Silicate minerals structural organization

Search of structure. Wiley, New York, 764 pp Holdren GR Jr, Berner RA (1979) Mechanism of feldspar weathering. I. Experimental studies. Geochim Cosmochim Acta 43 1161-1171 Huang WH, Keller WD (1970) Dissolution of rock-forming silicate minerals in organic acids simulated first-stage weathering of fresh mineral surfaces. Am Mineral 55 2076-2094... 

The metal phases which are presumed to be sequentially extracted are step 1, soluble, exchangeable and carbonate-bound metals step 2, metals occluded in easily reducible manganese and iron oxides step 3, organically bound metals and sulphides step 4 , metals in non-silicate minerals lattice structure. It is important to emphasise that these metal phases are nominal target only, operationally defined by the extraction used. Consequently, is highly desirable and recommended to refer to the sequentially extracted metal fractions as easily extractable , reducible , oxidi-sable and residual , respectively. 

The importance of carbon in organic chemistry results from its ability to form carbon-carbon bonds, permitting complex molecules, with the most varied properties, to exist. The importance of silicon in the inorganic world results from a different property of the element —a few coiiipounds are known in which silicon atoms are connected to one another by covalent bonds, but these compounds are relatively unimportant. The characteristic feature of the silicate minerals is the existence of chains and more complex structures (layers, three-dimen sional frameworks) in which the silicon atoms are not bonded directly to one another but are connected by oxygen atoms. 1 he nature of these structures is described briefly in later sections of this chapter. 

NMR and FTIR results indicate that both the organic matter associated with carbonate minerals and that associated with silicate minerals is predominantly of paraffinic composition. In addition, both groups contain approximately equal relative concentrations of aromatic structures. The latter result is in disagreement with Spiro s (2) data which indicate that the organic matter associated with silicates contains more aromatic as well as more polar structures as compared with the organics associated with carbonate minerals. This apparent difference in aromatic... 

It was determined that approximately four times more bitumen-free organic matter is associated with silicate minerals than with carbonate minerals. The major structural constituent of the various filtrate fractions was paraffinic compounds. However, spectroscopic analysis indicated relatively larger concentrations of olefins and branched paraffins with the acid/ether extracted... 

Most analytical measurements are performed on solutions (usually aqueous) of the analyte. While some samples dissolve readily in water or aqueous solutions of the common acids or bases, others require powerful reagents and rigorous treatment. For example, when sulfur or halogens are to be determined in an organic compound, the sample must be subjected to high temperatures and potent reagents to rupture the strong bonds between these elements and carbon. Similarly, drastic conditions are usually required to destroy the silicate structure of a siliceous mineral and to free the ions for analysis. 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

Chromium in soils potentially occurs in the +3 (chromic) oxidation state as the Cr " cation, and in the -f 6 oxidation state as chromate, Cr04 . However, soil conditions generally favor the Cr form, a very immobile cation that complexes strongly with organic matter and chemisorbs on oxides and silicate clays, even at quite low pH. Furthermore, Cr readily substitutes for Fe in mineral structures, and precipitates as insoluble Cr(OH)3 at higher pH. The chromic form is, therefore, very immobile in most soils and generally unavailable to plants, at least if the soil is not exceedingly acid. 

As soils weather and Si, Ca, Mg, Na, and K are leached away, the soil s colloidal fraction becomes enriched with Al, Fe, Mn, and Ti oxides and hydroxyoxides. The structural organization of these hydroxyoxides ranges from amorphous to crystalline. These Al, Fe, and Ti oxides and allophane are prominent nonlayer silicate minerals in most soils and then- content in soils increases with increased weathering. 

With the exception of the mineral melanophlogite, all porosils are synthetic products. All porosils were directly synthesized with structure-directing agents (SDA) as templates, in general, organic molecules. There are a few more cases where postsynthesis treatment of alu-minosilicate zeolites leads to the complete dealumina-tion of silicate framework structures, yielding all-silica... 

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