Solubility products silicate minerals

Some silicate minerals are also formed in a similar manner. The process is very slow, slower than even carbonate formation, because of the very low solubility of silicate minerals. In clay minerals, or in lateritic soils, silicates dissolve very slowly to form an intermediate product, silicic acid (H4Si04), which subsequently will react with other sparsely soluble compounds and form silicate bonding phases. Thus, a dissolution-precipitation process seems to be crucial to forming some silicate minerals. 

Dissolution of CaCOs is a congruent reaction the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of CO2 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach... 

Silicate minerals, 25 56t Silicate modifications, via silylation, 22 696 Silicate products, prices of, 22 468t Silicates, 26 145-146, 5 640. See also Anthropogenic silicas and silicates commercial soluble, 22 451-452... 

Loughnan (1969) discussed the solubility in relation to pH of some of the common products of chemical weathering of silicate minerals, In general, the hydroxides of Na, K and Ca are soluble at all pH s, and Mg(OH)2 is soluble at pH < 10. Aluminium oxide is soluble at pH s < 4 and >10, whereeis SiOj is slightly soluble at pH < 9 and increasingly soluble at higher pH values. Titanium hydroxide is soluble at pH < 5, but TiOz is soluble only at pH < 2. The hydroxide of trivalent iron is soluble only below pH 2.5, but Fe(OH)2 is soluble below about pH 8.5. 

Many common substances—notably silicates, some mineral oxides, and a few iron alloys—are attacked slowly, if at all, by the methods just considered. In such cases, recourse to use of a fused-salt medium is indicated. Here, the sample is mixed with an alkali metal salt, called the jliLX, and the combination is then fused to form a water-soluble product called the melt. Fluxes decompose most substances by virtue of the high temperatures required for their use (300°C to 1000 C) and the high con-eentrations of reagents brought into contact with the sample. 

This concept may be used to derive simple solubility expressions for silicate minerals in melts. Consider, for example, the crystallization of enstatite from a melt. Since the structvire of enstatite can only accommodate middle groups the "solubility product" for enstatite in a silicate melt can be written in the form... 

Thus, by combining what is already known of the nearly ideal mixing properties of silicate structural units in the melt (Masson, 1972) with the requirement that crystallizing mineral phases can only accommodate certain of those present in the polymers, it may be possible to construct tables of approximate solubility products for silicate minerals in melts similar to those for salts in aqueous solution. 

As hydrolysis of the silicates and aluminosilicates continued (equation 1), dissolved sodium ions were continually produced, and acid was consumed. The pH of the water gradually increased. Formation of kaolinite was replaced by formation of montmoril-lonite and finally by production of illite. Silica was more soluble than alumina and as aluminosilicates were attacked by the water a protective coating of A1(0H) formed around the particles. This coating helped transport mineral particulates to the sediment. The reactions indicated by equation 1 were replaced by the family of reactions indicated in equation 2, written using the hydrolysis of albite as an example. 

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