Transcript from Epic of Evolution: Life, the Earth and the Cosmos (BEP 210A)
March 20, 2000 - Lecture by Michael Wysession
For the next section of the course I’m going to talk about Earth materials. The last time I talked, I focused on Earth forces, and so I talked about the large-scale motions within the Earth. I talked about plates, the plates moving, convection, and I talked about a large set of cycles of rock. We’re going to look at the rock cycle in a couple of new and different ways this week. I’m going to start out with a little bit of inorganic chemistry (you’ve already gotten some organic chemistry). We need to do this because the Earth is made of matter, and the atoms that make up the matter of the Earth have arranged themselves into minerals, and these make up rock. I’ve put the definitions on the board, so we have a framework from which to work. This will be the foundation for the whole week. The Earth is primarily made of rock. There is some fluid, liquid water and magma, which is liquid rock, but it’s mostly rock. A rock is a “solid, cohesive aggregate of grains of one or more mineral”. Well, okay, so now we have to go what a mineral is. A mineral is a “naturally occurring solid, inorganic element or compound with a definite composition or range of compositions usually possessing a regular internal crystalline structure. A crystal is a “mineral grain displaying the characteristics of its atomic structure”. There are three types of rocks that we’ll get to this week: igneous, sedimentary and metamorphic -- and they are the basic foundation of the rock cycle. You can change rock from one of these forms to another. Igneous rock is formed by solidification from a lava or magma. Sedimentary rock is formed by chemical precipitation of the dissolved remnants of previous rocks or by the compaction of the eroded sediments of previous rocks. And metamorphic rock is formed by altering previous rocks through increased temperature and/or pressure. These are just definitions right now, but during this week I hope to give you a sense of the dynamic connections between the different states of the materials of the Earth.
What our Earth is made of is defined by what the starting solar abundances were. Claude made all these atoms in the dying moments of a star last week and this is what we have to work with. This is what came together to form our solar system and the Earth. The sorts of minerals and rocks we see is entirely dependent upon the relative abundances of those elements. The process of fusion burning helium to make more complicated atoms favors the production of some elements over others. It produced, for example, a lot of oxygen and silicon relative to certain other elements. Well, oxygen and silicon make up the foundation of almost all of the rock within the Earth. There are also other factors that are involved, and I will touch upon. It turns out that if you take the jumble of elements that we have within the Earth and you put them together in different ways -- you change the temperature a little bit, you change the pressure a little bit -- you create a variety of different combinations of atoms. Those combinations of atoms, how the atoms have frozen together, are minerals. That’s what a mineral is. A mineral is the particular way in which a group of atoms have frozen together. We’ve got liquids and we have solids and we have gases. And solids represent the frozen phase of any material. Take water, for example. It exists as a vapor or a liquid. Freeze it, you get solid water. Rocks are frozen magma, but the freezing point tends to be at a very high temperature, so we don’t necessarily think of rocks as being frozen the way ice is. Relative to the liquid and gas phases, most of the crust and mantle of the Earth are quite cold, so that the rock is solid. The way that the atoms that happen to be there came together to solidify defines what a mineral is. If you change the temperature and pressure, you create different minerals. And in fact, there are well more than 3,800 named minerals, and many other combinations of atoms that create minerals that aren’t even named yet. There are still papers being written on the discovery of new minerals. And what makes them different? Well, to us minerals are different because their physical properties are different. They have different color. They have different hardness. A diamond is the hardest mineral we know: it will scratch any other mineral. Talc for example is one of the softest minerals we know. Any other mineral will scratch talc when in contact with it. Hardness is one examples of a distinguishing characteristic for minerals. Density is another good example. Gold is very heavy. That’s how people have found it for years, by techniques like panning river sediments. You shake a pan full of sand and you wash off all the lighter grains of sand and what’s left at the bottom is the heaviest material, and that is where you find the gold. Minerals with heavy elements like iron and lead also tend to be very heavy. Luster is another characteristic. Many of you have seen opals that seem to shimmer, that have a particular shimmering appearance to them. This is called luster. Some minerals are magnetic. Some are transparent or translucent. Some fracture in particular ways. Common table salt (NaCl) always fractures in cubes. Mica peels off in flat layers. All of these are characteristics of minerals, and what’s remarkable is that if you take away a few atoms of one element and add a few atoms of another element, the whole set of characteristics of the mineral change. It turns out, however, that you cannot put any random set of atoms together and make a mineral. Most such agglomerations of atoms are not stable, and as I will show, there are particular requirements for stable minerals that allow them to exist.
