Harrow and Hillingdon Geological Society

Previous Meetings

Magic rocks: from Tutenkhamun to curing cancer in 50 minutes (via oil and nuclear power)

Dewi Lewis
(University College, London)

Zeolites have been around forever and scientific knowledge of them is quite old. They have been used in industrial processes for 60-70 years. The speaker was interested in how they formed.

Normal rocks and minerals such as rock salt are solids, with atoms packed close together and if the packing is ordered they form crystals. Pure compounds are not very interesting, Al₂O3, for example is white, dull and boring. However, add Cr impurities and it becomes ruby, add Fe or Ti and it becomes sapphire. Different impurities result in different properties. Alternatively if material of the same composition is arranged in different ways it also results in different properties.

Baron Cronstedt, a Swede, noted that Stilbite, an aluminium silicate, appears to boil on heating … repeatedly, ie it releases water on heating and then takes it in again and the process can be repeated. He called zeolites “boiling stones”.

What are zeolites?

Zeolites comprise little tetrahedral of Si in the middle surrounded by 4 O atoms and these can be linked together in various ways, as chains, rings, cages and channels, resulting in microporous solids. An example is sodalite, also known as ultramarine or lapis lazuli. Zeolites can be synthesised, eg Zeolite A is used in washing powder and zeolite Y in oil refining. Structures with channels can have 3-dimensional channels (eg ZSM-5) and 2-dimensional channels (eg mordernite).

There are 40 known zeolite minerals. They are formed from volcanic ash or in basalt all over the world and over 140 different structures had been synthesised 10 years ago and now 260. Some of the synthetic zeolites are direct analogues of natural ones but others have no natural counterpart (yet!). Zeolites can be in large deposits (whole mountains) or small ones – eg Boggsite has one known sample about 1mm across, which was found in Oregon and is now in the Smithsonian Museum in Washington, DC.

Lapis lazuli is a sodalite structure with trapped S^3- anions, very large deposits of which are found in Afghanistan. Tutankhamun’s funeral mask is made of gold and lapis lazuli, which was much more expensive than gold at that time. The colour of lapis lazuli remains blue forever because the S^3- anions cannot escape.

The question arises as to whether zeolites are only found on earth. If there was water on Mars, where did it go? And is it still there? Zeolites can hold up to 20% of their own mass as water at temperatures up to 250^oC without effect. Further exploration may reveal that at least some of the water on Mars is trapped in zeolites.

Why are zeolites interesting?

It is significant that the pore dimensions (about 5.5Å) are the same size as organic molecules. The zeolites behave like a sieve with all holes the same size and regular, because they are crystals. Their main interest to the speaker is because they exist.

What is in washing powder? The major component (15-30%) is zeolite and soap is the 5th least abundant constituent. The zeolite is there to soften the water by ion exchange and a cold wash is longer than a hot wash because the first process is the softening of the water. Hard water + soap results in scum. However, zeolite A has lots of holes and lots of Na but it loves Ca and removes it from the water by ion exchange. This is the largest tonnage usage of zeolites (about 500M tonnes). The speaker remembered attending a conference of the International Zeolite Association when there was an enormous bust-up between Unilever and the PQ Corporation because Unilever was proposing replacing Zeolite A (manufactured by PQ) with another zeolite in detergents.

In nuclear waste clean-up, zeolites are used to remove 137Cs and 90Sr from pond water to reduce the medium-waste level of power plants. They are also used for accident containment and zeolites were added to the concrete used to entomb the reactor at Chernobyl. They are also used as an addition to aniumal feeds and to effluent waters.

A catalyst is “a substance that increases the rate of approach to equilibrium of a chemical reaction without being consumed by the reaction”. Zeolites are selective catalysts. Zeolites are solid acids 10 times stronger than concentrated H₂SO₄ but the acidity is contained so it is non-toxic, non-corrosive and is easy to transport and store. Valuable properties are reactant selectivity and product selectivity. Every litre of petrol has been through a zeolite since it is used for cracking crude oil to produce petroleum products. 30,000 tonnes per year of zeolites at a cost of £450M result in the production of £35,000M of petrol.

Other uses include as slow-release agents, eg zeoponic synthetic soils, which were used to grow cabbages and radishes on the Mir space station.

Normally applying pressure to a material (eg squeezing a sponge) results in contraction. However, natrolite has a composition of Na₁₆Al₁₆Si₂₄O₈₀.16H₂O at low pressure but it has 32H₂O at a high pressure of 40GPa. This is because pressure results in water going into some zeolites and they expand. Most things expand when heated but zeolites shrink on heating – or at least they expand on first heating but above about 120^oC they contract.

On the internet one can find claims that zeolites (eg Waiora – water containing zeolites) cure cancer, autism, remove toxins and help to fight ageing at source from a cellular and biological level. The speaker recommended that these claims are taken with a large pinch of salt.

Conclusion

The speaker concluded that zeolites truly deserve the name magic rocks, which arose because Cuba has large deposits of zeolites and they use them for lots of things and these multitudinous uses caused newspapers to refer to them as “Magic rocks”.

Back to Top