As Stated by Cassie Fenton:
Mineral Separate Preparation for 3He Cosmogenic samples:
STEP 1: Remove only top 4 cm of the rock (for cosmogenic samples). (If you only use the top 3 cm or top 1 cm, etc., make a note of this. It affects correction of the 3He content in the rocks.) Break the top 4 cm of the rock sample into pieces small enough to fit the crusher (generally the size of a golfball, no bigger). I use a rock hammer and chisel for this process. In some cases, for harder samples, a rock saw is needed, but rarely.
STEP 2: Make sure you clean the crusher in between samples to avoid any contamination. This includes using a dry paintbrush, a vacuum, and/or compressed air to get any/all of the previous sample out of the crusher and out of containers used to catch/transport the sample.
Always save at least one golf-ball sized hand sample of your whole rock for an archive sample. You might need this for chemical analyses etc. in the future. Start with the crusher at its widest position and throw the rock pieces in, reducing the width of the crushing gap each time until you have a variety of grain sizes. You want the majority of your sample to fall in the [250-500 μm] and [500-1000 μm] mesh range (sieve sizes in the US are labeled differently – I used to use US sizes [20-60] or so), so you just have to eye it. Grains in the [125-250] range are okay for analyses too, but makes things a bit more complicated because the grains are much smaller (i.e. handpicking this grain size is slow and tedious). There will still be pieces larger than that, up to 1 cm or so, that’s okay. You want to keep some bigger pieces in case you need to crush more in order to obtain the amount of the mineral you’re looking for. Just don’t overcrush, or you’ll end up with a lot of powder (too small!!) and not mineral grains of desirable size.
STEP 3: Sieving. Here’s a short list of sieve sizes we use:
(From Comparison Table of U.S., Tyler, Canadian, British, French, and German Standard Sieve Series)
| U.S. Standard | U.S. Alternate | Tyler mesh designation |
| 2.00 mm | 10 | 9 |
| 1.68 mm | 12 | 10 |
| 841 micron | 20 | 20 |
| 420 micron | 40 | 35 |
| 250 micron | 60 | 60 |
The sieves we use are the U.S. alternate 10, 20, 40, and 60.
Set the sieves up descending in mesh size from top to bottom. (i.e. the 10 should be on the top, followed by the 20, 40 and 60, with a screen-free container (fitting the stack) at the bottom) to catch the finest part of the sample. You can either dry-sieve or wet-sieve. Both have their benefits. With dry-sieving, you don’t have to wait for the samples to dry, before moving onto the next step. Wet-sieving tends to clean the samples up, so the smaller grain sizes (<>
Either way, place your crushed sample in the top sieve and shake until you have a decent separation (Usually takes ~5 minutes). Take each container and empty it into a properly labelled bag. The labels are as follows (sample # followed by sieve size range; ex: 040609-01 [10-20]):
Clean sieves between samples, using a toothbrush and a thin tool to remove grains lodged in the screen/mesh.
STEP 4: Cleaning: Rinse the sample in a beaker and slowly pour off the dirty water until it runs fairly clear (don’t lose any sample!). Allow to dry. Oven should not be above 55º C (to minimize loss of He from crystals).
STEP 5: Magnetic Separation: I start with a hand magnet and mesh sizes [20–40] and [40-60], unless there are olivines large enough to fall in the [10-20] range, then I use that fraction as well. Often, in my basalt samples, the matrix is very iron-rich and comes off easily with a fairly strong hand magnet. If the hand magnet provides a good separate, put the magnetic material back in the sample bag and save it. Save the non-magnetic part of the sample (olivine, pyroxene, probably some plag and calcite).
Details:
1) Hand magnet: Obtain a typical hand magnet (they’re fairly weak, but work well). Dump your minerals grains on a piece of 8X11.5 paper with a clean piece of paper adjacent to it. Place a plastic baggie or weighing paper over your magnet to keep magnetic grains from sticking to the magnet, thereby decreasing contamination between samples. Run the magnet over the mineral grains, placng the magnetic grains on the empty paper, leaving the nonmagnetic grains behind. In our case, we are generally interested in olivines, pyroxenes, and feldspar. These generally stay on the nonmagnetic side, unless they have lots of inclusions or are coated with magnetic material.
