Muscovite, layered silicate

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... 

Layer silicates or /j/iy/fo-silicates Two-dimensional sheets of [Si04] tetrahedra sharing three oxygen atoms Kaolinite, ALi(OH)8(Si4010) Talc, Mg3(OH)2(Si4O10) Muscovite, KAl2(OH)2(AlSi3O10)... 

The analysis of 29Si MAS NMR spectra of layer silicates with a wide range of tetrahedral compositions (Si/Al ratio 2.7-1.7) indicates (483, 486) that the distribution of Si and Al in these materials is indeed determined by (i) the local balance of charges, and (ii) the Loewenstein rule. In muscovite, phlogopite, and vermiculite, aluminum is randomly distributed in... 

The most common class of pearlescent pigments today is based on thin platelets of mica (see Fig. 15.5). Mica itself is a natural mineral and belongs to the sheet layer silicates. Nacreous pigments are usually based on natural, transparent muscovite and only in some cases on synthetic phlogopite. Muscovite occurs worldwide, but only few deposits are suitable for pigment production. Mica is biologically inert and approved for use as a filler and colorant. 

The dominant class of pearl luster pigments is based on platelets of natural mica coated with thin films of transparent metal oxides [5.122-5.125, 5.127-5.130, 5.137]. The mica substrate acts as a template for the synthesis and as a mechanical support for the deposited thin optical layers of the pearl luster pigments. Mica minerals are sheet layer silicates. Pearl luster pigments are usually based on transparent muscovite mica only some are based on synthetic phlogopite. Although muscovite occurs worldwide, few deposits are suitable for pigments. Natural mica is biologically inert and approved for use as a filler and colorant. 

Fig. 16.3 Layered silicate structures (a) talc, Mgj(OH)3(SLO o) (b) muscovite (a mica) KAI (OH) (Si)AIO i,). IMne (I) Electroneutrality is maintained by balance of K(l). Mg(ll). AUMI). and Si(IV). (2) The repealing layers in muscovite are bound together by the K cations.) [From Adams. D. M. Inorfianic Solic/s-. Wiley New York. 1974. Reproduced with permission. ...

In general, weathering conditions tend to favor the gradual conversion of trioctahedral layer silicate clays (with dominantly Mg " and Fe " in octahedral sites) to dioctahedral layer silicates (with dominantly AP and Fe in octahedral sites). Layer charge reduction accompanies this process. Expected weathering sequences of the two common micas, biotite and muscovite, in leaching environments are ... 

PhyllosHicates Layer silicates (micas and clays) 2 5 ratio of Sl 0 Kaolinite (china clay) Al2(Si205)(0H)4 Muscovite (mica) KAl2(AISi30.,o)(OH)2 75% sharing ... 

Kudus, M.H.A., Akil, H.M., Rasyid, M.F.A., 2012. Muscovite-MWCNT hybrid as a potential filler for layered silicate nanocomposite. Materials Letters 79, 92—95. 

The refinement of the structural data for muscovite by Radoslovich [1960] advanced our understanding of this and other layer silicate structures considerably, even though it was later shown by Gatineau [1963] to be incorrect in one respect because the calculations had been terminated before the coordinates had fully converged to the best possible values. 

There have been several detailed and careful studies in this field recently. For example. Farmer and Russell [1964] have studied the infrared absorption bands arising from the structural hydroxyl groups of a number of layer silicates, i.e., in the region from 3500 to 3700 cm" . They attempted to assign the two frequencies they observed in terms of either octahedral substitutions or of Al-for-Si substitution tetrahedrally. They confirmed earlier work that in the dioctahedral micas such as muscovite, the OH dipole oscillations lie at around 16° to the plane of the sheets, whereas in the trioctahedral micas, the O—4 bond is normal to the plane of the sheets. 

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). 

Fig. 14. Compensation plot for dehydroxylation of kaolinite ( ) and other layer-type silicates (X = montmorillonite, illite and muscovite) data and sources given in Table 11. (Redrawn, with permission, from Advances in Catalysis, ref. 36).

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