Today I’m a consultant investigating and cleaning up soil and groundwater contamination (click here for more information); I also have a podcast called That’s So Second Millennium where I talk about science, geology and physics in particular, as well as religion and philosophy.
As far as how I got into geoscience in the first place… I was always that little boy who was really interested in math, and that expanded to include chemistry and minerals in high school. Over time the elements came to have personalities for me. I love color, so minerals were natural things for me to love as well. Years later, when I taught mineralogy, I assigned lists of elements – oxidation states – colors for quizzes. Unfortunately, it seems that students never enjoy anything as much when they’re going to be tested over it as I did when I was reading it for fun.
Hopefully you’re reading this blog post for fun, though, so let’s give it another go.
Elements, color, and minerals
You may have picked up in high school or college chemistry that the periodic table has the shape that it does because of the quantum behavior of electrons. They sort themselves out into shells and subshells. The elements in each row of the periodic table have their outermost electrons (in ground state, the lowest energy configuration) in a given shell: 1 in the first row, H and He, 2 in the second row, Li to Ne, and so on. Each shell has one or more subshells–those are those s, p, d, f letters you learn about.
How does that translate to light and color? Well, light comes to us as little bits of energy called photons. The whole electron structure business is about energy, and the jumps in energy electrons need if they are going to jump from one subshell to another. Visible light is made up of photons with a particular range of energies. Those energies happen to be about the right size to coax electrons to jump around inside the d subshells of atoms big enough to HAVE d subshells, but not completely full ones. The elements that fit that description are down there in the low spot in the middle of the periodic table, the transition elements, or you might nowadays call it the “d-block.” The rare earths, or lanthanides and actinides, or “f-block” elements also work.
If you run your eyes along the top line of the d-block, you see all in a row chromium, manganese, iron, cobalt, nickel, and copper. All of those are important elements in geochemistry and in industry, iron of course being a major element and the most abundant. They also all happen to be “willing” to lose variable numbers of electrons, go into different oxidation states, and exhibit different colors:
As you can see with cobalt and nickel, the oxidation state is not the only thing that controls the color. The ligands – molecules or ions – bonded to the metal change the behavior of the electrons and produce a whole spectrum of colors. Thus, this table is only an attempt to note some of the most common colors. You can explore the subject in a number of different directions, for an example click here.
Meanwhile, most compounds of non-transition elements, especially the “s-block” elements to the left of the periodic table like sodium and calcium, are colorless or white. It takes more energy to jerk around s and p electrons, and those energies correspond to ultraviolet photons.
Having d or f-block elements is not the only way for a mineral to wind up colored, by any stretch, but it is very common. Here are some of my favorite colored minerals and the elements that make them so, along with mugshots from mindat.org:
Uranium and nuclear waste
My criteria for choice of dissertation topic and therefore advisor and graduate school essentially came down to this. When I ran into Peter Burns (yes, Simpsons fans, I learned about uranium from Dr. Burns, go figure) at Notre Dame, and found out that I could work at the lunatic fringe of the periodic table, I decided to go for it. I’d recommend broadening the thought process beyond just the subject matter if you’re choosing a graduate program, but I can definitely report that uranium geochemistry is not boring.
At that time, 15 years ago, this place called Yucca Mountain in Nevada was in the news as the one place under consideration for storing the U.S. high level nuclear waste from power plants. I can’t possibly go into all the issues surrounding high level nuclear waste – weapons work generates different wastes than power plants, there’s the whole reprocessing question, the security problem so that waste doesn’t get stolen and made into dirty bombs, it goes on and on.
Let’s focus on a few key issues. Whether it was the best idea or not, nations around the world built quite a few nuclear power plants. We have dozens here in the U.S., and NONE of their high level waste has ever been permanently disposed of.
Although nuclear waste is nasty stuff to deal with, nuclear power has one big advantage today: it gives you juice without having to burn fossil fuels. Wait, let me make that two advantages: unlike renewable energy from solar and wind, nuclear power plants provide baseline power regardless of the weather. So it might not be the best solution to move completely away from nuclear power just yet.
(Really, they need to get fusion plants working so that we can stop dealing with uranium, but we’ve been waiting an awful long time for that. We may have working Star Trek transporter beams before we have fusion reactors at this rate.)
So we really, really need places to put all this high level waste safely. That means we need to understand how uranium geochemistry works well enough to put together reliable models. That means we need to know what uranium species are in solution at particular geochemical conditions.
