Silicon dioxide
`
`rom Wikipedia, the free encyclopedia
`
`Silicon dioxide, also known as silica (from the Latin silex), is a
`chemical compound that is an oxide of silicon with the chemical
`formula SiO2. It has been known since ancient times. Silica is
`
`ost commonly found in nature as quartz, as well as in various
`living organisms.[5][6] In many parts of the world, silica is the
`ajor constituent of sand. Silica is one of the most complex and
`u ost abundant families of materials, existing both as several
`inerals and being produced synthetically. Notable examples
`include fused quartz, crystal, fumed silica, silica gel, and
`aerogels. Applications range from structural materials to
`u
`icroelectronics to components used in the food industry.
`
`Contents
`
`I
`
`1 Production
`
`1.1 Fumed silica
`
`1.2 Silica fume
`
`1.3 Precipitated silica
`1.4 On microchips
`1.5 Laboratory or special methods
`I 1.5.1 From silicate esters
`
`I 1.5.2 Other methods
`
`Uses
`
`I 2.1 Precursor to glass and silicon
`I 2.2 Major component used in sand casting
`I 2.3 Food and pharmaceutical applications
`I 2.4 Other
`Structure
`
`Fused quartz
`Chemical reactions
`
`I 5.1 Solubility in water
`Occurrence
`
`I 6.1 Biology
`Health effects
`
`Crystalline forms
`See also
`
`;
`
`9
`
`10 References
`
`11 External links
`
`,
`Production
`
`Silicon dioxide
`
`IUPAC name
`
`Silicon dioxide
`
`Other names
`
`Quartz
`
`Silica
`Silicic oxide
`
`Silicon(I\/3 oxide
`
`Crystalline silica
`
`CAS Number
`
`Identifiers
`
`7631-86-9 (http://www.commo
`
`nchemistry. org/ChemicalDetail.
`
`aspx?ref=7631-86-9) "'
`
`CHEBI:30563 (https://wWw.eb
`i. ac. uk/chebi/searchld. do?chebi
`
`Id=30563)’
`
`ChemSpider
`
`22683 (http://www.chemspider.
`com/Chemical-Structure.22683.
`
`EC Number
`
`htrnl) 1-’
`
`231-545-4
`
`Gmelin Reference
`
`200274
`
`BUECKE
`
`C16459 (http://www.kegg.jp/en
`
`Silicon dioxide is mostly obtained by mining and purification of MCSH
`quartz. Quartz comprises more than 10% by mass of the earth's
`crust.[7] This product is suitable for many purposes while for
`
`try/C16459) 3'
`
`Silicon+dioxide (https://www.n
`
`lm.nih.goV/cgi/mesh/2014/MB_
`
`Elm Exhibit 21
`
`Elm Exhibit 2163
`Samsung, Micron, SK hynix v. Elm
`IPR2016-00387
`
`

`
`others chemical processing is required to make a purer or
`otherwise more suitable (e. g. more reactive or fine-grained)
`
`is a very fine particulate or colloidal form of silicon dioxide. It is
`repared by burning SiCl4 in an oxygen rich hydrocarbon flame
`
`SiC14 + 2 H2 + 02 —> S102 + 4 HC1.
`
`Silica fume
`
`This product is obtained as byproduct from hot processes like
`ferro-silicon production. It is less pure than fumed silica and
`should not be confused with that product. The production
`rocess, particle characteristics and fields of application of
`fumed silica are all different from those of silica fume.
