Clay Separations focus on separating the clay size fraction, <2 µm, from the rest of the material. In order to get only that size fraction we prepare the sample in a different way compared to bulk analyses. Do not freeze dry samples waiting for clay separations.
If the samples have already been dried the treatments will still work but using fresh, wet samples is easier. Below are outlined various treatments and methods we do on board the JOIDES Resolution.
Clay separation requires the use of hazardous chemical reagents. Please read carefully the following before starting.
Ethylene glycol is toxic and should not be ingested. It is also harmful if inhaled or absorbed through the skin and eyes. Proper personal protective equipment (PPE) should be used when handling this compound. Ethylene glycol is kept in a safety cabinet in the Thin Section Lab (Figure 1).
Figure 1. Safety cabinet in the Thin Section Lab
Concentrated hydrochloric acid HCl (~12M) is highly dangerous. It can cause severe tissue damage on contact, is highly toxic, and the fumes present similar risks of poisoning and chemical burns. When mixed with water, hydrochloric acid liberates large quantities of heat, so appropriate care should be used when diluting this compound. Note that the 2M hydrochloric acid used in the carbonate dissolution procedure is still dangerous and should be treated with the appropriate care.
Glacial acetic acid (~100%) is highly dangerous. It can cause severe tissue damage on contact. When mixed with water, glacial acetic acid liberates a lot of heat, so appropriate care should be used when diluting this compound. When diluted to ~10% concentration, it is very similar to white vinegar, so while it is still acidic and could cause tissue damage, it is not as hazardous. Acetic acid (10%) is kept with Borax below the sink in the ICP preparation part of the Thin Section Lab (Figure 2).
Figure 2. Acetic acid (10%) stored in the Thin Section Lab
This chemical largely consists of potassium sulfate and is not expected to be a health hazard. It is used as a laundry booster (Figure 3). It is stored in the Thin Section Lab with the Acetic acid (Figure 2).
Figure 3. Borax
Technique for clay separation requires the use of acetic acid. To dispose acid solution properly and environment-friendly, use the black sink in the Chemistry Lab or in the Thin Section Lab (Figure 4). These sinks are directly connected to a specific container in the ship dedicated to acid treatment. DO NOT USE OTHER SINKS !!! Use flowing water to dilute the solution.
Figure 4. Black sink to dispose acid solution
XRD analyses on clay separation requires several preparation steps:
It may be necessary to dissolve the carbonates in the sediment to better identify the clay minerals. The goal is to remove as much carbonate as possible to isolate the material contained within the carbonate for analysis. There are two standard methods for removing carbonate aboard the JOIDES Resolution: (i) hydrochloric acid (HCl) and (ii) acetic acid. Ask the Science party which method they prefer. If there is no preference, use acetic acid.
This is the recommended treatment for carbonate removal. The process is slightly more involved than the HCl procedure, but far less damage is done to the mineral structure. The following steps are from Kitty Milliken (UT-Austin) and are shown in Figures 5 to 8.
Note: If the centrifuge in the Chemistry Lab is busy/used by Scientists, there is another centrifuge in the Paleo Prep room of the Core Deck.
Figure 5. Decarbonation with acetic acid.
Figure 6. Using the shaker to help decarbonation reaction.
Figure 7. Setting the centrifuge.
Figure 8. Disposal of acid solution and DI washing cycle.
HCl is the simplest method for removing carbonate from sediment but does have severe drawbacks. Strong acids damage the mineral structure, especially within trioctahedral minerals. Before proceeding, be aware that this treatment may affect clay crystallinity.
There are various methods for separating clay from coarser material involving a series of centrifuging or gravity settling. Those listed below are methods used on-board. If you removed carbonates first, start here after your water washes are finished and the water has been decanted. If you did not remove carbonates, take approximately 5 mL of sample material and put into a centrifuge tube.
Get the sonic dismembrator case and probe power source (either in the Thin Section Lab or Chemistry Lab). Using the dismembrator is a very effective way to fully and randomly suspend the material. Suspended material can then separate out according to size, with the largest grain size on the bottom and the very small clay size fraction on top.
The dismembrator setup is shown in Figure 10. The power source is connected to the probe. The probe fits into the case through a hole in the top.
Figure 9. Adding Borax and Suspending Material with the dismembrator. Make sure to adjust the probe half-way into the sample material
Figure 10. Dismembrator set up and Probe with power source. Dismembrator power source control Panel. (A) On/Off switch (B) Start button (C) Set button (D) Mode button (E) Start/Stop button
Clay mounts are different either you are using the D4 Bruker or the Aeris for the analyses.
Clay mounts are put onto a zero-background silicon disk that fits into a 2 mm steel sample holder (Figure 11). Only put the disks into sample holders that have a hole drilled in the bottom (Figure 11). The hole allows the disks to be taken out, otherwise they are stuck inside the holder.
