Muscovite interlayer cation

With respect to trioctahedral micas, dioctahedral muscovite and celadonitic muscovite have smaller interlayer separations but similar a values. In dioctahedral micas, the proton position results in part from repulsion by the interlayer cation and the cations in the M(2) sites. Thus, the proton is located in that portion of the structure with minimal local positive-charge concentration, near the M(l) site (Radoslovich 1960 Guggenheim et al. 1987). The six-fold coordination of the interlayer cation with the basal inner O atom is distorted and elongated parallel to c. Both effects (i.e., the distorted coordination of the interlayer cation and the smaller H -K repulsion) thus control the interlayer separation. 

Kato T, Mima Y, Yoshii M, Maeda K (1979) The crystal stracture of IM-kinoshitalite, a new barium brittle mica and IM-manganese trioctahedral micas. Mineral J 9 392-408 Keppler H (1990) Ion exchange reactions between dehydroxylated micas and salt melts and the crystal chemistry of the interlayer cations in micas. Am Mineral 75 529-538 Keusen HR, Peters T (1980) Preiswerkite, an Al-rich trioctahedral sodium mica from the Geisspfad ultramafic complex (Penninic Alps). Am Mineral 65 1134-1137 Knurr RA, Bailey SW (1986) Refinement of Mn-substituted muscovite and phlogopite. Clays Clay Minerals 34 7-16... 

Figure 6. Bulk modulus as a function of interlayer cation size, calculated using the interlayer chemical composition. Na(Pg) paragonite (Comodi and Zanazzi 1997) Na(mu) Na-rich muscovite (Comodi and Zanazzi 1995) K(phl) phlogopite (Hazen and Finger 1978) K(mu) K-muscovite (Comodi and Zanazzi 1995) Cs(Cs-tfa) Cs-tetra-ferri-annite (Comodi et al. 1999) Rb(Rb-tfa) Rb-tetra-feni-annite (Comodi et al. 2001).

Figure 3.9 A clinographic view of the (dioctahedral) muscovite 2Mi structure (upper) showing the positions of the interlayer cations (large spheres) and a projection of a single 2 1 layer (lower, without the interlayer cations). 

One of the great advantages of studying phyllosilicate minerals lies in the fact that the structure of the major external faces, the 001 surfaces, is well known. This arises from our knowledge of the atomic arrangement of e.g., micas and related minerals. The perfect 001 cleavage ensures that the external smface is populated by a known arrangement of atoms. The caveats in this are the uncertainty of the placement of the interlayer cations. When muscovite mica is cleaved, the interlayer potassium ions (see Chapter 3) must distribute themselves between the two new smfaces. Presumably, about half the potassium ions go with one new surface and the rest with the other. The exact positions of the potassium on the surfaces are not known. 

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). 

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