`
`Precipitated silica
`
`‘ morphous silica, silica gel, is produced by the acidification of
`solutions of sodium silicate. The gelatinous precipitate is first
`ashed and then dehydrated to produce colorless microporous
`silica.[8] Idealized equation involving a trisilicate and sulfuric
`acid is shown:
`
`Na2Si3O7 + H2804 —> 3
`
`+ NEIZSO4 + H20
`
`pproximately one billion kilograms/year (1999) of silica was
`roduced in this manner, mainly for use for polymer composites
`
`tires and shoe soles.[7]
`
`On microchips
`
`Thin films of silica grow spontaneously on silicon wafers via
`hermal oxidation. This route gives a very shallow layer
`(approximately 1 nm or 10 A) of so-called native oxide.[9]
`I
`igher temperatures and alternative environments are used to
`grow well-controlled layers of silicon dioxide on silicon, for
`example at temperatures between 600 and 1200 °C, using so-
`called dry or wet oxidation with 02 or H20, respectively.[10] The
`
`depth of the layer of silicon replaced by the dioxide is 44% of
`the depth of the silicon dioxide layer produced.[10]
`
`PubChem
`
`cgi?mode=&term=Silicon+diox
`
`ide)
`
`24261 (https://pubchem.ncbi.nl
`
`m.nih.gov/compound/24261)
`
`RTECS number
`
`VV75 65 000
`
`UNII
`
`InChI
`
`ETJ7Z6XBU4 (http://fdasis.nl
`
`m.nih. gov/srs/srsdirect.j sp?regn
`
`o=ETJ7Z6XBU4) -1'
`
`Properties
`
`Chemical formula
`
`S102
`
`Molar mass
`
`Appearance
`
`60.08 g/mol
`
`Transparent solid (Amorphous)
`WhiteNVhitish Yellow
`
`(Powder/Sand)
`
`Density
`
`2.648 (on-quartz), 2.196
`
`(amorphous) g-cm_3[1]
`
`Melting point
`
`1,713 °C (3,115 °F; 1,986 K)
`
`(amorphous)[1](P4‘88) to
`
`Boiling point
`
`2,950 °C (5,340 °F; 3,220
`
`K)[1]
`
`12 (|| c-axis), 6.8 (J. c-axis),
`
`1.4 (am.) w/(m-K)[11<P12-213)
`
`1.544 (0), 1.553 (e)l11<P4~143>
`
`Hazards
`
`Thermal
`
`conductivity
`
`Refractive index
`
`(711))
`
`NFPA 704
`
`US health exposure limits (NIOSH):
`
`PEL (Permissible) TWA 20 mppcf (80
`
`REL
`
`(Recommended)
`
`mg/m3/%SiO2) (amorphous)[2]
`
`TWA 6 mg/m3 (amorphous)[2]
`
`Ca TWA 0.05 mg/m3[31
`
`IDLH (Immediate
`danger)
`
`mg/m3 (am0rphOuS)[2]
`
`Ca [25 mg/m3 (cristobalite,
`
`tridymite); 50 mg/m3
`
`(quartz)] [3]
`
`Related compounds
`
`Related diones
`
`Carbon dioxide
`
`Elm Exhibit 2163, Page 2
`
`

`
`The native oxide layer can be beneficial in microelectronics,
`here it acts as electric insulators with high chemical stability.
`In electrical applications, it can protect the silicon, store charge,
`lock current, and even act as a controlled pathway to limit
`
`current flow.[11]
`
`Germanium di0Xid€
`Tin di°_Xid_e
`Lead dloxlde
`
`Laboratory or special methods
`
`From silicate esters
`
`any routes to silicon dioxide start with silicate esters, the best
`I nown being tetraethyl orthosilicate (TEOS). Simply heating
`TEOS at 680-730 °C gives the dioxide:
`
`Si(OC2H5)4 —’ Sioz + 2 O(C2H5)2
`
`Similarly TEOS combusts around 400 °C:
`
`Si(OC2H5)4 + 12 0, _> sio, + 10 H20 + 8 CO2
`
`Related compounds Silicon monoxide
`
`Silicon Sulfide
`
`Thermochemistry
`
`42 J.m01—l .K—l [4]
`
`Std molar
`entropy (59298)
`
`Std enihalpy of
`formation (AfH°298)
`
`‘911 kJ'm°1_1[4]
`
`Except where otherwise noted, data are given for
`materials in their standard state (at 25 °C [77 °F],
`100 kPa)-
`
`.
