Condensation silicate

A. Determination of silica in a soluble silicate Discussion. Most of the silicates which come within the classification of soluble silicates are the orthosilicates formed from SiO units in combination with just one or two cations. More highly condensed silicate structures give rise to the insoluble silicates. 

Parquet (layered) H Si40io 4 T ale—Mg3Si40i o(OH)2 Mica—KAl3Si30io(OH)2 Kaolinite—Al2Si205(0H)4 (Condensed) Silicic acid—H2S1205... 

Stephens, J. R., 1980. Visible and ultraviolet (800-130 nm) extinction of vapor-condensed silicate, carbon, and silicon carbide smokes and the interstellar extinction curve, Astrophys. J., 23H, 450-461. 

We often ask the question Why does such and such a compound exist but more rarely do we ask the question Why does such and such a compound not exist , yet the answer to the second question may be more revealing than the answer to the first. A number of years ago, Dent-Glasser (1979) published a paper entitled Non-existent silicates in which she noted that there were many stoichiometrically allowed silicate structures that had never been observed. In particular she pointed out that the alkali metals rarely formed orthosilicates such as Na4Si04, and transition metals rarely formed highly condensed silicates such as NiSi40g. Dent-Glasser s list of known silicates is shown in Fig. 4.6... 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

The inner surface of the silicalemma, i.e., the limiting membrane within which silica deposition occurs549 consists of a protein template enriched in serine and threonine. This protein will present a layer of hydroxyl groups which can undergo condensation reaction with silicic acid molecules with a consequent loss of water (Fig. 48). As a result, the initial layer of condensed silicic acid will be held fixed to the protein template of silicic acid (Fig. 49). Such a situation is kinetically more favorable than simply allowing the silicic acid molecules to come together by random collision. 

The simplest of the condensed silicate anions are formed by combining two or more Si04 tetrahedra by sharing oxygen atoms. The pyrosilicate anion, 51207 ", is present in thortveitite (Sc2Si207) and hemimorphite (Zn4(0H)2Si207). 

Previously, we have speculated on the possibility that some particularly highly condensed silicate anions, the double-n-ring (DnR) silicates, may be likely candidates for zeolite precursors (2,4,11). On the basis of an observed relationship between the extent of depolymerization of D4R silicates in the synthesis gel and the structure of the zeolites obtained from that gel, a possible precursor role of the D4R silicate has been discussed fll l. ... 

Generally the first step is the preparation of the reaction mixture at low temperature (< 60°C). The different ingredients are mixed in this step, which in most cases results in the formation of the so-called synthesis gel. In this gel silicate and aluminate monomers and oligomers in solution are in equilibrium with condensed silicate and aluminate units in the gel phase. In some cases a digestion period is necessary to reach this equilibrium. 

The IR absorption spectra of CjS, alite or P-C S, pastes (H29.L22,S44) are relatively diffuse, as would be expected from the poor crystallinity. Si-0 stretching is represented by a single, broad band peaking at about 970 cm this is compatible with the existence of condensed silicate anions. Small amounts of carbonate are readily detectable by the absorption near 1400 cm . 

Most of the minerals which constitute rocks and soil are silicates. Many of these minerals have complex formulas, corresponding to the complex condensed silicic acids from which they aie derived. These minerals can be divided into three principal classes, the framework minerals (hard minerals similar in their properties to quartz), the layer minerals (such as mica), and the fibrous minerals (such as asbestos)... 

Minerals with Layer Structures. By a condensation reaction involving three of the four hydroxyl groups of each silicic acid molecule, a condensed silicic acid can be made, rvith composition (H.SisOs), which has the form of an infinite layer, as shown in Figure 31-4. The... 

Phosphate minerals, like silicate minerals, are found with a great variety of cations. Unlike the latter group, which contains numerous types of condensed silicate anions, almost all phosphate minerals are orthophosphates which contain the PO " anion. 

A concentrated solution of sodium or potassium silicate with SiOjiMjO molar ratios of 1 1 to 1 2 consists mainly of SiOa " and HSiO ions. However, in solutions of higher ratios it is proposed that an increasing proportion of the silica forms extremely small three-dimensional, internally condensed silicate polymer-ions or charged particles. These are generally equiaxed or spheroidal in shape. The silicon... 

A gelatinous precipitate of condensed silicic acids (SiO JCH2O) is obtained when an ordinary acid, such as hydrochloric acid, is added to a solution of sodium silicate. When this precipitate is partially dehydrated it forms a porous product called silica gel. This material has great powers of adsorption for water and other molecules and is used as a drying agent and decolorizing agent. 

The P—O—P link in polyphosphates is readily hydrolysed in excess water, metaphosphates revert to orthophosphate. So, unlike the condensed silicates, polyphosphates are never found as minerals. 

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