Uranium is a weird element – I did not call it the lunatic fringe of the periodic table for nothing. Uranium(VI), the oxidation state of uranium when it’s in equilibrium with all this nasty oxygen stuff we have in Earth’s atmosphere, is nearly always in the form of a weird complex cation called the uranyl ion, UO22+. Those two oxygens stick off into space to make this sort of three-ball dumbbell.
You may be aware that there are a lot of carbonate minerals… most metal carbonates are insoluble in water. Not the uranyl ion. Uranyl carbonate is mad soluble. There are also uranyl hydroxide ions in water solution at a variety of pH conditions. All this was known reasonably well from studies dating way back, some in geology (especially related to ore deposits of uranium) and some from chemical engineering. So in the run up to deciding on whether to do the Yucca Mountain repository or not, these existing studies were used to model the geochemistry and how long it would take the uranium to escape and how far it would go. Like all engineers and bureaucrats, the people involved were pretty confident about their answers.
For a trace element, uranium forms a lot of distinct minerals. That tends to happen when your chemistry is weird and you don’t fit into the sites of other elements in ordinary minerals. There were and are many of these minerals whose structures are not yet known. At the time, my research group (not me personally) was interested in a weird pair of minerals called studtite and metastudtite. Their structures weren’t known. Their bulk chemistry seemed to indicate peroxide ions, which would be very strange; there aren’t any other peroxide minerals, because the peroxide ion is really unstable. As I recall, Peter didn’t think they were really peroxides once they were crystalline, although he might remember it differently.
In any case, as it turns out, you can use peroxide to synthesize studtite and it is, in fact, a peroxide. The peroxide must be generated by radioactivity chewing up water molecules to make peroxide in the intense environment around other uranium minerals.
But as it turns out, on the way to making studtite, the real science happened.
If you jack uranium and peroxide into solution at certain pH conditions, you get crystals of studtite. At other conditions… well, you get a solution, and if you evaporate it down, depending on the counter ion (you need some cations like sodium, lithium, etc. for charge balance) you get something delightfully frightening:
Nobody knew these things existed. They’re actually pretty stable in solution. In a nuclear waste repository, like oh say Yucca Mountain, with MAD amounts of radiation from not just uranium but a whole bunch of hot, hot fission products, there could be oceans of peroxide and the conditions could be just right for making these things, which would traipse off into the Nevada groundwater and do things those previous geochemical models did not suspect.
Yucca Mountain died because of politics, not because of these studies. It may be just as well. Maybe we dodged a bullet there. In any case, we need to do something else with all that waste, and there may be some more craziness lurking out here on the lunatic fringe that we’d better put into our models before we pull the trigger.
For my first postdoc, I studied the interaction between clay minerals and high-pressure carbon dioxide. This research was funded by Shell in the Netherlands and was aimed at discovering whether carbon sequestration in deep aquifers is a viable option. An aquifer is a permeable rock with water in it, and deep aquifers have caps of less permeable rock called aquitards. Clays tend to be the dominant minerals in these aquitards. Many clays have the ability to expand or contract their crystal lattice and are called swelling clays.
Carbon sequestration involves scavenging carbon dioxide from power plant emissions and compressing it into a liquid or supercritical fluid. Carbon dioxide below the critical point liquifies at around 60 atmospheres, not a very high pressure. It’s actually very easy to make supercritical carbon dioxide, as the critical point is only around 30 C.
This fluid is then injected into a deep aquifer to get it away from the atmosphere. By the time it gets into that aquifer, it will be warm enough to be supercritical even if it was not at the surface. The supercritical fluid is lighter than water, so it rises, and the caprock will have to hold it in place if the sequestration effort is to work.
The following website and figure from Shell may help make more sense of this process. Click here for information on carbon capture and storage and here for an explanatory figure.
When we started the experiments, we were concerned that the carbon dioxide would suck water right out of the clay and cause the caprock to shrink and crack. Remarkably, the opposite was what we mostly observed. If anything, carbon dioxide entered the clay and swelled it. This is mostly good news: although swelling could also destabilize the caprock, a modest amount of swelling will actually close cracks and make the caprock better at holding in the carbon dioxide.
The best advice I could give to young scientists is to ask questions. Ask all kinds of questions and just talk to people. Get specific about what you can expect from a career in academia, in environmental consulting, in mining, in geotechnical, in whatever industry. Make friends and be a friend. Tell people about the things that light you up and also the things that make you sad or afraid, and be a welcoming person when other people respond in kind. This was immensely hard for me when I was in college: I was definitely a loner and pretty depressed most of the time. I had to learn eventually that I had to talk to people whether I felt up to it or not.
At the same time, be gentle on yourself. You’ve got plenty to offer the world, whatever your problems or family issues or your relationship status.