`a
`x
`*7 V°“f-" (What ‘S I 7)
`Infobox references
`
`TEOS undergoes hydrolysis via the so-called sol-gel process.
`
`The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:
`
`[12]
`
`‘l’ 2 H20 —>
`
`‘l‘ 4
`
`Other methods
`
`I: eing highly stable, silicon dioxide arises from many methods. Conceptually simple, but of little practical value,
`combustion of silane gives silicon dioxide. This reaction is analogous to the combustion of methane:
`
`+ 2 02 —>
`
`‘l’ 2
`
`Uses
`
`estimated 95% of silicon dioxide produced is consumed in the construction industry, e.g. for the production of
`ortland cement.[7] Other major applications are listed below.
`
`Precursor to glass and silicon
`
`Silica is used primarily in the production of glass for windows, drinking glasses, beverage bottles, and many other
`ses. The majority of optical fibers for telecommunication are also made from silica. It is a primary raw material
`for many ceramics such as earthenware, stoneware, and porcelain.
`
`Silicon dioxide is used to produce elemental silicon. The process involves carbothermic reduction in an electric arc
`
`furnace:[13]
`
`SiO2+2C—>Si+2CO
`
`Major component used in sand casting
`
`Elm Exhibit 2163, Page 3
`
`

`
`Silica, in the form of sand is used as the main ingredient in sand casting for the manufacture of a large number of
`etallic components in engineering and other applications. The high melting point of silica enables it to be used in
`such applications.
`
`Food and pharmaceutical applications
`
`Silica is a common additive in the production of foods, where it is used primarily as a flow agent in powdered
`foods, or to adsorb water in hygroscopic applications. It is the primary component of diatomaceous earth. Colloidal
`silica is also used as a wine, beer, and juice fining agent.[7]
`
`In pharmaceutical products, silica aids powder flow when tablets are formed.
`
`Other
`
`‘ silica-based aerogel was used in the Stardust spacecraft to collect extraterrestrial particles. Silica is also used in
`he extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
`I ydrophobic silica is used as a defoamer component. In hydrated form, it is used in toothpaste as a hard abrasive
`to remove tooth plaque.
`
`In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric. In
`cosmetics, it is useful for its light-diffusing properties and natural absorbency. It is also used as a thermal
`enhancement compound in ground source heat pump industry.
`
`Structure
`
`In the majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen
`atoms surrounding a central Si atom. The most common example is seen in the quartz
`crystalline form of silica SiO2. In each of the most thermodynamically stable
`
`crystalline forms of silica, on average, all 4 of the vertices (or oxygen atoms) of the
`SiO4 tetrahedra are shared with others, yielding the net chemical formula: SiO2.
`
`Structural motif found in
`
`or-quartz, but also found
`in almost all forms of
`silicon dioxide.
`
`For example, in the unit cell of oi-quartz, the
`central tetrahedron shares all 4 of its corner O
`atoms, the 2 face-centered tetrahedra share 2 of
`their comer O atoms, and the 4 edge-centered
`tetrahedra share just one of their 0 atoms with
`other SiO4 tetrahedra. This leaves a net average of 12 out of 24 total
`
`I-5»:-
`“E:
`u ,_._c;
`- we-.
`__ "*1
`lif-
`I Hi:
`
`_
`.' '5'-~*=' *-
`' '°""'"
`
`ertices for that portion of the 7 S104 tetrahedra that are considered to be a
`
`|| art of the unit cell for silica (see 3-D Unit Cell (http://www.mindat.org/mi
`-3337.html)).
`
`_
`_
`_
`_
`_
`_
`_
`S102 has a number of distinct crystalline forms (polymorphs) in addition to
`
`I -1':
`-
`.
