Muscovite structure

The average Al203/Mg0 ratio for 24 illites is 9.6 and for 101 montmorillonites 6.7. Attapulgite values range from 2.5 to 0.48. The ratios of octahedral Al/octahedral Mg are respectively 5.4, 4.3 and 1.8-0.4. Radoslovich (1963b) found that the 2M muscovite structure required a minimum of 1.7 of the three octahedral sites be filled with Al. The Al occurs in the two symmetrically related sites and the larger divalent cation occurs in the distinctive or unoccupied site. The lower limit of 1.7 Al is equivalent to 85% of the two symmetrically related or occupied sites being filled in a stable muscovite structure. A similar restriction is reported for the trioctahedral micas where an upper limit of 1.00 (R3++ R4+)per three sites was found by Foster (1960). 

For 24 illites an average of 1.53 Al per three octahedral sites was found. This is 77% occupancy of the two occupied sites. Total trivalent ions (A1+ Fe3+) averaged 1.76. Thus, most illites have near the minimum or less than the minimum number of Al per three sites required to maintain a stable 2M, muscovite structure. 

I-inure 12. The proton- and oxalate-promoted dissolution of muscovite. The slow weathering kinetics is a characteristic of micas. Oxalate affects the stoichiometry of A1 and Si release, but has not i significant catalytic effect. Figure 12c displays a schematic representation of the muscovite structure. It reveals the 2 1 structure. For example, an A1 layers (black) exists in an octahedral sheet between two tetrahedral sheets (white) whose cations are composed or25% A1 and 75% Si. Siioxane and edge surfaces are exposed lo solution. 

Configurational entropy due to Al/Si disordering in muscovite structure ignored ( A1/Si ordered )... 

Preferred value (Robie et al.,1976, see Table I), includes an ideal molar configurational entropy of I8.7O J/ K due to disordering of tetrahedral Al and Si in muscovite structure, t. O... 

Figure 3. The muscovite structure, viewed along the a axis. Approximately one-half of the unit cell is shown. Open circles in increasing size indicate, respectively, silicon, aluminium, tetrahedral oxygen, octahedral oxygen, and potassium. Hatched circles are hydroxyls (from Brindley and MacEwan [1953]). (Quoted in Brown [1961].)...

Brown, J. L., and C. I. Rich, 1968. High resolution microscopy of muscovite structure. Science 161 1135-1137. 

Silicates with layer. structures include some of the most familiar and important minerals known to man, partieularly the clay minerals [such as kaolinite (china clay), montmorillonite (bentonite, fuller s earth), and vermiculite], the micas (e.g. muscovite, phlogopite, and biotite), and others such as chrysotile (white asbestos). 

Figure 9.12 Schematic representation of the structures of muscovite mica, (K2Al4(Si6Ali)02o(OH)4], hydrated montmorillonite, [Al4Sig02o(OH)4].xH20 and chlorite, (MgioAl2(Si6Al2)02o(6H)i6], see text.

Stimulated by these observations, Odelius et al. [73] performed molecular dynamic (MD) simulations of water adsorption at the surface of muscovite mica. They found that at monolayer coverage, water forms a fully connected two-dimensional hydrogen-bonded network in epitaxy with the mica lattice, which is stable at room temperature. A model of the calculated structure is shown in Figure 26. The icelike monolayer (actually a warped molecular bilayer) corresponds to what we have called phase-I. The model is in line with the observed hexagonal shape of the boundaries between phase-I and phase-II. Another result of the MD simulations is that no free OH bonds stick out of the surface and that on average the dipole moment of the water molecules points downward toward the surface, giving a ferroelectric character to the water bilayer. 

Why might one be specifically interested in the chemistry of a particular species in soil From a positive perspective, it is desirable to provide plants with nutrients in forms that are available and yet are not present in concentrations high enough to cause environmental harm. Potassium ions are found in the soil solution and on exchange sites. Both species are available to plants. It may also be part of the mineral structure of soil, as in muscovite,... 

The minerals of the pyrophyllite [(Al2Si40io(OH)2)]-ferripyrophyllite [(Fe 2Si40io(OH)2)] series are related to the mica group. The structure of these minerals closely approximates that of muscovite with the two tetrahedral sheets and octahedrally coordinated intercalated ions, a 2 1 layering (Fig. 2.13) but there is little substitution of Si " by Al Only two-thirds... 

The problem with limited selectivity includes some of the minerals which are problems for XRD illite, muscovite, smectites and mixed-layer clays. Poor crystallinity creates problems with both XRD and FTIR. The IR spectrum of an amorphous material lacks sharp distinguishing features but retains spectral intensity in the regions typical of its composition. The X-ray diffraction pattern shows low intensity relative to well-defined crystalline structures. The major problem for IR is selectivity for XRD it is sensitivity. In an interlaboratory FTIR comparison (7), two laboratories gave similar results for kaolinite, calcite, and illite, but substantially different results for montmorillonite and quartz. 

Conformations and aggregate structures of sorbed natural organic matter on muscovite and hematite. Geochim. Cosmo-chim. Ada 65 1047-1057 Nanzyo, M. Watanabe,Y. (1981) Material balance associated with phosphate sorption by amorphous clays, silica-alumina gel and synthetic goethite. Soil Sd. Plant Nutr. 27 329-337... 

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