Mica group sheet silicates

Aluminium group Chlorite group Clay minerals group Feldspar group Mica group Sheet silicates group Silicates group ... 

Aluminium group Chlorite group Mica group Sheet silicates... 

Mica A large group of silicates of varying composition, but similar in physical properties. All have excellent cleavage and can be split into very thin sheets. Used in electrical insulation. 

The chlorites are a group of sheet silicate minerals similar to the micas and talc. The general formula, A5 6T40io(OH) j, is deceptively simple. The range of possible substitutions is enormous, for example, A = Mg, Al, Fe, ... 

Note that Al + sometimes substitutes for in tetrahedrai sites but never more than 50%. Silicates tend to cieave between the siiicate groups, ieaving the strong Si-0 bonds intact. Amphiboies cieave in fibers micas into sheets. ... 

If, in three-layer sheet silicates, part of the Si in the tetrahedral layer is substituted by Al, then negative surface charges will occur that are compensated for by alkali cations, rather weakly bound between the three-layer stacks. In that way the large group of mica is formed. On the other hand, the formal partial substitution of Al by Mg in the pyrophyllite lattice produces a charge deficiency that will be compensated for by monovalent or divalent atoms such as Na or Ca to create the minerals of the smectite group. 

The micas are characterized by extended silicate sheets rather than chains. Their structures resemble the serpentine mineral group in that they are dom-... 

The three-sheet or 2 1 layer lattice silicates consist of two silica tetrahedral sheets between which is an octahedral sheet. These three sheets form a layer approximately 10 A thick. The oxygens at the tips of the tetrahedra point towards the center octahedral sheet and substitute for two-thirds of the octahedrally coordinated hydroxyls. The 2 1 clay minerals include the mica and smectite groups which are by far the most abundant of the clay minerals. The pure end members of this type are talc, a hydrous magnesium silicate pyrophyllite, a hydrous aluminum silicate and minnesotaite, a hydrous iron silicate. 

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