`-
`-
`-
`""1. ,,,,.,_,,.,_: .,
`:n:-
`:5
`
`',
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`3'»:
`Den-5»1;.- g-'n11'
`
`4'»:
`
`'
`
`.1‘:
`
`'
`
`Relation between refractive index and
`
`density for some SiO2 forms.[14]
`
`amorphous forms. With the exception of stishovite and fibrous silica, all of
`he crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements.
`
`Silicon—oxygen bond lengths vary between the different crystal forms, for example in on-quartz the bond length is
`161 pm, whereas in on-tridymite it is in the range 154-171 pm. The Si-O-Si angle also varies between a low value
`of 140° in ot-tridymite, up to 180° in B-tridymite. In ot-quartz the Si-O-Si angle is l44°.[15]
`
`Elm Exhibit 2163, Page 4
`
`

`
`I ibrous silica has a structure similar to that of SiS2 with chains of edge-sharing SiO4 tetrahedra. Stishovite, the
`
`I igher-pressure form, in contrast has a rutile-like structure where silicon is 6-coordinate. The density of stishovite
`is 4.287 g/cm3, which compares to or-quartz, the densest of the low-pressure forms, which has a density of
`.648 g/cm3.[8] The difference in density can be ascribed to the increase in coordination as the six shortest Si-O
`ond lengths in stishovite (four Si-O bond lengths of 176 pm and two others of 181 pm) are greater than the Si-O
`ond length (161 pm) in or-quartz.[16] The change in the coordination increases the ionicity of the Si-O bond.[17]
`ut more important is the observation that any deviations from these standard parameters constitute
`icrostructural differences or variations, which represent an approach to an amorphous, vitreous or glassy solid.
`
`I
`
`The only stable form under normal conditions is or-quartz and this is the form in which crystalline silicon dioxide is
`sually encountered. In nature impurities in crystalline or-quartz can give rise to colors (see list). The high
`emperature minerals, cristobalite and tridymite, have both a lower density and index of refraction than quartz.
`Since the composition is identical, the reason for the discrepancies must be in the increased spacing in the high
`emperature minerals. As is common with many substances, the higher the temperature the farther apart the atoms
`due to the increased vibration energy.
`
`The transformation from on-quartz to beta-quartz takes place abruptly at 5 73 °C. Since the transformation is
`accompanied by a significant change in volume it can easily induce fracturing of ceramics or rocks passing through
`his temperature limit.
`
`The high-pressure minerals, seifertite, stishovite, and coesite, on the other hand, have a higher density and index of
`I efraction when compared to quartz. This is probably due to the intense compression of the atoms that must occur
`during their formation, resulting in a more condensed structure.
`
`I aujasite silica is another form of crystalline silica. It is obtained by dealumination of a low-sodium, ultra-stable Y
`zeolite with a combined acid and thermal treatment. The resulting product contains over 99% silica, has high
`crystallinity and high surface area (over 800 m2/g). Faujasite-silica has very high thermal and acid stability. For
`example, it maintains a high degree of long-range molecular order (or crystallinity) even after boiling in
`concentrated hydrochloric acid.[18]
`
`I olten silica exhibits several peculiar physical characteristics that are similar to the ones observed in liquid water:
`I egative temperature expansion, density maximum (at temperatures ~5000 °C), and a heat capacity minimum.[19]
`ts density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm-3 at 2200 °C.[2°] When molecular silicon monoxide,
`SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave
`discharge, molecular SiO2 is produced with a linear structure. Dimeric silicon dioxide, (SiO2)2 has been prepared
`
`y reacting 02 with matrix isolated dimeric silicon monoxide, (Si2O2). In dimeric silicon dioxide there are two
`
`oxygen atoms bridging between the silicon atoms with an Si-O-Si angle of 94° and bond length of 164.6 pm and
`he terminal Si-O bond length is 150.2 pm. The Si-O bond length is 148.3 pm, which compares with the length of
`161 pm in ct-quartz. The bond energy is estimated at 621.7 kJ/mol.[21]
`
`Fused quartz
`
`en molten silicon dioxide SiO2 is rapidly cooled, it does not crystallize but solidifies as a glass. The geometry
`
`of the silicon and oxygen centers in glass is similar to that in quartz and most other crystalline forms of the same
`composition, i.e., silicon is surrounded by a regular tetrahedra of oxygen centers. The difference between the glass
`and the crystalline forms arise from the connectivity of these tetrahedral units. Although there is no long range
`periodicity in the glassy network there remains significant ordering at length scales well beyond the SiO bond
`length. One example of this ordering is found in the preference of the network to form rings of 6-tetrahedra.[22]
`
`Elm Exhibit 2163, Page 5
`
`

`
`The glass transition temperature of pure SiO2 is about 1475 K.[23]
`
`Chemical reactions
`
`Silica is converted to silicon by reduction with carbon.
