IUPAC group, and the 3rd period of the periodic table of the elements. In terms of mass fraction (ppmw), it is the second most abundant element in the earth's shell after oxygen.

Silicon is a classic semi-metal, therefore has properties of both metals and non-metals and is an element semiconductor. Pure, elemental silicon is gray-black in color and has a typically metallic, often bronze to bluish sheen. In the standard language, the element 'silicon' is written, the spelling with 'c' is mainly used in chemical terminology. The English translation for silicon is silicon, e.g. B. can be found in the term Silicon Valley (dt. 'Silicon Valley'). The occasional translation of silicone, silicone, is a wrong friend. The difference to English is probably due to the fact that the ending -on, which was changed in 1817, correctly refers to the chemical properties that are similar to carbon (carbon) and boron (boron), whereas in many non-English languages ​​the ending -ium refers to a suggesting supposed metal has been preserved to this day.


Silicon in inanimate nature

The entire earth consists of about 15 percent by mass of silicon; In particular, the earth's mantle is composed to a considerable extent of silicate rock melts. The earth's crust consists of about 25.8 percent by weight of silicon; this makes it the second most common chemical element after oxygen. Here silicon occurs mainly in the form of silicate minerals or as pure silicon dioxide.

Sand consists mainly of silicon dioxide. Quartz is pure silicon dioxide. Many gemstones consist of silicon dioxide and more or less admixtures of other substances, such as amethyst, rose and smoky quartz, agate, jasper and opal. Silicon forms silicates with many metals. The oceans also represent a huge reservoir of silicon: in the form of monomeric silica, it is dissolved in considerable quantities in all oceans. A total of 1437 silicon minerals are known to date (as of 2011), with the rare moissanite with a content of up to 70% having the highest silicon content (for comparison: mineral quartz has a silicon content of up to 46.7%).

Since silicon also occurs in elemental form in nature, it is recognized by the IMA as a mineral and is listed in Strunz’s mineral systematics (9th edition) under system no. 1.CB.15 (8th edition: I / B.05-10) in the department of semi-metals and non-metals. In the systematics of minerals according to Dana, which is mainly known in the English-speaking area, the element mineral has the system number. 01.03.07.01.

Solid silicon has so far (as of 2011) been detected at 15 sites, including the first in the Nuevo Potosí deposit in Cuba. Other locations are in the People's Republic of China, Russia, Turkey and the United States. (Source: wikipedia; the text is available under the license "Creative Commons Attribution / Share Alike")


Silicates

Silicates are the salts and esters of orthosilicic acid (Si (OH)4) and their condensates. The esters are described under silicic acid esters, for the condensates see silicas. All salts are through SiO4−Tetrahedra are compounds that have tetrahedra that can be linked together in different ways. Unlinked parts of the tetrahedron contribute to the charge balance as metal cations or may be present as hydroxide ions (OH−). With the exception of alkali silicates, silicates are insoluble in water or other solvents.

Natural silicates play a major role in mineralogy, as a large number of minerals can be assigned to this group of substances. The earth's crust consists of over 90 percent, the earth's mantle almost entirely of silicates

(Danger: Unfortunately, there is currently no consensus on the definition of quartz: According to Strunz, quartz is an oxide, but in the Anglo-American world, according to Dana, it is a silicate. In this respect, general statistical statements such as Frequency of silicates in the earth's crust etc.) should be viewed with caution. For some, these are pure silicates without quartz (Strunz), for others (Dana) silicates including quartz!


Silicon in living nature

In addition to the already mentioned essential nature of silicon, there are a number of living things that produce structures containing silicon dioxide. The best known are the diatoms, sponges (Porifera, Spongiaria) and radiolarians, which build up an exoskeleton from silicon dioxide through the enzyme-catalyzed condensation of orthosilicic acid Si (OH) ⁠4. Many plants also contain silicon dioxide in their stems and leaves. Well-known examples here are the horsetail and the bamboo plant. The built-up silicon dioxide framework gives them additional stability.

Physical properties of silicon

Silicon is an element semiconductor. According to the band model, the energetic distance between the valence band and the conduction band is 1.107 eV (at room temperature). By doping with suitable doping elements such as boron or arsenic, the conductivity can be increased by a factor of 106. In silicon doped in this way, the impurity conduction caused by foreign atoms and lattice defects is significantly greater than that of the intrinsic conduction, which is why such materials are referred to as impurity semiconductors. The lattice parameter is 543 pm. Spectrum of the complex index of refraction (N = n + i k) of silicon

The complex refractive index, which depends on the wavelength of the light, is shown in the adjacent picture. Here, too, information about the band structure can be read. A direct band transition at 370 nm (EÀ1 = 3.4 eV) can be seen on the basis of the strongly increasing curve of the extinction coefficient k. Another direct band transition can be observed at ≈ 300 nm (EΓ2 = 4.2 eV). The indirect band transition of silicon (Eg = 1.1 eV) can only be guessed at. The fact that there are further indirect band transitions can be seen from the wide curve of k for wavelengths> 400 nm.

