Silicate SheetsLet's review the silicon-oxygen units of a silica sheet. Each silicon is bonded to 4 oxygen atoms in a tetrahedral array. Image that 3 of the oxygen atoms are in a plane on the bottom and 1 is above the silicon atom. An aluminum atom can substitute for the silicon atom and that changes only the total charge.
The tetrahedra condense so that the 3 oxygen atoms in the bottom plane are shared with other silicon atoms. The unshared oxygen atom is represented by the green dot in the figure below. The structure extends in 2-dimensions.
The empirical formula for a silica sheet is [Si2O5]2- so there is one minus charge per silicon atom in the sheet (and an additional negative charge for every aluminum that substitutes for a silicon).
Octahedral Element OxidesWe've seen that aluminum(III) can substitute for silicon(IV) in all of the silicate structures. It can also form octahedral structures.
Condensation of Octahedra with TetrahedraTake one of these octahedra and condense it with the silica sheet above. The aluminum is bonded to a triangle of 3, adjacent oxygen atoms sticking up from that surface.
Next take two silica sheets that are parallel but opposite so that the unshared oxygen atoms of 1 sheet point at the unshared oxygen atoms of the other.
Add an aluminum(III) to the structure so that the aluminum bonds to 3 oxygens on one sheet and 3 oxygens on another.
Finally, add additional Al3+ cations so that all of the unshared oxygens on each sheet are bonded to an aluminum.
What is the charge? Remember that each silicon atom in a sheet donates 1 oxygen atom and is responsible for a charge of -1. Each octahedral aluminum bonds with 6 of these oxygen atoms so there are 6 Si(O)(O3/2-1 for every Al3+. This gives us a net charge of -1/2 for every silicon atom in the two sheets.
We can think of the top silicate sheet + condensed alumina + bottom silicate sheet as a single unit. They are strongly and covalently bonded together. I'll refer to this as the silicate "sandwich" structure.
The extra 1/2 unit of negative charge per silicon atom must be balanced by positively charged ions external to the sandwich.
TalcTalc has a structure related to the one described above with 2 silicate sheets connected by Mg+2 cations. There are 3 of these bridging Mg+2 cations for every 2 [Si2O5]2-.
The formula for this mineral is Mg3Si4O10(OH)2. The additional hydroxide groups (that are shared between magnesium cations) are necessary for charge balance.
The neutral, tightly bonded "sandwich" units are only loosely connected in the structure so the layers slide by one another easily. This gives talc (and talcum powder that is made from it) the slippery feel. Two representations of the structure of talc are shown below.
MicaTake the talc structure and replace 1/4 of the silicon(IV) with aluminum(III) in the silica sheets. Instead of [Si4O10]4-, there is [AlSi3O10]5-.
For each of these aluminosilicate units there are, like in talc, 3 octahedral cations. Two of these are Mg+2 and one is Fe+2.
For every aluminum(III) in the silicate, there must be one additional cation. In mica these are K+ and the potassium cations are weakly coordinated to oxygen between the sandwiches.
Mica, like talc, is a layered mineral but the layers don't slide easily against one another. The structure, with those potassium cations connecting the layers, tells us why.