`
`I luorine reacts with silicon dioxide to form SiF4 and 02 whereas the other
`
`I alogen gases (C12, Brz, 12) are essentially unreactive.[8]
`
`Silicon dioxide is attacked by hydrofluoric acid (HF) to produce
`
`exafluorosilicic acid: [15]
`
`sio, + 6 HF _) H2SiF6 + 2 H20.
`
`F is used to remove or pattern silicon dioxide in the semiconductor
`industry.
`
`Silicon dioxide acts as a Lux-Flood acid, being able to react with bases
`nder certain conditions. As it does not contain any hydrogen, it cannot act
`as a Bronsted—Lowry acid. While not soluble in water, some strong bases
`ill react with glass and have to be stored in plastic bottles as a result.[24]
`
`Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide, as
`described in this idealized equation:[8]
`
`-i’ 2
`
`—> Na2SiO3 +
`
`Manufactured Silica fume at
`maximum surface area of 380 m2/g
`
`Silicon dioxide will neutralise basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide,
`or mixtures of oxides, forming silicates and glasses as the Si-O-Si bonds in silica are broken successively).[15] As
`an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate, and glasses,
`
`dependent on the proportions of reactants:[8]
`
`2 Na2O + SiO2 —> Na4SiO4;
`
`Na2O + SiO2 —> Na2SiO3;
`
`(0.25—0.8)Na2O + SiO2 —> glass.
`
`Examples of such glasses have commercial significance, e. g. soda-lime glass, borosilicate glass, lead glass. In
`these glasses, silica is termed the network former or lattice former.[15] The reaction is also used in blast furnaces to
`I emove sand impurities in the ore by neutralisation with calcium oxide, forming calcium silicate slag.
`
`Silicon dioxide reacts in heated reflux under dinitrogen with ethylene glycol and an alkali metal base to produce
`I ighly reactive, pentacoordinate silicates which provide access to a wide variety of new silicon compounds.[25] The
`silicates are essentially insoluble in all polar solvent except methanol.
`
`Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:[15]
`
`S102 + Si —» 2 SiO
`
`Elm Exhibit 2163, Page 6
`
`

`
`Solubility in water
`
`The solubility of silicon dioxide in water strongly depends on its crystalline
`form and is 3-4 times higher for silica than quartz; as a function of
`emperature, it peaks at about 340 °C.[26] This property is used to grow
`single crystals of quartz in a hydrothermal process where natural quartz is
`dissolved in superheated water in a pressure vessel that is cooler at the top.
`Crystals of 0.5-1 kg can be grown over a period of 1-2 months.[15] These
`crystals are a source of very pure quartz for use in electronic applications.[8]
`
`Occurrence
`
`Even though it is poorly soluble, silica occurs widely in many plants. Plant
`u aterials with high silica phytolith content appear to be of importance to
`grazing animals, from chewing insects to ungulates. Studies have shown
`hat it accelerates tooth wear, and high levels of silica in plants frequently
`eaten by herbivores may have developed as a defense mechanism against
`redation.[27][28]
`
`Bundle of optical fibers composed of
`high purity silica.
