Muscovite octahedral sheet

MgOn(OH)j -] units (Fig. 7.5), and the illite type in which the octahedral sheet is sandwiched between two layers of tetrahedra (cf. micas such as muscovite, Fig. 7.4). Many important clay minerals such as vermiculite, biotite, and smectites (notably montmorillonite and beidellite, the princi-... 

This attempt at size adjustment between the two types of sheets is illustrated by the fact that very few Fe-rich 2 1 clays contain less than 0.2 tetrahedral R3+. When increased Al in the tetrahedral sheets is compensated by substitution of Mg in the octahedral sheet, the octahedral charge increases and the overall layer charge is increased. This allows strong K-bonds to be developed and exert an influence on layer dimensions. Once the layer charge is larger than 0.7 and predominantly tetrahedral in origin, the Al dominated octahedral sheet represents the stable phase such a clay approaches muscovite in composition and charge distribution and its stability is deter-... 

As the predominant charge shifts from the tetrahedral to the octahedral sheet (illite to glauconite and celadonite), anion-anion repulsion is increased (octahedral sheet of celadonite 2.48 A thick as compared to 2.21 A for muscovite) and the larger Fe3+ ion in octahedral coordination represents the more stable phase. 

The main feature of clay minerals and micas is the layered crystallographic structure. Muscovite is a 2 1 [tetrahedral-octahedral-tetrahedral(T O-T)] phyllosilicate. In an ideal structure, aluminum exists in the octahedral sheet (=0) between two tetrahedral sheets ( = T), whose cations are composed of 25% Al and 75% Si. Interlayer K + cations balance the resulting negative charge (see schematic representation in Fig. 12, below). 

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. 

Figure 28. A fragment of the crystal structure of an Al-rich dioctahedral mica with the tram octahedral sites vacant. Part (A) shows the octahedral sheet of mnscovite, part (B) shows the corresponding portion of the muscovite dehydroxylate, and part (C) is muscovite in transition between a hydroxylate to a dehydroxylate stracture. Values refer to the surmnations of the contributing positive charge of neighboring cations to the anion. For simphcity, tetrahedral sites (not shown) have an occupancy that is considered to be Si only (from Guggenheim et al. 1987).

The compressibility of the octahedra is greater than that of the tetrahedra. This results in an increased dimensional misfit between tetrahedral and octahedral sheets, so that there is an increase in the tetrahedral rotation angle, a, with P (from 16.0 to 18.4° at 41 kbar for paragonite, and from 11.5 to 12.7° at 28 kbar for muscovite). 

Thirty years passed before another structural determination of a mica was made by Radoslovich (I960), a refinement of the structure of muscovite, in particular 2Mj polytype. The purpose was to clarify some points left open by Jackson and West (1933) a - the presence of reflections forbidden the space group C2/c, b - a monoclinic angle different from the ideal of 13 = cos i(-a/3c), c - the non-correspondence between the dimensions of the tetrahedral sheet and the octahedral sheet, d - the too-large K-0 distance, e - the order or disorder in the tetrahedral sites, f - an explanation of the frequency of the polymorphs of the micas. 

Moyes LN, Parkman RH, Chamock JM, Vaughan DJ, Livens FR, Hughes CR, Braithwaite A (2000) Uranium uptake from aqueous solution by interaction with goethite, lepidocrocite, muscovite, and mackinawaite An X-ray absorption spectroscopy study. Environ Sci Technol 34 1062-1068 Muller F, Besson G, Manceau A, Drits V-A (1997) Distribution of isomorphous cations within octahedral sheets in montmorillonite from Camp-Bertaux. Phys Chem Minerals 24 159-166 Muller J-P, Manceau A, Calas G, Allard T, Ildefonse P, Hazemann, J-L (1995) Crystal chemistry of kaolinite and Fe-Mn oxides relation with formation conditions of low temperature systems. Am J Sci 295 1115-1155... 

Smectite clays are a family of complex layered oxides with 2 1 layer lattice structures analogous to those of muscovite, phlogopite and other mica minerals [6]. Figure 2 illustrates the 2 1 structure wherein a central M04(0H)2 octahedral sheet is symmetrically cross-linked above and below to two tetrahedral MO4 sheets. Aluminum, iron, magnesiiun and sometimes lithium... 

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... 

Polytypism in layered silicates has been well characterized. There are as many as 19 polytypes known among micas. The repeat unit in muscovite mica, [KAl2(OH)2(Si3Al)0 o], for instance, consists of a sheet of octahedrally coordinated aluminium ions sandwiched between two identical sheets of (Si, AOO tetrahedra, the large ions being located in interlayer positions. Surface oxygens of the tetrahedra in... 

The micas have layer structures in which silicate sheets are combined with aluminate units the aluminum ions can be octahedrally as well as tetrahedrally coordinated. For example, the mica muscovite contains both octahedral and tetrahedral Al3+ ... 

In the dioctahedral 2 1 sheet-structure silicate with the occupied sites more than 85% occupied by Al, the structure seems to be able to compensate for the internal strain and can grow to a considerable size. The Al octahedral occupancy values of muscovite (>1.7) and the 2 1 dioctahedral clays (1.3—1.7) indicate that there is little overlap. It is likely that the decreased amount of tetrahedral twist induced by increasing the size of the octahedral cations and octahedral charge (decreasing Al) determines that a clay-size rather than a larger mineral will form. The R3+ occupancy value can be less than 1.3 when the larger Fe3+ is substituted for Al. When Al occupancy values are less than 1.3 (65%), in the absence of appreciable iron, the internal strain is such that growth is in only one direction. The width of the layer is restricted to five octahedral sites. Sufficient layer strain accumulates within this five-site interval such that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

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