Let me give you a couple of examples of what I’m talking about with minerals. Minerals have a definite atomic structure. This is an example of a very common mineral. Actually a lot of minerals have this structure, but the most common in our lives would be the table salt I just mentioned (NaCl: sodium chloride - half sodium, half chloride). The sodium and chlorine atoms arrange themselves in a nice cubic structure. If you go and look at a piece of salt, a crystal of sodium chloride, it tends to break along these same nice sharp square angles. The way that the mineral breaks reflects its atomic structure. That’s a very important point of minerals -- their macroscopic behavior, how they behave on a very large scale, mimics their atomic structure. Another classic example would be quartz. We will deal with quartz a lot because of its importance within the Earth. Here are a couple of examples of quartz crystals, and I can hand them around. What you will notice when you look at this is that it has six sides: quartz crystals generally will have six sides. In this one the sides aren’t symmetric, but nonetheless there are six sides. No one cut this into a six-sided rock. It grew this way naturally because at an atomic level the atomic structure of the silicon and oxygen that make up quartz form a six-sided structure.
Here’s another example of quartz, though here there is the addition of small amounts of other elements, and now it has a shimmer to it. This is called tiger’s eye, and it has a particular luster that is due to imperfections that have worked themselves into the quartz structure. Amethyst, opal, agate, jasper: these are all quartz with a little bit of different materials, elements added in. You get a variety of different colors and textures to the rock as a result.
Here is some basic inorganic chemistry here. An element is determined by the number of protons it has, but we can change the number of neutrons and electrons and change the characteristics of the element in a geological sense. If we change the number of neutrons (this is a review from what Claude talked about) you get isotopes. For instance, if a carbon atom has two extra neutrons, we call it carbon-14. It’s not stable, and it will radioactively decay, but the only difference between carbon-14 and carbon-12 is that you’ve got two added neutrons: the number of protons is the same (6). If you change the number of electrons, then you have ions and they are very important, because when you change the number of electrons you change the electrical charge of the atom, and you change the way it bonds with other ions. When atoms bond, they attempt to achieve a greater degree of stability. Part of this has to do with the filling up the electron orbitals. Here are examples of some major elements in terms of Earth compositions, and their preferred ions. Electrons gather in distinct orbitals, or energy levels, and the first one has two electrons, and the second one has eight electrons. So that gives a total of 10. The third group (orbital) of electrons also has eight in it, and that gives a total of 18. The next one has 18 and that gives a total of 36. These are very important numbers. Now, what is an orbital? When I was in elementary school, I learned that an atom was a nucleus with a bunch of little electrons orbiting it, like planets. And then when I got to junior high school, they weren’t really orbiting it like planets: there were shells of electrons. And then when I got to high school, there weren’t really shells of electrons, the electrons were in orbitals, which were different geometrically-shaped clouds where electrons preferentially hung out. And then when I got to college, they weren’t really physical entities, they were more like quantum energy level probability distributions. And I don’t know what they were in graduate school because by that point I gave up on electrons, and I went into studying earthquakes.
Nonetheless, an atom is “happy” (this is a term from my junior high chemistry) when it has 2, 10, 18 or 36 (and more for higher, larger elements) electrons, because this means that its orbitals are full. All the possible electrons that can exist at that particular energy level are there. And atoms will gain or lose electrons so that they have those numbers that fill out the orbitals. Well, let’s take an example. Hydrogen normally would have one proton, one neutron and one electron: one of everything. However, instead of gaining an electron, it tends to lose an electron, and so it now has one proton and no electron, which gives it a + 1 charge. We call this H+. The electrical charge means it is now attractive to other ions that have a negative charge. Sodium plays a big part in Earth rocks. I just showed you sodium chloride. Sodium normally has 11 electrons, so it has two full orbitals and a single electron in the third orbital. The first orbital has two electrons in it, the next one has eight, and the third has one. That is not a stable situation, so it loses its last electron, and now it has two full electron orbitals, but also a positive charge of +1, which means it’s going to actively interact with other ions. Magnesium also plays an important part. It has 12 electrons. It loses two electrons, again to go down to two full orbitals, but this gives it a +2 charge. Oxygen is missing two electrons. It normally has eight, so it has two electrons in the first orbital and six in the second orbital, and it grabs hold of two electrons if it can to fill out its second orbital of electrons. This gives it a -2 charge. It has two extra electrons relative to the number of protons in it. Chlorine normally has 17 electrons, and it goes up to 18 by gaining an electron and filling out the third orbital, giving it a -1 charge. Okay, you get the idea here. The preferred ion states of atoms involving changing the number of electrons to fill out orbitals.
