Unit2 Silicate Structures无机非金属专业英语.doc

Unit2 Silicate StructuresAtomic 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 as corners are shared in several ways. There are four general types. In orthosilicates, SiO44-, tetrahedra are independent of one another; in pyrosilicates ,Si2O76- ions are composed of two tetrahedra with one corner shared; in metasilicates, SiO3(SiO3)n, two corners are shared to form a variety of ring or chain structures; in layer structures,(Si2O5)n, layers are made up of tetrahedra with three shares 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 combining tetrahedral groups with all corners shared. Three basic structuresquartz、tridymite、cristobaliteeach exists in two or three modifications . The most stable forms are low quartz , below 573C ;high quartz ,573 to 867C;high tridymite,867 to 1470C;high cristobalite ,1470 to 1710C;and liquid ,above 1710C;and liquid , above 1710C .The low temperature modifications are distorted derivative structures of the basic hightemperature 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 hightemperature 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 hightemperature forms, tridymites(p=2.26)and cristobalite(p=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 Mg2+ in octahedral and Si4+ in tetrahedral sites. (From a coordination point of view this assembly can also be considered an array of SiO4 tetrahedra with Mg2+ ions in the octahedral holes) .Each oxygen ion is coordinated with one Si4+ and three Mg2+ or with two Si4+. The structure of kyanite, Al2SiO5, consists of nearly cubic close-packed oxygen ions with Si4+ in tetrahedrl and Al3+ in octahedral sites. However, in the polmorphic forms andalusite and sillimanite have much more open structures, with SiO4 tetrahedra coordinated with AlO6 octahedral. Mullite, Al6Si2O13 , a common constituent of fired clay, has a structure similar to that of sillimanite (compare Al16Si8O40 and Al18Si6O39) .Pyrosilicate. Crystalline silicates containing Si2O76- ions are rare.Metasilicates. Silicates containing (SiO3)n2n- ions are of two types-cyclic or chain arrangements of the silica tetrahedra. Some of the discrete cyclic ions observed are the Si3O96- (such as in wollastonite, CaSiO3) and Si6O1812- (in bery1,BeAl2Si6O18) ions. Minerals with chain structures comprise a large group. Those with compositions corresponding to single chain,(SiO3)n2n-, are the pyroxenes , and those with double chains,(Si4O11)n6n-,the amphiboles. The pyroxenes include enstatite, MgSiO3 ; dropsied ,MgCa(SiO3)2; spodumene,LiAl(SiO3)2; and jadeite .The amphiboles include tremolite ,(OH)2CaMg5(Si4O11)2, in which isomorhic 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 Al3+ replacing some of the Si4+ 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.硅酸盐结构单元原子排列在数以百计的硅酸盐有复杂的化学成分的基本结构一个美丽的简单性和秩序。在同一时间的细节,许多硅酸盐结构复杂和难以说明没有三维模型,我们不会试图给精确的结构信息。为硅氧的半径比为0.29,对应于四面体协调,和四个氧离子几乎总是排列在一个中央硅。与粘结强度为1、氧离子可能只有两个硅原子协调在石英;这低配位数让拥挤不堪的结构不可能在通用硅酸盐二氧化矽,和有更多的开放结构比前面所讨论的。可以链接的tetrahedra SiO4在化合物如角落在几个方面是共享的。通常有以下四个类型。在orthosilicates,SiO44 -,tetrahedra都是相互独立的;在pyrosilicates,Si2O76 -离子是由两个tetrahedra与一个角落里,SiO3共享;在偏硅酸盐(SiO3)n,两个角落是共享,形成多种环或链结构;在层结构,(Si2O5)n,层是由三股tetrahedra角落;在各种形式的二氧化硅,二氧化硅,四个角是共享的。二氧化硅。结晶二氧化硅,二氧化硅,存在于几个不同的多态形式对应的不同组合方式四面体组织与所有角落共享。三种基本结构石英、鳞石英、方石英每个存在于两个或三个修改。最稳定的形式是低石英,低于573 c;高石英,573到867 c;高鳞石英,867到1470 c;高方石英,1470到1710 c;液体,超过1710 c;液体,超过1710 c。低温的修改是扭曲的衍生品结构基本高温形式。(一个衍生品结构在某种意义上是一个由一个基本结构,通过扭曲更对称结构在空间而不是替换不同的化学物种)我们把我们的注意力基本高温形式。高石英具有结构可看作是由连接链硅tetrahedra。密集结构相比上一节中讨论的,这是一个相对开放的结构,例如,石英的密度是2.65克/立方厘米,而采用3.59和3.96氧化铝。然而,石英具有较大的密度和更紧密的包装比前两种形式,tridymites高温(p = 2.26)和方石英(p = 2.32)Orthosilicates。这群包括橄榄石矿物(镁橄榄石,Mg2SiO4,固体的解决方案与Fe2SiO4)、石榴石、锆石、硅酸铝蓝晶石、红柱石、莫来石silimanite,。镁橄榄石的结构,Mg2SiO4,类似于发现,Al2BeO4金绿玉。氧离子几乎是在一个六角拥挤不堪的结构与Mg2 +在八面体和四面体网站+在李四吧。(从一个协调的观点这组装也可以看作是一个数组的tetrahedra SiO4与Mg2 +离子的八面体孔)。每个氧离子是配合一个李四吧+和三个Mg2 +或+两个李四吧。蓝晶石的结构,Al2SiO5,包括近立方拥挤不堪的氧离子与李四吧+在+在八面体tetrahedrl和与网站。然而,在polmorphic形式红柱石、硅线石有了更多的开