Structural linkage among Mica group silicates

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The general formula of Micas can be written as: I2M4-6□2-0T8O20A4, where:\nI : K or Na, but Cs, NH4, Rb, Ca, Ba and other cations are present;\nM : Mg, Fe2+, Fe3+, Al, Li, but also Ti, Mn, Zn, Cr, V and other cations occur;\n□ : a vacant space;\nT : Si, Al, and less commonly Fe3+;\nA : (OH), F but can also be Cl, O and S.\nThe basic structural feature of mica is a composite layer in which an octahedrally coordinated M cation sheet (mainly Al, Mg, Fe) is sandwiched between two identical and opposing sheets of linked (Si,Al)O4 tetrahedra. In the micas, these layers (sometimes referred to as TOT or 2:1 layers) have a net negative charge which is balanced by planes of I cations (commonly K, Na - in 12-fold coordination) positioned between them.\nNote: The color code used in these models is CPK.\n–\nWebsite: http://geology.uaic.ro/muzee/mineralogie/\nConcept & 3D-modelling by dr. Andrei Ionut APOPEI