Separating minerals has the end goal of pure piles of individual minerals. You are often searching for relatively small and not all that abundant. Mineral separation techniques are equivalent to burning the haystack to find the needle; taking advantage of various properties that remove minerals you don't want, and leave what you do.
The first step is almost always turning your rock sample into a pile of individual minerals. There are two primary ways people do this.
- Crushing and grinding. Jaw crushers (like the Bico Chipmunk) reduce fist sized rock chunks into small pellets. Disc mills, or pulverizers (like the Bico UA Pulverizer) break these small pellets into sand. The hope is that the minerals preferentially break along grain boundaries, but themselves stay realtively intact. Realistically it is difficult to tell what percentage of the desired minerals are broken beyond recognition (a real problem in the (U-Th)/He world, as I blogged about here and here), but I would not be surprised if the total yield from a standard pulverizer was 50%. As far as the actual machines go, there really aren't that many options when it comes to the jaw crusher, you can buy large ones and small ones, but they operate in the same way. With the pulverizers, there are two main options in style; you can have them belt-driven or direct-drive. Unfortunately Bico no longer makes the direct drive model, and if anyone knows of another supplier, please let me know. The belt drive models are OK, but much less powerful and much more difficult to set up. Pulverizers work by grinding your sample between two metal plates, and the quality of your grind depends on how far apart the two plates are set. Belt drive plates tend to drift during grinding, whereas the direct drive plates stay set. Think of coffee, you want a standard drip grind, but the belt drives give you some drip, some espresso, and some french press. The other option with pulverizers is the material the grinding plates are made of. You can get various steel alloys, iron, and even ceramic. I have recently started using the super expensive Mo-steel plates and have now become a complete convert. Iron plates leave filings in your sample, which have to then be removed. The Mo-steel plates do not "shed" and consequently do not wear down as fast. Again, I'd be interested in other other experiences.
- Electric Pulse Disaggregation (EPD). EPD machines are now commercially available, although they are still pretty pricey. They were originally developed for use on lunar samples, where the waste generated during standard crushing and grinding would have just been unacceptable. The company marketing them now is called SelFrag. They have a pretty good website (complete with downloadable video), but don't show enough examples of separated crystals. EPD works by sending an enormous pulse of electricity through your sample, causing it to fall apart along grain boundaries. EPD separates whole crystals, even preserving delicate surface features. Bernhardt Saini-Eidukat at North Dakota State University has a nice page showing images of EPD separated minerals, I think made using a home made device.
Once your rock sample has been turned into a pile of sand, the order of the steps becomes somewhat arbitrary. It depends on preference, what mineral you are aiming to separate, rock type, and sample size. So I'll present the options in the order I tend to do them. The next step, for me, is to concentrate the dense minerals (namely zircon and apatite, but monazite and sphene also count). This is typically done hydrodynamically, using machines that are basically big gold pans. Gemeni tables and Wilfley tables work on the same principle: your sample is slowly introduced onto a sloped and vibrating grooved table that has a constant stream of water running over it. The "heavy minerals" are preferentially caught in the grooves, while the "light minerals" get washed away. You then collect the heavy and light fraction in buckets, and can effectively reduce your pile of sand from a big bucket to a small beaker. At one point in grad school I collected and separated some of the Fish Canyon Tuff. I was unimpressed with the total amount of apatite and zircon I ended up with, and decided to go back to the light fraction from the gemeni table and see if I had missed some substantial amount of apatite. I tried everything and ended up discovering that there was absolutely no apatite or zircon in the gemeni light fraction; they are pretty efficient machines.
I've also heard rumors of skilled geologists using actual gold pans to separate minerals. I am terrible at gold panning, and have never tried it with anything but river sand.
After hydrodynamic separation, some samples may need to be cleaned and/or washed. For granitoids, this means just time in an ultrasonic and rinsings with ultra-pure water. But you may also want to soak in acetic acid (to get rid of carbonate cements), hydrogen peroxide (do dissolve organics), or some other chemical (again, I'd love to hear more examples).
Two of the primary minerals geochronologists are interested in are non-magnetic (apatite and zircon). Minerals have slightly different magnetic properties, so the next step in separations is to take your cleaned heavy fraction from the gemeni table and run it through a Franz magnetic separator. Using a Franz is simple, your sand is slowly let into a vibrating metal channel that runs through a large electromaget. The magnetic field acts on the grains as they move down the channel, pushing the "magnetc" fraction to one side of the channel. You end up with two different streams of mineral grains, the magnetic and the non-magnetic, which can easily be collected once they exit the magnet. You can vary the power of the magnet, and really skilled users can effectively separate out very pure piles of magnetic minerals, including monazite, sphene (yes, I still call it sphene), different micas, amphiboles, etc... When you are done with all of the magnet powers, you are left with the non-magnetic fraction. This is hopefully mainly apatite and zircon, but is usually contaminated with quartz and feldspar that made it through the gemeni table. That means, time for the heavy liquids.
