 So, in an earlier set of videos we talked about or we developed parameters to help us classify soils. In this set of videos we're going to round off that theme of classification by talking about clay minerals. Now clay minerals are used to describe a size fraction in soil as we've already discussed, anything under two micrometers we might describe as a clay. Quite confusingly though clay is also a term used to describe a range of minerals that exist within soil. So these things can be mutually exclusive. We can have particles that are not made up of clays be part of the clay size fraction be under two micrometers and we can also have an agglomeration of clay minerals that would apparently be greater than two micrometers. But usually in soils the bulk of the clay size fraction is made up of clay minerals. So clay minerals are silicate minerals and that means that they're based around this arrangement of atoms and the arrangement of atoms is called a silicate tetrahedron where we have four oxygen atoms surrounding a silicon atom. So that's the building block of all silicate minerals and clay minerals form sheets of these silicate tetrahedron. So the sheets look something like this. If I look from above down onto one of these sheets of silicate tetrahedron, the arrangement looks something like this where we have individual silicate tetrahedron joined together to the other silicate tetrahedron by their oxygen atoms and in the middle of each of these triangles we'll have a silicon atom. So this is what it looks like looking down onto it. So silicates that form sheets like this are given a special name called phylo-silicate. So this sheet arrangement in clays is really what drives some of the interesting properties that clay minerals have. So you know clays might be on the order of micrometer in width or length but actually if you look at the thickness that could be on the order of nanometers. So a hundred or a thousand times less. So you can imagine a sheet that's a thousand times wider or longer than a day's thicker and that's really what we're talking about when we talk about clay mineral. So in addition to layers of silica clay minerals also have layers of alumina and those layers are composed of octahedra of alumina. What that is is you have six atoms or well six groups of hydroxyl surrounding an aluminium atom and sometimes those hydroxyl groups are swapped with oxygen atoms when they're making up these sheets. But it looks something like this. If I were to draw it as a sheet it's a little bit confusing so I won't attempt that my skills are not on good enough but the single arrangement looks something like this. Sometimes the aluminium atom is also replaced by magnesium as well. So the different arrangement of these sheets is really what gives rise to the different range of clay minerals that we can have. So for instance a montmorillonite which is a part of the smectite group of clays and it's sort of what makes a bentonite clay has an arrangement that looks like this. So instead of drawing out all of the atoms I've just done it as a schematic where this blue trapezium represents the silica layers and the red a cuboid represents the alumina layers. So for montmorillonite the arrangement looks like this where we have essentially a silica layer, a lumina layer sandwiched between two silica layers. So what joins these layers together is the sharing of oxygen atoms or hydroxyl groups. So in smectite clays like montmorillonite we have three layers and they're joined together to another three layers by an interstitial gap here. Now inside that gap you can have either calcium, atoms, magnesium, sodium or water. So the expansive properties that smectite clays have like montmorillonite arise because of the relative size of the particle that fits in this interstitial gap relative to its charge. So magnesium and sodium atoms are relatively the same size but magnesium has two charges for every one of sodium and calcium is almost well twice the size of a magnesium atom. So and water has a relatively low charge to the particle size so you need a lot of water in there to meet the charge requirements of the structure. So that's why when you add water to these types of clays you get expansive properties. So together these six layers make up one crystal of clay but the thickness of one of these three layers is around let's say 10 angstroms. Now one angstrom is equal to 0.1 nanometres. So each layer is probably about one nanometre, each sort of set of layers is about one nanometre in thickness. So Ily's clay minerals have the same arrangement as montmorillonite with three layers connected to another three layers but the difference here is that we now have really much larger potassium ions sitting in the interstitial gap. So potassium has a much larger diameter even than calcium but it has only one charge so they're now sitting in the interstitial gap and that's what gives rise to Ily's clay minerals. So the arrangement of sheets for kaolinite is completely different from Ily's or montmorillonite and the arrangement looks something like this where we have alternating silica and alumina sheets and those are joined together mainly through hydrogen bonding but you'll notice that in this arrangement we don't have the interstitial zone anymore and that's the reason why kaolinite has much less expansive properties than smactite type clays. So what governs this structure change in clay minerals is really the underlying need for minerals to maintain charge balance within the structure and they have to maintain that charge balance with changes in element substitution. So for some of the Ily's and montmorillonite type clays the aluminium is replaced by magnesium and you have a three plus charge being replaced by two plus charge. Let me show you what I mean. So let's say we have kaolinite here and we have maybe a typical chemical formula for kaolinite would be Si4, Al4, O10, OH8. Now let's count up all of the positive and negative charges within that. So silicon atoms by themselves have a four plus charge so we have four times four plus charge. Aluminium atoms have a three plus charge so we have four times three plus. And those two together give us, well here it's sixteen positive charges and here's twelve positive charges. Oxygen has a negative, a two negative charge so we have ten times two negative and the OH group has a one negative charge so we have eight times one negative. So we have here twenty negative charges and eight negative charges. So if we add up all of those together we actually get a zero net of charge and this would be the case for kaolinite but what would happen if we swapped some of that aluminium for magnesium? Well instead of our aluminium three plus we would go to magnesium two plus. So if we swapped all of the aluminium and kaolinite to magnesium we would be left with, well here it would be four times two plus so we'd be left with eight positive charges. So we'd be left with an overall surplus of four negative charges. So to try and counteract that the illites in montmorillon and I rearrange the arrangement of sheets and crystal structures and also include the interstitial cations, the positive cations to counteract that excess negative charge.