`
`Silicification in and by cells has been common in the biological world for well over a billion years. In the modern
`orld it occurs in bacteria, single-celled organisms, plants, and animals (invertebrates and vertebrates). Prominent
`examples include:
`
`I Tests or frustules (i.e. shells) of diatoms, Radiolaria and testate amoebae.[6]
`I Silica phytoliths in the cells of many plants, including Equisetaceae, practically all grasses, and a wide range
`of dicotyledons.
`I The spicules forming the skeleton of many sponges.
`
`Crystalline minerals formed in the physiological environment often show exceptional physical properties (e.g.,
`strength, hardness, fracture toughness) and tend to form hierarchical structures that exhibit microstructural order
`over a range of scales. The minerals are crystallized from an environment that is undersaturated with respect to
`silicon, and under conditions of neutral pH and low temperature (0-40 °C).
`
`I ormation of the mineral may occur either within the cell wall of an organism (such as with phytoliths), or outside
`he cell wall, as typically happens with tests. Specific biochemical reactions exist for mineral deposition. Such
`ueactions include those that involve lipids, proteins, and carbohydrates.
`
`It is unclear in what ways silica is important in the nutrition of animals. This field of research is challenging
`ecause silica is ubiquitous and in most circumstances dissolves in trace quantities only. All the same it certainly
`does occur in the living body, leaving us with the problem that it is hard to create proper silica-free controls for
`urposes of research. This makes it difficult to be sure when the silica present has had operative beneficial effects,
`and when its presence is coincidental, or even harmful. The current consensus is that it certainly seems important
`in the growth, strength, and management of many connective tissues. This is true not only for hard connective
`issues such as bone and tooth but possibly in the biochemistry of the subcellular enzyme-containing structures as
`ell.[29]
`
`Elm Exhibit 2163, Page 7
`
`

`
`Health effects
`
`Silica ingested orally is essentially nontoxic, with an LD50 of 5000 mg/kg
`
`(5 g/kg).[7] On the other hand, inhaling finely divided crystalline silica dust
`can lead to silicosis, bronchitis, or cancer, as the dust becomes lodged in
`he lungs and continuously irritates them, reducing lung capacities.[30]
`Studies of workers with exposure to crystalline silica have shown l0-fold
`igher than expected rates of lupus and other systemic autoimmune
`diseases compared to expected rates in the general population.[31] Prior to
`ew rules issued in 2013, OSHA allowed 100 pg per cubic meter of air.
`The new regulations reduce the amount to 50 pg/m3. The exposure limit for
`he construction industry is also set at 50 pg/m3 down from 250 pg/m3.[32]
`
`Quartz sand (silica) as main raw
`
`material for commercial glass
`
`In the body, crystalline silica particles do not dissolve over clinically
`elevant periods. Silica crystals inside the lungs can activate the NLRP3
`inflammasome inside macrophages and dendritic cells and thereby result in
`rocessing of pro-Interleukin 1 beta into its mature form. Chronic exposure
`0 silica may thereby account for some of its health hazards, as interleukin-1 is a highly pro-inflammatory cytokine
`in the immune system.[33][34][35] This effect can create an occupational hazard for people working with
`sandblasting equipment, products that contain powdered crystalline silica and so on. Children, asthmatics of any
`age, allergy sufferers, and the elderly (all of whom have reduced lung capacity) can be affected in much less time.
`‘ morphous silica, such as fumed silica is not associated with development of silicosis, but may cause irreversible
`lung damage in some cases.[36] Laws restricting silica exposure with respect to the silicosis hazard specify that
`hey are concerned only with silica that is both crystalline and dust-forming.
