Silicates containing chain ions

Single chains formed from tetrahedral MX4 groups sharing two vertices adopt various configurations in crystals. Although there are different numbers of tetrahedra in the repeat unit, namely, 1, 2, and 3 for the chains (a), (b), and (c) of Fig. 23.12 (described as IT, 2T, 3T, etc. chains) the composition is in all cases MX3 

Single and double chains formed from Si04 tetrahedra. 

NaAl(Si03)j, and spodumene, LiAl(Si03)2. Details of the chain in Na2Si03 are shown inset in the various crystals the parallel chains pack so as to provide suitable environments for the cations-6-coordination of 8-coordination of Ca (in diopside). 

Silicates M Si2 0s prefer the hexagonal layer to the double chain ion (see next section). Sillimanite is strictly an orthosilicate, for it contains discrete Si04 groups. However, since one-half of the A1 atoms are 4-coordinated it may alternatively be regarded as an aluminosilicate in which the Si atoms and these A1 atoms form double chains of type (d). The octahedrally coordinated A1 atoms lie between these chains, and the compound may be formulated Al(AlSi05). In the mineral world the most important compounds containing double chains are the amphiboles, of which 

Crystals containing single and double chain ions 

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Under the appropriate conditions, monomers occupy virtually the entire space of pores or the whole interlayer space. Subsequent oxidative polymerization was carried out in the presence of molecular oxygen (as an electron acceptor) and a redox-active host that catalyzes electron transfer. Layered silicates containing metal ions are of particular interest because they initiate polymerization of an intercalated monomer. Thus when Na ions in hectorite are replaced by Cu or Fe, styrene can be polymerized both in the pores and on the smface. The polymer has a brushlike structure, which indicates that the inorganic smface possesses an orienting effect. This effect decreases as the chain grows away from the smface. 

The silicate anion in the mineral kinoite is a chain of three Si04 tetrahedra that share corners with adjacent tetrahedra. The mineral also contains Ca2+ ions, Cu2+ ions, and water molecules in a 1 1 1 ratio. 

Two typical and important classes of chain silicates are the pyroxenes and the amphiboles, containing chains as represented schematically in Figure 17-4 the structure has been somewhat distorted to show all atoms. Note that in these chains also, aluminum ions may replace silicon ions if... 

It is helpful in the discussion to describe silicate structures using the Q nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO ] " are represented as Q and those fully connected to other Q units as Q. In general, minerals based on Q , Q and units are decomposed by acids. Such minerals are those containing isolated silicate ions, the orthosilicates, SiO (Q ) the pyrosilicates, Si O " (Q ) ring and chain silicates, (SiOg) (Q ). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2 3 when attack occurs at A1 sites, with scission of the network (Murata, 1943). 

Both sepiolite and palygorskite contain tetrahedral silicate sheets (with a variety of substitutions for the Si " ), but the apicies of the tetrahedra are thought to point up or down with the transition areas containing Ca and Mg ions, and bound or associated H2O. The arrangement produces a continuous basal oxygen plane that is compartmentalized. Laths or ribbons three chains wide in sepiolite and two chains wide in palygorskite are separated by discontinuous octahedral areas (Fig. 2.16A and B). 

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