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Zeolite and related solids

Dear Reader,

Ages ago I wrote a lot about Prussian blue which is a miracle substance, now I am going to write about something which is related to that cesium catching blue solid. It is the main group version made with aluminum and silicon, the zeolites. These are a collection of microporous solids which have anionic frameworks made of aluminum, silicon and oxygen, the ultimate non-zeolite zeolite is porous silica such as a MCM48. Here we have a porous solid which has the empirical formula of SiO2. Such a solid made from Si(IV) centers and oxygen(-II) ions (oxide anions) is electrically neutral, as a result the silicas are not normally ion exchange materials. Equally if a pure alumina is made it has an empirical formula of Al2O3, as it is made of Al(III) ions and oxide anions then again it forms a neutral lattice which does not make a good ion exchanger. However if a silica has some of the silicons replaced with aluminum to form a aluminosilicate then a lattice can be formed in which the solid retains the “silica’s metal to oxygen ratio” which now having an excess of oxygen anions. This charge imbalance is normally resolved by including cations which are not part of the covalently bonded lattice. In some cases these cations are fixed deeply within the lattice in places where they cannot escape from while in other cases such as the microporous zeolites these charge balancing cations are in sites which are accessible to the solvent. In some cases the charge inbalanace is so great that some of the oxides are converted into hydroxides or are replaced with fluoride anions to form solids such as the mica mineral biotite.

Biotite is a mineral which I have been using in a project funded by SSM on the effects of surgar acids on the absorption of radionuclides onto mineral surfaces. I will save these results for another day. Now before we get onto the zeolite lets look at the biotite, it has the following data for the cell a = 5.343 Å, b = 9.258 Å, c = 10.227 Å Z = 2; beta = 100.26° V = 497.79 Den(Calc)= 2.89

The solid has the empirical formula KMg2.5Fe2+0.5AlSi3O10(OH)1.75F0.25 If we were to replace all the aluminium with silicon (getting rid of the K atom) and all the hydroxides with fluorides we would get a formula of Mg3Si4O10F2

This would be 3 x Mg and 4 x Si (+22 charge in total) and ten oxides and finally two flourides (-22 in total). In terms of charge balance the solid would be the same as Si11O11 but it might not be possible to get the right layered strucutre without the magnesium atoms. Here is a picture of a unit cell of biotite, the orange atoms are oxygens, the pale blue are magnesiums, the pink ones are silicon and the purple ones are potassium atoms.

A unit cell of biotite

A unit cell of biotite

The centre of the layer contains a mixture of iron, magnesium and titanium atoms, while the outer part of the layer contains silicon and aluminum atoms. Here is a picture of the cell where only oxygen, silicons and potassium atoms are shown.

Biotite cell with only potassium, oxygen and silicon atoms shown

Biotite cell with only potassium, oxygen and silicon atoms shown

Now if we show a spacefilling diagram of several layers of biotite, it should be clear that the solid has layers of potassium atoms which allow the solid to bind metals by ion exchange.

 

The layers of biotite showing the potassium in purple, oxygen in red, silicon/aluminium in gold and magnesium sites in blue

The layers of biotite showing the potassium in purple, oxygen in red, silicon/aluminium in gold and magnesium sites in blue

We will get back onto biotite and other zeolites soon.

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