Minerals, silicate

Silicate minerals frequently contain iron. The presence of chains, layers, or networks of sihcon bonding to oxygen is one of the most significant structural features. Electric neutrality is maintained by including the appropriate number of metal cations, usually in sites with fourfold or sixfold coordination. 

The rigidity of the basic silicate structures exerts a greater controlled influence on cation site symmetries than do the individual cation charges, an important difference from the oxides where the anions have more freedom of movement. The ease with which Fe and Fe + cations can be distinguished and site occupancies determined in oxides by Mossbauer spectroscopy suggests a logical extension to similar studies in silicates, and considerable progress has been made in this direction. 

The application of MOssbauer spectroscopy to silicate mineralogy has been well described in two papers by Bancroft et al, in which the influences of electronic configuration, oxidation state, and coordination symmetry of the iron cations were correlated firstly with silicates of known structures [212], and latterly with silicates of unknown and complex structures [213]. Most of the data discussed here are taken from these works, but references are given where appropriate to other data available. Silicate minerals of lunar origin are discussed on p. 294. 

Silicate minerals provide a wide range of site symmetries, some of which are not usually found in conventional oxide systems, and frequently the phases have wide ranges of composition enabling studies of cation substitution effects. The principal types of silicate are listed in Table 10.3. 

Name of mineral or series Formula Oxidation S (Fe) state of Fe /(mm s ) A/(mm s ) Coordination number of Fe 

For a fuller discussion of the effect of the various silicate structural groups on the Si chemical, the reader is referred to Engelhardt and Michel (1987). 

A list of the reported chemical shift for other inorganic silicates is given in Table 4.3. 

Howie and J. Zussman, An Introduction to the Rock-forming Minerals, Longmans, London, 1966, 528 pp. B. Mason and L. G. Berry, Elements of Mineralogy. W. H. Freeman, San Francisco, 1968, 550 pp. 

Liebau, Silicon, element 14, in K. H. Wedepohl (ed.). Handbook of Geochemistry, Vol. 11-2, Chap. 14, Springer-Verlag, Berlin, 1978. F. Liebau, Structural Chemistry of Silicates, Springer-Verlag, Berlin, 1985, 347 pp. 

The quoted radii, whieh in turn depend on the CN, are the empirieal effeetive ionie radii dedueed by R. D. Shannon (and C. T. Prewitt) and do not imply full eharge separation sueh as Si (0 )4, ete. Note that AF ean oeeupy either 4- or 6-eoordinate sites so that it ean replaee either Si or M in the lattiee — this is partieularly important in diseussing the struetures of the aluminosilieates. Several other eations ean oeeupy sites of differing CN, e.g. Li (4 and 6), Na (6 and 8), K (6-12), though they are most eommonly observed in the CN shown. 

As with the borates (p. 205) and to a lesser extent the phosphates (p. 526), the Si04 units ean build up into ehains, multiple ehains (or ribbons), rings, sheets and three-dimensional networks as summarized below and elaborated in the following paragraphs. 

Acio-silicates discrete (Si04) no 0 atoms shared 

The quoted radii, which in turn depend on the CN, are the empirical effective ionic radii deduced by R. D. Shannon (and 

Quartz is the dominant form in which silica is found in coals, and it is ubiquitous. There is some distinction between clastic grains of quartz introduced by wind or water and authigenic quartz deposited from solutions. Quartz is also a major component of clay and siltstone partings in coal that are of detrital origin. 

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Silicon makes up 25.7% of the earth s crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous silicate minerals. 

Silicate [12627-13-3] Silicate adhesives Silicate esters Silicate grouts Silicate minerals Silicates... 

Many other silicate minerals contain zirconium as a constituent (164,165). These minerals may be altered or metamict zircons such as cyrtoHte or malacon. 

C. E. Marshall, The Colloid Chemistry of the Silicate Minerals, Academic Press, Inc., New York, 1949. 

Asbestos, V natural fibrous form of several silicate minerals of the following types. 

Some commonly used insulating materials are calcium silicate, mineral slagwool, glass fiber, cellular glass, and polyurethane. 

The structural complexity of borate minerals (p. 205) is surpassed only by that of silicate minerals (p. 347). Even more complex are the structures of the metal borides and the various allotropic modifications of boron itself. These factors, together with the unique structural and bonding problems of the boron hydrides, dictate that boron should be treated in a separate chapter. 

Asbestos It is not the name of a distinct mineral species but is a commercial term applied to fibrous varieties of several silicate minerals such as amosite and crocido-lite. These extremely fine fibers are useful as fillers and/or reinforcements in plastics. Property performances include withstanding wear and high temperatures, chemical resistance, and strengths with high modulus of elasticity. When not properly handled or used, like other fibrous materials, they can be hazardous. 

Many studies have been made of the rates of water evolution from layer-type silicate minerals which contain structural hydroxyl groups (clays and micas). Variations in composition of mineral specimens from different sources hinders comparison of the results of different workers. Furthermore, the small crystallite sizes and poor crystallinity that are features of clays limit and sometimes prevent the collection of ancillary observations (e.g. microscopic examination and diffraction measurements). 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

Layer-silicate structure, as in other silicate minerals, is dominated by the strong Si-O bond, which accounts for the relative insolubility of these minerals. Other elements involved in the building of layer silicates are Al, Mg, or Fe coordinated with O and OH. The spatial arrangement of Si and these metals with O and OH results in the formation of tetrahedral and octahedral sheets (see Fig. 8-2). The combination of the tetrahedral and octahedral sheets in different groupings, and in conjunction with different metal oxide sheets, generates a number of different layer silicate clays (see Table 8-1). 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

A variety of other structures are possible with silicate minerals, including sheets and three-dimensional frameworks. In all cases, the structure includes bonds that are predominantly covalent and directional. They can therefore be viewed as being based on increasingly crosslinked inorganic polymers. 

The eleetronic configuration of the group-IIA elements, [inert gas] ns, render them so reactive that they never occur native but are always combined with other elements. Thus, Be is found in complex silicate minerals Mg, Ca, Sr and Ba, however, occur in carbonate, sulfate or phosphate ores. Consequently, whereas the extractive metallurgy of Be is relatively complex, that for the other elements is quite straightforward. 

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