In my last post I mentioned a lot of heavy liquids, but I realized afterwards I should be more systematic in my presentation. So I'll try. First with a list of the heavy liquids I know about, then a brief discussion of the different ways to use them. All heavy liquids separate minerals by floating things less dense than the liquid, and letting the rest sink.
- The Tungstates: Sodium polytungstate (SPT), lithium polytungstate (LST of FastFloat) and lithium metatungstate (LMT now discontinued) : ρ=2.5-3.1 g/cm3. These liquids have the distinct advantage over all other products in the fact that they are non-toxic. Many of the other liquids I'll mention are nasty things, but the tungstates don't even require a fume hood. Their only downside is their relatively high viscosity, which means it takes a while for your heavy minerals to sink, and filtering the liquid is kind of a pain. But I don't care, the safety and freedom from the hood is well worth it. SPT, LPT, and LMT will float quartz and feldspar, and sink apatite, zircon, and pyrite (argh, pyrite).
- Tribromomethane or Bromoform : ρ=2.85 g/cm3. Bromoform is not pleasant to work with; it is very toxic and you have to avoid both skin contact and inhalation. It can be especially bad for your liver and kidneys, oh, and even better news, it might be a carcinogen. Bromoform has the same use as the tungstates, which begs the question, why does anyone buy bromoform?
- Thallium foimate or Clerici Solution : ρ=4.32 g/cm3. I've only heard of this in legend, well, and I've seen a locked cabinet with a "Warning, Clerici Solution" on it. I am guessing you use it to sink zircon and float apatite. Or you use it to destroy your enemies, I am not sure. I think I'd rather hand pick apatites from a pile of sand than use it. From the MSDS "May be fatal if swallowed. May be fatal if absorbed through the skin. Causes respiratory tract irritation. Causes eye and skin irritation. May cause digestive tract irritation. May cause central nervous system effects. May cause liver and kidney damage. May cause cardiac disturbances."
- Acetylene Tetrabromide or Tetrabromoethane (TBE) : ρ=2.96 g/cm3. TBE, by the way, is also called Muthmann's Liquid, I like that name, had never heard it before tonight, and thought I'd mention it. (could diet coke be called Thermochronic's Solution?) TBE is nasty, but allegedly less nasty than Bromoform, but is dangerous in similar ways, it attacks organs, is an inhalation hazard, and can be easily absorbed through the skin. TBE has the same general uses as Bromoform and the tungstates (separating apatite and zircon from quartz and feldspar). I've used TBE to make "feldspar juice" ρ=2.58 g/cm3, which lets you float k-feldspar and separate them from quartz and plagioclase feldspar. I've been able to get very pure feldspar separates, some of that data I'll be showing later.
- Methylene Iodide or Diiodomethane (MEI) : ρ=3.32 g/cm3. MEI is also nasty, but it has a really low viscosity and you typically don't need to work with large volumes, thereby decreasing the hazard. MEI will float apatite and let zircon sink, which is what it is mainly used for. Using MEI isn't too bad, the real danger is that you wash it with acetone, and the mixture of MEI and acetone is very flammable. If it catches on fire you would rather not be in the vicinity. But that is easy to avoid.
So as far as I can tell there are three primary ways people use the heavy liquids. I have only tried two of them, but here we go:
- Separatory funnels. These are straightforward to use, the have a valve at the bottom, you fill them with the liquid, dump in your sample, and let things settle. You can then open the valve, let out the dense minerals that have sunk to the bottom, but leaving the light minerals in the funnel. Separatory funnels have the advantage of being simple and easy to buy, but they use a large amount of liquid (50 - 100 mL), and because of their design can often leak. In addition, many are made with plastic valves, which get abraded and can actually have lots of little mineral grains stuck in them, that are almost impossible to clean.
- Constriction tubes and knitting needles. If you've never tried this, check out the classic paper Dumitru, T.A. and Stockli, D.F., 1998, A Better Way to Separate Apatite From Zircon Using Constriction Tubes, in P. van den Haute and F. De Corte (eds) Advances in Fission-Track Geochronology, p.325-330. These allow you to separate small samples using only a few mL of liquid. Check out the article for a description, and when the web resource describing the technique become available I'll post a link. Any description I try to do will just be confusing.
- Liquid Nitrogen. I've never tried this, but Ain't From Around Here says she's going to try it, so I am eagerly awaiting the results. The idea is that you put your sample and the liquid in a tube. Some people then centrifuge the tube, but even if you don't, you give things time to settle and separate. You then stick the bottom in liquid nitrogen, freezing the bottom liquid and effectively trapping the heavy fraction. The light fraction is then poured off, and you then just have to wait for the frozen liquid to thaw, and then pour off the heavies. Allegedly the liquids are not damaged by the freezing.
After this you are left with piles of pretty pure individual minerals. Some phases are easier to separate than others, but this is at least a good place to start. Each technique you use, at least for geochronology, has more steps, but they all begin with pure separates. If I missed something please comment or email. Mineral separation is really an amazing thing to watch. Parts of it are a pain, but some of the steps are just incredible. My favorites are the Franz and using MEI. In both of those you immediately see the separation....very gratifying.