If for whatever reason, the hand magnet doesn’t yield a very good separation, move on to the Frantz magnetic separator.
You can also use a Frantz if the hand magnet doesn’t work. Make sure your sample is washed before using the Frantz.
2) Frantz Magnetic separator: It is best to work with a [40-60] range on this machine, but the [20-40] range is feasible. The larger grains just tend to clog up the sample cup every now and again.
Adjust the desired angle on the ramp by turning the proper knob/wheel. I usually use an angle between 15-20 degrees tilting away from me (15 degree works best). A shallow angle tilting down to the sample collection cups also seems most productive. It allows the grains to interact with the magnet for a longer period of time, allowing for a cleaner separate. Take your clean sample fraction and pour a small amount of it into the sample cup. Turn on all appropriate switches, adjusting the strength of the magnet by increasing or decreasing the amps. I usually start with 0.25 amps, run the sample through 2-3 times. This will separate out the very magnetic material from the less magnetic material. I then switch to a higher amp (somewhere between 1.0 and 1.5) and run the magnetic fraction (when run at 0.25) through (2-3 times). This will separate off the very nonmagnetic. Then I just play around with values in between until I get a nice separate of my mineral of interest. Before changing the amps, I check each collection cup under a microscope to see which has the largest amount of my mineral of interest. These values will vary with different samples. You just have to pick and choose, use what best suits your sample.
STEP 6: Heavy liquid separation: We use lithium metatungstate with a density of 3.0 g/cc. At this density, olivines and pyroxenes (“heavies”) sink and everything you don’t want (“lights”) floats to the surface.
Here’s the address for the company we order from. We use litium metatungstate (density = 3.0). It’s water soluble, allowing for a density variation and recycling.
GEOLIQUIDS
15 E. Palatine Rd., Suite 109
Prospects Heights, Illinois 60070
1-800-827-2411
We recently ordered 3 lbs at a cost of $144/lb, I think.
Here’s a list of equipment we use to
CRU-5000 Centrifuge
50 ml polypropylene, graduated, conical centrifuge tubes with caps
3-piece Whatman filter funnel (9 cm in diameter, 200 ml volume, 17.9 cm height, Whatman no. 1950-009)
Whatman filter papers (medium-fast (1), 9 cm in diameter)
- fill the bottom of centrifuge tubes with sample grains. (Don’t fill it past the 5 ml mark). Label clearly.
- add lithium metatungstate (at a density of at least 2.95, preferably 3.0 or greater) to the 25 or 30 ml line (depends on how much sample you use and how much heavy liquid you have to work with).
- Cap and shake all the tubes
- place in centrifuge at 1.5X1000 rpm for at least 10 minutes.
- have a doer of liquid nitrogen set aside and set up funnel/filtration apparatus.
- remove tubes from centrifuge,
STEP 7: Acids: I usually use HNO3, HCl , and HF to clean up the “heavies” samples. To get rid of any secondary calcite, start with dilute (5-10%) HNO3 and your “heavies” in a beaker placed in a sonicator for 15 to 20 minutes. Then use dilute (5-10%) HCl (in sonicator 15-20 minutes). Sometimes the minerals are well-oxidized. For example, oxidized olivines tend to have an iddingsite “rind” (a red coating) around them. Sometimes HCL will get rid of this. If it doesn’t, move onto a 5% HF solution.
HF treatment: WEAR HEAVY GLOVES, A LAB COAT, AND A FACE MASK. HF is very nasty stuff. Pour your minerals into a small glass or Nalgene beaker (HF etches glass, so if you use glass beakers, set them aside after that for HF use only) and add a 5% HF solution until it submerges your minerals. Don’t use too much. It isn’t necessary to fill the beaker, just top your minerals with the solution. Place in an ultrasound for 15-20 minutes, depending on how oxidized your sample is. Check your sample regularly to make sure you don’t dissolve you minerals completely. Pour off HF into waste container, rinse your sample, and dry. Repeat as necessary, until the majority of your sample is clean (iddingsite-free).
STEP 8: Microscope check. After isolating the olivine/pyroxene, and cleaning them up with acids etc, you should have a good separate. I always check under a microscope and handpick out what looks like crappy mineral grains (grains that are still dirty, or that are obviously not oliv/pyroxene). You want the purest separate of olivine or pyroxene possible.