Well, how do these materials bond within minerals? Well, we actually have a couple of different types of bonds, and both Ursula and Claude have talked about some of these already. We have ionic bonds, we have covalent bonds, we have in some cases metallic bonds, and we also have funny things called van der Waals bonds. I’m not sure of the details of how van der Waals bonds work, because as I said, I cut out of physics before I got to graduate school. Ionic bonds are the simplest to think of conceptually. Here is an example using the sodium chloride I just mentioned. Sodium loses its electron and has a +1 charge. Chlorine gains an electron and has a -1 charge. Opposites attract, plus and minus, they bond together, and this creates the atomic structure that I showed you. This has always been amazing to me because if you take either of these separately, they are incredibly unstable. Sodium for instance is an incredibly volatile ion. I remember my high school chemistry teacher taking a strip of sodium metal and dropping it in a beaker of water. It burst into flames. It bounced on top of the water, burning and flaring whenever it came in contact with the water. We dump chlorine in swimming pools to kill any germs in there. In detergents, it dissolves stains in your clothes. In hydrochloric acid, it is highly corrosive. We use this to dissolve all sorts of things. In fact, the way we test if a rock is limestone (Calcium carbonate) is that we put a little drop of hydrochloric acid on it, and it dissolves the rock right before you eyes, so you know you have limestone. Marine animal shells are made of calcium carbonate and they will dissolve as well. But put them together (the sodium and the chloride), and they create this incredibly stable mineral that is totally inert – salt. It’s not erupting into flames in contact with the liquid of our stomach. And it’s a tribute to how well these atoms fit together. I’m going to talk in a moment about the second aspect of the stability of an atom.
The second type of atomic bonds is that of covalent bonds, and let me use diamond as an example. Diamond is an incredibly stable, hard mineral. There is not an electrical charge that’s binding these atoms of carbon in a diamond structure. Rather, the carbon atoms are sharing their electrons among them. Each of the carbon atoms involved only keeps to itself the first orbital of electrons, so it shares most of its electrons with a neighboring carbon atom. The fact that all of the atoms of the second orbital are shared provides for a very tight bond that holds these atoms together in a very strong manner. The most important example for us in the rest of our discussion has to do with silicon and oxygen. Silicon normally should have 14 protons, neutrons and electrons. But 14 is right in between 10 and 18 so it could go either way. It could gain four electrons or it could lose four electrons in order to fill out the orbitals, but it prefers to lose the electrons and go to Si+4, and it commonly then bonds with O-2 ions. However, instead of forming ionic bonds with the O-2 ions, a silicon ion will covalently bond with four O-2 ions, forming a tetrahedron. However, since the tetrahedron contains one Si+4 ion and four O-2 ions, this cluster has a net charge of –4, and will then ionically bond with positive ions like sodium, aluminum, potassium, iron and magnesium. There’s a lot of silicon and a lot of oxygen, and this little mini pyramid, this tetrahedron, provides the foundation for almost all of the rocks in the Earth. We call these rocks silicates, and they make up most of the Earth. On the order of 70 percent of the Earth is comprised of these silicate rocks. Now, if you just have the silicon and oxygen, you create quartz, but as I just mentioned, you can add in a variety of different elements and make a variety of different minerals. Most of our upper mantle is a mineral called olivine, which is the silicon and oxygen tetrahedra combined with magnesium and iron ions. For instance, pure magnesium olivine is written as Mg2SiO4.
Some of the rocks behave very differently from each other. For example, here’s a mineral you may have seen before called mica. Mica is a sheet silicate. It tends to have very strong bonds within the sheet, but very weak van der Waals bonds that connect the sheets, so the material just peels off in sheets. In fact, it’s practically transparent, and people used to use mica for windows in the days before glass was readily available. Asbestos is another example of a sheet silicate. What happens, however, is that the different sides, the different sheets, are not balanced in terms of their atomic structures, and this causes the sheets to bend and warp and roll up into very fine fibers. So in asbestos, the sheets roll up into fibers, and that’s why they present a health hazard. These little fibers can lodge themselves between the cilia that line your throat, and can cause cancer.
Let me show you the atomic structures of two silicate minerals. Here’s an example of mica. The atoms look randomly placed, but if I turn the structure in this direction, you can see that there is a very clear layering. The black balls are the silicon atoms, and the red balls are oxygen. So here we have examples of silicon tetrahedra (one silicon surrounded by four oxygens) making up a whole layer of the structure. But you also notice that there are other ions in here as well, and that’s because the tetrahedra have net negative electronic charges, and they grab positive ions. You’ll also notice the layer of gold atoms that’s in the middle. These longer bonds here are the weaker van der Waals bonds, and this unit is very weakly bonded to the adjacent layer, and will slide off it and break away easily. Talc is another example of a sheet silicate. The reason talc feels so greasy is that the van der Waals bonds are so weak that the layers just slide right off each other.
