Disilicate ion

The structure formed by two tetrahedral units joined at a corner is known as the pyrosilicate or disilicate ion (see Figure 14.1b). 

Draw a Lewis electron-dot diagram for the disilicate ion (Si207 ). What changes in this structure would be necessary to produce the structure of the pyrophosphate ion (P207 ) and the pyrosulfate ion (8207 ) What is the analogous compound of chlorine ... 

According to the above equations, the disilicate ions SijO and HSiaO " should also be present in much higher concentrations than HSiOa". Since it is well known that disilicic acid reacts completely with molybdic acid in less than 5 min, the total silica reacting in 5 min should have been much higher than observed if the equations relating to disilicate ions were applicable to the present system. 

It is therefore concluded that Roller s and Ervin s (27) observations regarding the formation of disilicate ions were valid only in the calcium silicate system in which their studies were made. It would not be surprising to find that the divalent calcium ion promotes the formation of complex disilicate ions in which Si-O-Ca linkages are involved. 

Utilizing this description of silicate glass formation, we suggest models to explain the incorporation of 4+ and 6+ actinide ions in sodium disilicate glass. During cooldown,... 

Figure 1. Structures of (a) orthosilicate, (b) disilicate, (c) cyclotrisilicate, and (d) cyclohexasilicate ions.

The rule reqiiires that in silicates the silicon tetrahedra share no elements with each other if the oxygen-silicon ratio is equal to or greater than four. This is found t o be vrue in general (topaz, zircon, olivine, other orthosilicates), Most of the few exceptional substances now known contain extra oxygen atoms in the form of hydroxide ions. These include the clay minerals, micas, and chlorites, discussed above, and also the mineral hemimorphite, Zn[Pg.561]

Lochhead and Bray (1995) studied Eu3+ doped sodium disilicate glass with a high-pressure fluorescence line-narrowing technique. This technique was used to characterize the local structure of the Eu3+ ions up to a pressure of 21 GPa. For the crystal-field analysis they assumed a C2v site symmetry which allowed for a complete splitting of the crystal-field components. The crystal-field strength was determined according to eq. (11). The effect of pressure... 

Fig. 16.1 Various silicate structures (a) Si04 tetrahedron. When carrying a -4 charge, this is the orthosilicate ion. (b) The disilicate anion, (c) Portion of an infinite single chain. (SiOdw""- (d) Portion of an infinite double chain or band. (Si40u] . (e) Portion of a sheet or layer structure, [SiiOsl, " .

Minerals containing the pyrosilicate or disilicate anion, Si O (Fig. 16.1b). are not common thortveitite, Sc SiiO. hemimorphite, Zn4(OH)2Si 07-H2O (does not contain discrete SijO ions), and barysilite, MnPbxISiiOy), as well as vesuvianite and epidote which contain both SiOj" and SiiO " ions. Linear tri- and tetrasilicates are almost unknown. 

Figure 5 shows that the solubility of amorphous silica is independent of pH between 4 and 9 above pH 9 the solubility increases because of the formation of monosilicate, disilicate, and multimer ions. DS coatings are deposited at 90 °C—equilibrium constants are not available at this temperature. Silica solubility data are available (24), and hence it is... 

---