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Update on your new best friend

Dear Reader,

Slightly more than one year ago I wrote about the new best friend of the Japanese people. Rather than a person who will go out and go for a karaoke session with you, drink beer with you or go for a walk in the park with you this new friend is one who can clean up your drinking water and keep your fruit and veg safe. Also this friend can help keep the milk healthy. Now you might ask what sort of super nice person can do all these things, or what sort of imp or kappa is able to do all these things.

The identity of this new friend is a bit more humble sounding, it is the clay in the soil. I have seen a recent paper by Naofumi Kozai, Toshihiko Ohnuki, Makoto Arisaka, Masayuki Watanabe, Fuminori Sakamoto, Shinya Yamasaki and Mingyu Jiang (Journal of Nuclear Science and Technology, 2012, 49(5), pages 473 to 478) in which the cesium behaviour in the soil is reported.

In this paper the soil was examined with X-ray diffraction. This found smectite, mica, hornblende, kaolinite, quartz, orthoclose, cistobalite, feldspar, stishovite, gibbsite, sodalite, olivine and sorosilicate minerals in the soils.

The smectite clay is rather similar to the illite clay which I showed a picture of recently. The two clays are related, I have seen a thesis which explains how the smectite clay transforms into illite clay. For your information the thesis is here. The difference between these two layered clays is in the anionic bread layers between the potassium jam. The smectite and the illite have slightly different layers.

A smectite clay, the oxygens are in orange red, the potassium ions in blue, the aluminium/silicon atoms are in sea green. Note that layers of potassium ions which look like slices of bread.

I think that both the smectite and illite are formed from mica, so I would say that the more mica in the rocks which formed the soil the better when you are considering cesium in soil. The mica is very similar to the two clays, again it is a layered solid.

Many of us know mica, it is a mineral which can be made into thin sheets. We will soon see why it is possible to split mica into thin sheets. It is easy to separate the mica by peeling apart the layers.

Here is a view of a mica (Muscovite) which was studied some time ago. (O.V. Sidorenko, B.B. Zvyagin and S.V. Soboleva, Kristallografiya, 1975, 20, 543-549). Should should see again that the solid is layered, the potassiums (blue) have anionic layers of alumo-magnesium silicate between them. I have shown all the non oxygen atoms in the layers as green.

Mica showing the layers of potassium ions between the anionic layers

The important thing about the mica is that the aluminium atoms are randomly mixed with magnesium and silicon atoms in the two layers. The authors of the paper I am working from expressed the view that one of the types of layers (the middle layer in the trilayer anionic layer) has 20 % magnesium and 80 % aluminium. While the two outer layers of the anionic trilayer have 28.4 % aluminium and 71.6 % silicon. The mica also contains some hydroxyl anions which I have been unable to locate in that solid. I have made a new drawing of the mica which shows the mixed aluminium/silicon and aluminium/magnesium layers in different colours to show you how the solid fits together.

Diagram of mica. The potassium ions are in blue. The mixed aluminium/magnesium sites are in grey. The mixed aluminium/silicon sites are in green. The oxygens are in a rather fetching shade of orange.

It is important to note that the green aluminium/silicon sites have a tetrahedral arrangement of oxygen atoms around the central atoms while the grey mixed magnesium/aluminium sites have a distorted octahedral arrangement of oxygens around the central atoms.

The hornblende clay is a very different mineral, I have looked at the crystal structure and it looks in some ways like a SBA-15 or MCM-41 to me. It has tube like holes which pass in one direction through the clay. These tubes are then filled with sodium and potassium cations. As the clay is so different, I suspect that it may behave differently to the layered clays which I showed you.

The mica based minerals can change the spacing between the anionic layers to suit a new cation which has a larger diameter than the potassium. In this way the larger cesium ions can be held in the solid. But in the case of the hornblende, I do not think it will be so easy to add a larger cation. I think that the holes will get plugged up and blocked by the larger cations.

Here is a view of several unit cells of the hornblende clay, the solid is a little disordered. It has a mixture of sodium (yellow) and potassium (blue) cations in the solid. Also note that it has grey calcium ions in the alumosilicate layers. The aluminiums are purple, the silicons are sea green and the iron atoms in the clay are orange. The oxygen is in a rather fetching orange/red as before.

A view of hornblende clay showing the holes in which the potassium/sodium ions go.

I will try and give you an update soon on the clay and the cesium.


5 Responses

  1. […] is also important when cesium gets into our soil. I recently wrote a little on the subject of the clay minerals in the soils in Japan. Here is a useful set of lecture notes on silicate minerals, most clays seem […]

  2. How did you get this picture? What program? Thanks

  3. I used ORTEP to make a POVray file, then I used POVray to draw the picture. The POVray will give you a bit map file. The Bitmap was then cropped using paint and saved as a png file which I then uploaded.

    If you want POVray then please go to http://www.povray.org

    If you want a copy of ORTEP then please go to the University of Glascow and look for it there, the crystallographer there (Louis Farrugia) has seen fit to provide the world with an easy to use version of the Oak Ridge code which also can write POVray files. We should all be very glad that Louis has done so !


  4. […] is perfectly reasonable in terms of chemistry, I have been saying since the accident occurred that the cesium will stick to soil minerals. I would say that it is […]

  5. […] On of the key parameters needed to understand how well the clay will trap radioactivity is the Kd value, to understand Kd please look at this post about plutonium in spanish soil. Also do bear in mind that not all clays are equally good at catching cesium for details of some different clays please see here. […]

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