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Here is another structure called beryl, a structure that includes minerals such as emeralds. An emerald is a type of beryl structure. In this particular structure we have beryllium and aluminum combining with the silicon and oxygen. Again, here are the densely-packed tetrahedra of silicon and oxygen. If you look through the structure in just the right way, you can see this hexagonal structure. There’s a six-sided hole right in the middle of the structure, revealing the mineral’s six-sided symmetry. Beryl tends to grow into tall mineral crystals, and I have some examples here. The crystal has six sides, just like the atomic structure. And the only difference between a mineral and a gem, is that a gem is a mineral that has very few imperfections. For instance, I’ve got a sapphire here from our collection, as well as a ruby and a spinel, and when you look through these, they are very vibrant in terms of their colors. If you don’t mind, I won’t pass these around. These have very few imperfections compared to the way the minerals usually appear in nature. They are examples of the mineral’s crystal structure without any other foreign atoms joining into the structure, and it’s rare that it happens this way, and that’s why they are worth so much - because they are so uncommon. Here is a huge garnet crystal, but it is not at all transparent. It has a reddish-black color due to having a lot of imperfections. It has a lot of other elements that have crept their way into it so it’s not clear and transparent, the way a gem-quality garnet crystals would be.
Most minerals just look like regular rocks. This one is geologically important. This is called a potassium feldspar, and you see this everyday, all the time. For all of the granite on the outside of all the buildings here at Wash U, the red color in the granite comes from this particular mineral. There’s nothing particularly special about it, except that it is a very stable combination of commonly found elements, so there is a lot of it in continental rock. This feldspar is still primarily silicon and oxygen, but instead of magnesium bonding to the silicon-oxygen tetrahedra, we now have potassium, and the potassium causes a slightly different shape and color to the mineral.
I mentioned that minerals need to be stable. What makes a mineral stable? There are two things. One is that the ionic charge has to add up to zero. For stability, you have to have a 0 sum ionic charge. For sodium chloride, the +1 charge of the sodium adds to the –1 charge of the chlorine, and they add up to 0. The second thing is that the ion sizes of the bonded elements must be compatible. Here’s an example of stacking tennis balls, and you can see that we can stack tennis balls a couple different ways, but some ways are more efficient than others. In the first example, we see that there are large gaps between the tennis balls, but if you tilt the layers over, the gaps between them become smaller. That’s a denser, heavier, more stable structure. Imagine, however, that you’re stacking tennis balls with small super balls. The super balls can fit into the gaps between the tennis balls. That would make for a stable atomic structure. You can fill in the gaps between large ions with smaller ions, and make a much more stable atomic structure.
If you look at the sizes of ions they tend to vary quite a bit. This picture shows them at surface temperatures and pressures. The negative ions tend to be larger than the positive ions. There are more electrons floating around that aren’t being pulled in by protons, so if you look at the atoms with a net negative charge, these ions tend to be much larger in size. If you look at the ions that are missing electrons, they’ve got extra protons, and they tend to be quite small. What you have to do in creating a mineral is not only make sure that the ion charges add up, but that these sizes are compatible with each other. Now, that seems fine and well and good, but things get a little more complicated because we’re constantly changing the rules within the Earth. As we go around the surface of the Earth, and as we go down into the Earth, we change the temperature and pressure, and when we do that we end up changing the actual structure of our minerals.
Let’s take water as an example. Now, we know that water can exist as ice or water vapor, and if we’re at the right point in both temperature and pressure, then we actually have what’s known as a triple point, a place where water vapor, liquid and ice can exist all together. I’ll come back to this. Notice, however, how we change the rules when we change the pressure. Right here at the surface, at one bar of pressure, ice will melt at 0 degrees Centigrade and become water, and if you increase the temperature to 100 degrees Centigrade, water will turn into steam. But if you increase your pressure, the temperature at which water will exist in a liquid phase is much lower and water will exist as a liquid at a much higher temperature. In fact, at mid-ocean ridges the water that’s coming out of the volcanic vents is 300 degrees Centigrade. It is under the pressure of all of the water on top of it, and so it is still in a liquid phase. Water remains a liquid all the way out to a much higher temperatures at a larger pressure. What is the point of a pressure cooker? Why do cooks use a pressure cooker? Well, the greatest temperature that you can heat water to in your kitchen (unless you happen to be cooking in a deep submarine) is 100 degrees Centigrade. But suppose you want to boil something at a much higher temperature, what do you do? You increase the pressure, so inside your pressure cooker the water can reach a much higher temperature. But be careful. Release the pressure, and the high-temperature water will turn instantly to steam.