`
`production
`
`‘ study that followed subjects for 15 years found that higher levels of silica in water appeared to decrease the risk
`of dementia. The study found an association between an increase of 10 milligram-per-day of the intake of silica in
`drinking water with a decreased risk of dementia of ll%.[37]
`
`Crystalline silica is used in hydraulic fracturing of formation which contain tight oil and shale gas, a use which
`resents a health hazard to workers. In 2013 OSHA announced tightened restrictions on the amount of crystalline
`silica which could be present and required "green completion" of fracked wells to reduce exposure.[32] Crystalline
`silica is an occupational hazard for those working with stone countertops, because the process of cutting and
`installing the countertops creates large amounts of airborne silica.[38]
`
`Crystalline forms
`
`SiO2, more so than almost any material, exists in many crystalline forms (called polymorphs).
`
`Elm Exhibit 2163, Page 8
`
`

`
`Crystalline forms of SiO2
`
`[15]
`
`Crystal symmetry
`Pearson symbol, group
`No.
`
`g/ems
`
`Notes
`
`Structure
`
`rhombohedral (trigonal)
`hP9, P3121 No.152[391
`
`2648
`
`Helical chains making individual single
`crystals optically active; (1-quartz converts to B-
`quartz at 846 K
`
`hexagonal
`hPl8, P6222, No.
`
`180[4°1
`
`Closely related to or-quartz (with an Si-O-Si
`angle of 155°) and optically active; B-quartz
`converts to B-tridymite at 1140 K
`
`?C'
`
`!
`
`>39
`as
`
`‘I
`
`1‘
`
`i‘
`
`I
`
`orthorhombic
`oS24, C2221, No.20[411 2265
`
`Metastable form under normal pressure
`
`hexagonal
`hP12, P63/mmc, No.
`
`194[411
`
`Closely related to on-tridymite; [3-tridymite
`converts to B-cristobalite at 2010 K
`
`X '
`
`-5.-
`
`ot-cristobalite
`
`tetragonal
`tPl2, P41212, No. 92[421 2-334
`
`Metastable form under normal pressure
`
`[3-cristobalite
`
`_
`cubic
`cF 1 04, Fd3m,
`No.227[431
`
`Closely related to oz-cristobalite; melts at 1978
`K
`
`tetragonal
`tP36, P41212, No. 92[441 30“
`
`Si5O10, Si4O14, Si8O16 rings; synthesised from
`
`glassy silica and alkali at 600-900 K and 40-
`400 MPa
`
`moganite
`
`monoclinic
`
`mS46, C2/c, No.15[451
`
`Sl4O8 and Sl6O12 I'lI1gS
`
`Elm Exhibit 2163, Page 9
`
`

`
`monoclinic
`
`mS48, C2/c, No.15[46]
`
`2_911 Si4O8 and Si8O16 rings; 900 K and 3-3.5 GPa
`
`stishovite
`
`Tetragonal
`tP6, P42/mnm,
`
`No.136[47]
`
`One of the densest (together with seifertite)
`polymorphs of silica; rutile-like with 6-fold
`coordinated Si; 7.5-8.5 GPa
`
`seifertite
`
`orthorhombic
`
`oP, Pbcn[48]
`
`One of the densest (together with stishovite)
`polymorphs of silica; is produced at pressures
`above 40 GPa.[49]
`
`cubic (cP*, P4232,
`
`melanophlogite No.208)[14] or
`tetragonal (P42/nbc)[50]
`
`Si5-O10, Si6O12 rings; mineral always found
`
`with hydrocarbons in interstitial spaces - a
`c1athrasi1[51]
`
`fibrous
`
`orthorhombic
`
`W-silica[8]
`
`0112, Ibam, No.72[52]
`
`1.97
`
`Like SiS2 consisting of edge sharing chains,
`melts at ~l700 K
`
`hexagonal
`
`Sheet-like bilayer structure
`
`‘Fm
`I-"I":-"I"'I-‘I"t-'1':-‘i
`‘I I‘ II II
`I‘
`K"a.K".n.H"a.%"a.‘u"
`
`See also
`
`I Fused quartz
`I Mesoporous silica
`
`I Thermal oxidation
`
`I Silicon carbide
`
`References
`
`Elm Exhibit 2163, Page 10
`
`

`
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