Silica minerals, structure

Silica minerals are a primary mineral classified as tectosilicates, characterized by repeating SiO units in a framelike stracture. Quartz, one of the most abundant minerals on earth, often comprises up to 95% of aU sand and silt fractions. It therefore is representative of the structure and properties of sihca minerals. 

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

Stishovite. Stishovite was first prepared (68) in the laboratory in 1961 at 1200—1400°C and pressures >16 GPa (158,000 atm). It was subsequendy discovered, along with natural coesite, in the Arizona meteor crater. It has been suggested that these minerals are geological indicators of meteorite impact structures. Stishovite (p = 4.35 g/cm3) is the densest known phase of silica. The structure, space group P42/nmn, is similar to that of... 

The crystal structure of the natural mineral tobermorite was partly worked out by Megaw and Kelsey (22). Taylor and coworkers (15, 17, 26, 27) have carried out detailed investigations of tobermorite prepared synthetically, and correlated their structural features with those of the natural mineral structure. However, little was known about the nature of the surface, except what may be deduced from the surface energy results, the surface energy being approximately the geometric mean of the surface energies of calcium hydroxide and hydrous amorphous silica. 

Is there any simple way to relate mineral structure to solubilities and release rates Will this ranking allow us to suggest the order of importance of these minerals with respect to input or removal of silica to the silica budget ... 

Crystalline silica includes the silica minerals quartz and its polymorphs such as cristobalite and tridymite, which have the same chemical formula but different crystal structures. Amorphous, noncrystalline silica can also occur in a wide variety of geologic environments (Ross, 1999). 

Florke, O.W., Graetsch, B., Martin, B., Roller, K. Wirth, R. (1991) Nomenclature of micro- and non-crystalline silica minerals, based on structure and microstructure. Neues Jahrbuch filr Mineralogie, Abhandlungen 163 19-42. 

Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. In Heaney, P.J., Prewitt, C.T. Gibbs, G.V. (Eds) Silica Physical Behaviour, Geochemistry and Materials Applications. Reviews in Mineralogy 29, Washington Mineralogical Society of America, pp. 209-232. 

Mineral structures are composed in an prdered fashion from silica and alumina .etrahedra (MO4), octahedra of Al, Fe, and Mg (MOg), and various other polyhedra w hQxe M symbolizes the metal cation). They form according to certain rules, deduced by Pauling, that minimize electrostatic repulsion forces and maximize attractive forces. These Pauling rules have been stated as follows ... 

The principal primary mineral groups are the silica minerals (including quartz), feldspars, feldspathoids, olivines, pyroxenes, amphiboles, and micas. All are silicates and can be classified structurally based on the arrangement of connected silica (Si04) tet-rahedra. Table 2.2 summarizes the classification system for these common rockforming minerals. 

The mineral structures will be described briefly, progressing from the least to most complex arrangement of silica tetrahedra. 

Feldspars. Like the silica minerals, feldspars are framework silicates with all tetrahedral corners shared. By replacing one in four of the Si atoms by Al, the anionic framework Si3A10g is obtained from Si02 If the framework charge is balanced by structural K, the formula of potassium feldspar, KSisAlOg, is obtained. This feldspar has two common structural polymorphs (different structures with the same chemical formula), orthoclase and microcline. If the charge is balanced by structural Na, the feldspar obtained is albite, NaSi3A10g. 

Steps 1, 2, and 3 account for the initially high rate of dissolution of alkali and alkaline earth metals (Na", K, Ca ) from feldspars, and the relative lack of silica and aluminum dissolution. This is termed incongruent (nonstoichiometric) dissolution, meaning that a portion of the mineral structure is dissolving selectively, leaving a residue enriched in silica and alumina. Step 4 accounts for the later stage of congruent (stoichiometric) dissolution, in which the elements are released into solution in proportion to their mole fractions in the structure. 

Silica, alumina, iron, and the various base cations that are dissolved by primary mineral weathering can precipitate as new low-temperature minerals—a process known as neoformation. This should be distinguished from weathering processes, termed alteration, in which part of the parent mineral structure is inherited by the weathering product. Examples of alteration are given in the previous section. 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

Various publications describe processes in which the raw material, before carbonization, is incorporated with mineral structures such as kieselguhr,65 silica,66 clay,67 bone.68 In some cases, the purpose is to provide a more rigid structural support for the carbon grains in others, it is to ensure advantages of two complementary adsorbents and in still others it is to improve the rate of filtration. Few, if any, of these specialties are produced on a commercial scale. 

Mn(IV) mineral will mask the detection of the Mn(III) or Mn(II) reduction products. This was the case for experiments performed with a lOx higher concentration of Mn02 coated silica (1.0% vs. 0.1% coating) and Co(II)EDTA (2.0 mM vs. 0.2 mM). Even though the ratio of the solid phase Mn and the reductant were constant, Mn reduction products were not detected due to the overwhelming signal of the Mn(IV) (data not shown). These sets of experiments add credibility to the notion that oxidation of Co(II)EDTA by Mn-oxides is a surface-mediated process that is restricted to the outer molecular layers of the mineral structure. 

In this context, we performed first-principles electronic structure calculations of a small cluster (Si04) and analyzed potential energy surfaces (PES s) corresponding to some deformation modes of the cluster. The interatomic potential derived from the PES s, which is ionic and pairwise, has been found to reproduce most structural properties of silica minerals quite satisfactorily against expectation. 

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