Unit 2 Silicate Structures
Atomic arrangement in hundreds of silicates having complex chemical compositions have in their basic structures a beautiful simplicity and order. At the same time the details of many of the silicate structures are complex and difficult to illustrate without three-dimensional models, and we will not attempt to give precise structure information.
The radius ratio for Si-O is 0.29, corresponding to tetrahedral coordination, and four oxygen ions are almost invariably arrayed around a central silicon. With a bond strength of 1, oxygen ions may be coordinated with only two silicon atoms in silica; this low coordination number makes close-packed structures impossible for SiO2, and in general silicates have more open structures than those discussed previously. The SiO4 tetrahedra can be linked in compounds such that corners are shared in several ways. There are four general types. In orthosilicates, SiO4[4-], tetrahedra are independent of one another; in pyrosilicates, Si2O7[6-], ions are composed of two tetrahedra with one corner shared; in metasilicates, SiO3[2-] (SiO3)n[2n-], layers are made up of tetrahedra with three shared corners; in the various forms of silica, SiO2, four corners are shared.
Silica
Crystalline silica, SiO2, exists in several different polymorphic forms corresponding to different ways of combing tetrahedral groups with all corners shared. Three basic structures---quartz, tridymite, cristobalite---each exists in two or three modifications. The most stable forms are low quartz, below 573C; high quartz, 573C to 867C; high tridymite, 867C to 1470C; high cristobalite, 1470C to 1710; and liquid above 1710C. The low temperature modifications are distorted derivative structures of the basic high-temperature forms. (A derivative structure in the sense is one that can be derived from a basic structure of greater symmetry by distorting the structure in space rather than substituting different chemical species) We confine our attention to the basic high-temperature forms.
High quartz has a structure which can be viewed as composed of connected chains of silica tetrahedra. Compared to the close packed structures discussed in the last section, this is a relatively open structure; for example, the density of quartz is 2.65g/cm3, compared with 3.59 for MgO and 3.96 for Al2O3. However, quartz has a greater density and closer packing than either of the high-temperature forms, tridymite (rho=2.26) and cristobalite (rho=2.32).
Orthosilicates
This group includes the olivine minerals (forsterite, Mg2SiO4, and solid solutions with Fe2SiO4), the garnets, zircon, and the aluminosilicates-kyanite, silimanite, andalusite, and mullite. The structure of forsterite, Mg2SiO4, is similar to that found for chrysoberyl, Al2BeO4. The oxygen ions are nearly in a hexagonal close-packed structure with Mg[2+] in octahedral and Si[4+] in tetrahedral sites. (From a coordination point of view this assembly can also be considered an array of SiO4 tetrahedra with Mg[2+] ions in the octahedral holes). Each oxygen ion is coordinated with one Si[4+] and three Mg[2+] or with two Si[4+].
The structure of kyanite, Al2SiO5, consists of nearly cubic close-packed oxygen ions with Si[4+] in tetrahedral and Al[3+] in octahedral sites. However, in the polymorphic forms and alusite and sillimanite have much more open structures, with SiO4 tetrahedra coordinated with AlO6 octahedra. Mullite, Al6Si2O13, a common constituent of fired clay products, has a structure similar to that of sillimanite (compare Al16Si8O40 and Al18Si6O39).
Pyrosilicates
Crystalline silicates containing Si2O7[6-] ions are rare.
Metasilicates
Silicates containing (SiO3)n[2n-] ions are of two types-cyclic or chain arrangements of the silica tetrahedra. Some of the discrete cyclic ions observed are the Si3O9[6-] (such as in wollastonite, CaSiO3) and Si6O18[12-] (in beryl, Be3Al2Si6O18) ions. Minerals with chain structures comprise a large group. Those with compositions corresponding to single chain, (SiO3)n[2n-], are the pyroxenes, and those with double chains, (Si4O11)n[6n-], the amphiboles. The pyroxenes include enstatite, MgSiO3; diopside, MgCa(SiO3)2; spodumene, LiAl(SiO3)2; and jadeite. The amphiboles include tremolite, (OH)2Ca2Mg5(Si4O11)2, in which isomorphic substitution is widespread. The asbestos minerals are amphiboles.
Framework Structures
Many important silicate structures are based on an infinite three-dimensional silica framework. Among these are the feldspars and the zeolites. The feldspars are characterized by a framework formed with Al[3+] replacing some of the Si[4+] to make a framework with a net negative charge that is balanced by large ions in interstitial positions, that is, albite, NaAlSi3O8; anorthite, CaAl2Si2O8; orthoclase, KAlSi3O8; celsian, BaAl2Si2O8; and the like. The network structures are similar in nature to the cristobalite structure, with the alkali or alkaline earth ions fitting into interstices. Only the large positive ions are from feldspars; smaller ones that enjoy octahedral coordination are from chains or layer silicates.
Much more open alumina-silica frameworks occur in the zeolites and ultramarines. In these compounds the framework is sufficiently open for there to be relatively large channels in the structure. The alkali and alkaline earth ions present can be exchanged in aqueous solutions, leading to there use as water softeners. In addition, these channels can be used as molecular sieves for filtering mixtures on the basis of molecular size. The size of the channels in the network depends on the composition.
Selected from "Introduction to Ceramics" 2nd Edition, W.D.Kingery, John. Wiley&Sons, 1976
Words and Expressions
close-packed structure 紧密堆积结构
orthosilicate n.正（原）硅酸盐
pyrosilicate n.焦硅酸盐
metasilicate n.偏硅酸盐
polymorphic a.多晶的
tridymite n.鳞石英
cristobalite n.方石英
derivative a.衍生的
modification n.改变
olivine n.橄榄石
forsterite n.镁橄榄石
garnet n.石榴石
zircon n.锆石
kyanite n.蓝晶石
sillimanite n.硅线石
andalusite n.红柱石
mullite n.莫来石
chrysoberyl n.金绿宝石
hexagonal a.六方晶系的
wollastonite n.硅灰石
beryl n.绿柱石
pyroxene n.辉石
amphibole n.闪石
enstatite n.顽火辉石
diopside n.透辉石
spondumene n.锂辉石
jadeite n.硬玉
tremolite n.透闪石
isomorphic a.同晶型的
asbestos n.石棉
feldspar n.长石
zeolite n.沸石
interstitial a.间隙的
ultramarine n.a.深蓝色（的）