Like water and a few other substances, silicon has a density anomaly: its density in liquid form (at Tm = 1685 K) is 10–11% higher than in solid, crystalline form (c-Si) at 300 K.

Chemical properties of silicon

In all naturally occurring and in the majority of synthetically produced compounds, silicon only forms single bonds. The stability of the Si-O single bond in contrast to the C-O double bond is due to its partial double bond character, which is created by the overlap of the lone electron pairs of oxygen with the empty d orbitals of silicon. The double bond rule that has been valid for many years, according to which silicon as an element of the 3rd period does not form multiple bonds, must now be regarded as outdated, since a large number of synthetically produced compounds with Si-Si double bonds are now known. In 2004 the first compound with a formal Si-Si triple bond was structurally characterized.

With the exception of hydrofluoric acid containing nitric acid (in which hexafluorosilicate is formed), silicon is insoluble in acids because passivation occurs through the formation of a solid silicon dioxide layer. On the other hand, it dissolves easily in hot alkaline solutions with hydrogen formation. Despite its negative normal potential (−0.81 V), it is relatively inert in compact form, as it is covered with a protective oxide layer in air.


Silicas - silicon dioxide

The oxygen acids of silicon are called silicic acids. The simplest silica is monosilicic acid (orthosilicic acid) Si (OH)4. It is a weak acid (pKa1 = 9.51; pKa2 = 11.74) and tends to condense. Dehydration leads to compounds such as disilicic acid (pyrosilicic acid) (HO)3Si – O – Si (OH)3 and tri-silica (HO)3Si – O – Si (OH)2–O – Si (OH)3. Cyclic (ring-shaped) silicas are e.g. B. Cyclotric silica and Cyclotetra silica with the general empirical formula [Si (OH)2–O–] n. Polymers are sometimes referred to as metasilicic acid (H.2SiO3, [–Si (OH)2-O-] n). If these low molecular weight silicas condense further, amorphous colloids (silica sol) are formed. The general formula of all silicas is H.2n + 2SinO3n + 1. SiO is often used as the empirical formula2 • n H2O stated; In the case of silicas, however, the water is not water of crystallization, but can only be split off through a chemical reaction and is formed from constitutionally bound hydroxyl groups.

In general, the lower water content of orthosilicic acid products are grouped under the term polysilicic acids. The formal end product of dehydration is silica anhydride SiO2. (Silicon dioxide). In nature, support structures made of silicic acid anhydride occur in plant and animal life, for example in the diatoms (diatoms) and radiolarians (radiolarians) and glass sponges (hexactinellida) that are widespread in the sea, as well as in horsetail. The silicic acid anhydride skeletons of dead diatoms and radiolucent animals sink to the sea floor, where they accumulate and form deposits of kieselguhr (diatomaceous earth) or radiolarian sludge. Deposits from the Miocene contain 70–90% SiO2, 3–12% water and traces of metal oxides. Silicic acid is also found in groundwater. The rainwater or seepage water that runs down through the soil layers and contains carbonic acid absorbs silica from the silicates of the soil minerals. Therefore, drinking water also contains small amounts of silica. (Source: wikipedia; the text is available under the license "Creative Commons Attribution / Share Alike")

(Danger: Silicon dioxide is a collective name for the modifications of the oxides of silicon with the empirical formula SiO2. In the German-speaking area, or according to Strunz, the term silicic acid is incorrectly used for silicon dioxide or, recently, "Silica", which is derived from Anglo-Saxon. Equating silicon dioxide with sand is also incorrect