Suppose I take that ice, and bring it to much higher pressures. This is where unusual things begin to happen. As you begin to compress the ice, the atoms of hydrogen and oxygen rearrange themselves. Why does that happen? Because as you squeeze the material you change the relative sizes of the ions. It’s a lot easier to compress those big, fluffy negative ions, which are much bigger and compress more quickly. The positive ions don’t change all that much in size, but as you go down in the Earth you shrink the size of the negative ions, and therefore a structure that might have been nice and stable at the surface, is no longer is stable down deep. You can think of the atoms as continuously vibrating, and if you give them any room to vibrate, they’re just going to bounce around until they fly apart. You want them all locked together nicely. Ice will go into a set of reorganizations as you go down into the Earth. We give the new structures names like ice-3, ice-4, etc. If you have ever read Kurt Vonnegut’s Cat’s Cradle you may remember ice-9. Ice-9 is a fictional material – we haven’t gone that high in naming ice structures. Anything that touched Ice-9 would also become ice-9, and the whole world eventually freezes over. This change in material structure happens with all minerals. Earlier, I showed you quartz. Quartz at surface temperature and pressure is actually called alpha quartz, the crystal that I handed around. However, if you increase the temperature and keep the pressure low, quartz turns into something called tridymite, and then cristobalite, and then it will melt as liquid magma, liquid silica. If you increase the pressure a little bit, the alpha quartz will rearrange its atoms into a mineral called beta quartz. If you keep the temperature low but squeeze it, to high pressures, quartz takes on a new mineral structure called coesite, and eventually becomes something called stishovite. All of the names are not important in this class, but we have learned some very important things about our world from these phase transitions. For instance, we do find stishovite at the Earth’s surface, yet there are no places on Earth that have such high pressures and low temperatures. If you go down into the Earth, yes, the pressure increases, but the temperature also increases, so you would never form stishovite. Where did this stishovite come from? Meteorite impacts. When a meteorite hits the Earth, such as the one that killed off the dinosaurs 65 million years ago, the high-pressure shock wave of the impact will send quartz right into a stishovite phase. There are many craters around the world that have been identified as being impact craters as opposed to fossil volcanic craters by displaying the presence of stishovite.
[Q: Well, yeah, I’m assuming like as soon as the pressure gets higher like it turns into stishovite, but when pressure gets lower why doesn’t it go back to quartz?]
Excellent question. You shock the material, and it became stishovite, but then when the impact is over, why doesn’t the stishovite turn right back into quartz? It turns out that it does turn to quartz, but it does so very slowly, so it continues to exist in a metastable state. It turns out that reactions occur very slowly when materials are cold, and the surface of the Earth is quite cold compared to the Earth’s interior. The rate of the reaction, the speed at which this stishovite converts to the quartz, is slow, on the order of tens of billions of years. Perhaps the best example of this is diamond. Diamonds are not stable at the surface. Don’t ever let a diamond get near a flame. It will burn like a piece of coal - a rather expensive source of fuel. The reason it will burn is that it’s not stable at these conditions. Diamonds actually form at least 150 kilometers beneath us, which is interesting because if you see a natural diamond at the surface it is a rock that has come up from at least 150 kilometers down. There are only a few places in the world where this process has occured. It just so happens that a lot of these regions are in South Africa, which is why so many of the diamond mines have been there. But the diamond is the same material as the graphite in pencils – it’s all carbon atoms. However, graphite involves very weak van der Waals bonds, just like in talc. As a result, the sheets of graphite just peel off from each other. When you compress the diamond to a very high level, however, all of those carbons are in covalent bonds with each other, and they are very tightly bound. I remember an episode from the old Superman television series. Lois Lane loses her diamond ring in quicksand, and when she’s not looking, Clark Kent takes a piece of coal and squeezes it in his hands to an inhumanly high pressure, and recreates her diamond. “Here, Lois, I found your diamond.” And this is essentially what the Earth does. It can take carbon in the form of graphite and squeeze it. The atoms rearrange themselves into a much denser structure: diamond. When you take the pressure away, however, (when Clark opens his hands) the diamond will slowly turn back into graphite, but this takes billions of years to do, and so during our lifetimes it’s not going to cause any appreciable damage.