Solubility of silicon dioxide

The solubility of silicon dioxide in water is heavily dependent on the modification or degree of order of the silicon dioxide. In the case of crystalline, highly ordered quartz, the solubility at 25 ° C is around 10 mg SiO2 per liter of water. It should be noted, however, that the solution equilibrium may only be established very slowly. The disordered amorphous silicas are much more soluble at the same temperature with approx. 120 mg / l water. The solubility increases with increasing temperature. For quartz at 100 ° C it is then approx. 60 mg / l water. In the case of amorphous silica, 330 ppm silicon dioxide are already dissolved in water at 75 ° C. The solubility also increases with increasing pH. Acids are capable of SiO2 practically impossible to dissolve, with the exception of hydrofluoric acid (HF), which forms gaseous silicon tetrafluoride (SiF4) is attacked. Alkaline melts and - to a lesser extent - aqueous alkali lye dissolve particularly amorphous silicon dioxide. In addition to silica, some natural waters contain colloidal silicon dioxide (SiO2), which does not hydrate to silica at normal temperatures in water. This colloidal SiO2, this also includes various compounds containing silicic acid, does not react with ammonium heptamolybdate to form the yellow colored heteropoly acid. (Source: wikipedia; the text is available under the license "Creative Commons Attribution / Share Alike")

Chemical composition of quartz (chemism)

Quartz is a very pure compound and only incorporates traces of other elements into the crystal lattice. Natural quartz can contain between 13 and 15,000 ppm (but mostly only a few 100 ppm) Al3+, between 9 and 1400 ppm Na+, between 3 and 300 ppm K+, as well as smaller amounts of Fe3+, Ti4+, P5+, H+ and Li+ contain.

These ions are usually incorporated via a coupled replacement (substitution) of a Si4+Ions by a trivalent and a monovalent ion, such as Al3+ and well+. The foreign ions are built into the Si positions in the lattice as well as in otherwise empty interstitial lattice spaces. The incorporation of iron and aluminum together with the effect of ionizing radiation is responsible for the different colors of the quartz varieties.

Changing contents of trace elements such as Al, Li, Na, K, Sb, Ti reflect changing growth conditions.


Reactions

All SiO behaves at room temperature2-Modifications are inert and do not react with most other substances. Even at moderately higher temperatures, SiO2 is chemically very stable. For this reason, fused quartz (SiO2-Glass) is often used for chemical apparatus construction when catalytic reactions of the metal cations in normal glass are to be avoided. The reason for this low reactivity of the SiO2 is the very strong Si-O bond, but also its macromolecular structure. Since quartz itself is an anhydride (orthosilicic acid H4SiO4), it is generally not attacked by acids. The exception to be emphasized is hydrofluoric acid (HF), which decomposes quartz and first silicon fluoride (HF4), then forms fluorosilicic acid (silicofluoric acid)

SiO2 + 6 HF → H2SiF6 + 2 H.2O

SiO2 is also attacked by alkaline substances (such as KOH, potassium hydroxide). The rate of reaction depends on the modification and the crystal size. Crystalline quartz only dissolves very slowly in hot aqueous alkali solution, while amorphous SiO2 dissolves relatively quickly at room temperature

SiO2 + 2 KOH → K2SiO3 + H2O

This reaction is important to the collector, who sometimes uses alkaline solutions and detergents to remove mineral deposits from moss and lichen. Quartz crystals are usually not an issue, but cleaning cryprocrystalline quartz can be problematic. A similar reaction leads to the formation of SiO2-Gel (silica gel) in concrete, if alkaline compounds of the concrete, typically Ca (OH)2, with amorphous SiO2 and cryprocrystalline quartz (e.g. opal and chalcedony). The SiO2-Gel attracts water, swells and breaks the concrete within a few decades.

All kinds of SiO2 dissolve in molten sodium bicarbonate (Na2CO3) or in potash (K2CO3) and form silicates SiO2 + K2CO3 → K2SiO3 + CO2 At higher temperatures in many geological processes, quartz acts as an acid and reacts with many alkaline minerals. A well-known example is the formation of the mineral wollastonite (Ca3Si3O9) made of quartz and calcite in contact metamorphic processes at temperatures above 600 ° C

3 SiO2 + 3 CaCO3 → Approx3Si3O9 + 3 CO2

The reverse reaction takes place with the weathering of silicate rocks; e.g., if in the presence of carbonic acid (H.2CO3) releases silica in meteoric waters and forms carbonates.
(Source of the capital "Reactions": Akhavan, A.C., The Quartz Page)


literature

  • Rusk, B., Brian Rusk, Koenig, A., Lowers, H., 2011; Visualizing trace element distribution in quartz using cathodoluminescence, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry. American Mineralogist vol. 96, 703-708.
  • Götte, T., Pettke, T., Ramseyer, K., Koch-Müller, M., Mullis, J., 2011; Cathodoluminescence properties and trace element signature of hydrothermal quartz: A fingerprint of growth dynamics. American Mineralogist 96, 802-813.

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