The disk should sit flush with the sample holder. Some of the zero-background silicon disks are at different depths, so a quartz insert disk (Figure 11, Step #1) can also be put in the bottom of a sample holder with the silicon disk on top (Figure 11, Step #2). Do not force the zero background disks into the holder, it may break.
Figure 11. Steel sample holder with quartz insert disk and zero-background silicon disk. Clay Mount for the D4 Bruker.
Insert a zero-background silicon disk in a sample holder in a 'back loading' way (Figure 12, Step #1). Add on top of it an Aluminum spacer disk (Figure 12, Step #2) and clip the whole thing (Figure 12, Step #3). Flip the sample holder (Figure 12, Step #4).
The zero-background silicon disk can be heated up to 550ºC, which makes it suitable for further clay treatments as described below. However, the rest of the sample holder assembly must not be heated (magnetic part of holders). Please remind to dismantle the mount and put only the Silicon disk in a muffle furnace for additional clay treatment (Figure 14). Only the treatment with Ethylene Glycol is not 'destructive' because the temperature does not exceed 70ºC.
Figure 12. Clay Mount with Aeris Sample Holders.
1. Remove the <2 µm size fraction by collecting the uppermost 1 cm of solution with an eye dropper (Figure 13). It helps to add a little isopropanol. If necessary, resuspend flocculated clay particles using the dismembrator and add more Borax solution.
2. If material is still very suspended, try centrifuging the samples for 4 minutes at 750 rpm. In this instance, the >2 µm size fraction will be the only fraction suspended in the liquid and all the larger grains will be packed in the bottom. Take the suspended material with an eyedropper and put it on the quartz disk.
3. Make an oriented clay mount by placing 2–3 drops (enough to cover the disk) of clay suspension directly onto the silicon disk (Figure 11, Step #3 and Figure 12, Step #5). If the material is not spreading evenly, add a drop or two of 70% isopropanol and spread the material around with a small glass rod (Figure 11, Step #4 and Figure 12, Step #6). Once spread, let the sample dry in the desiccator (Figure 14A).
The clay particles orient themselves as the solution dries on the disc. Note that it can be difficult to determine if there is enough material on the disc for a scan. Sometimes it may appear as if there is no sediment in the upper 1-2cm and thus nothing on the disk. Try scanning in the XRD before assuming there is nothing in the water drops added to the silicon disk. It is surprising what the XRD will return with very little sample. If the scan is not satisfactory add a few more drops from a bit deeper in the test tube and re-scan.
4. Once the sample is dry (Figure 14B), you are ready to run it through the D4 or the Aeris. If there are additional treatments requested, continue to the sections below after running the samples through the X-Ray diffractometers.
Figure 13. Collecting the suspended material with an eye dropper. Make sure to have enough clay material.
Figure 14. A. Dessicator and oven for glycolation. B. Dried sample preparation after dessication.
The following techniques are modified from the U.S. Geological Survey Open-File Report 01-041, A Laboratory Manual for X-Ray Powder Diffraction (hard copy available in the X-Ray Lab Library). Ethylene glycol can be used to expand swelling clays (e.g., smectites, montmorillonite, nontronite, and beidellite), some mixed-layer clays, and vermiculite as an aid to mineral identification. There are two ethylene glycol treatment methods:
The advantage of the vapor treatment is less disturbance of the sample and less amorphous scattering of X-rays by excess liquid than in the Quick method (below).
Figure 15. Glycolation: Vapor Treatment
Keep samples in the glycolator until ready to run through the XRDs. Glycolation only lasts for 4 hours after the samples are removed from the glycol atmosphere.
Before placing samples in the XRDs, wipe all sides of the sample holder with a Kim Wipe to remove any ethylene glycol. Ethylene glycol is sticky and can damage the grabber arm and sample spinner.
Several clays have intensity peaks at very similar angles making it difficult to distinguish one clay from another. Heating clays is a way to work around this. We can run a sample through one of our XRDs, heat the sample in the muffle furnace, run again, and then compare the scans. For example, kaolinite and chlorite have overlapping peaks, making it hard to differentiate one from another. Heat morphs kaolinite, and it develops an amorphous signal, essentially removing its presence in the scan and leaving only the chlorite signal. The amount of kaolinite and chlorite can be determined by comparing these scans.
When the muffle furnace has finished its program and is cooling down, wait until the temperature reaches between 100° and 200°C before removing the sample (Figure 16C). The sample can be placed in the desiccator until it reaches room temperature. Once it has completely cooled down it can be run in the XRDs.
Figure 16. A. Muffle furnace in the Chemistry Lab. B. Clay Mount in the muffle furnace. Note that only the Si disk of Aeris sample holder is put in the furnace. C. After heating.
For the double peaks of Kaolinite and Chlorite, heating the sample only suggest that one of the minerals is present, it will not give final and complete results for a sample that contains both Kaolinite and Chlorite. In order to determine which mineral is present, an additional treatment is necessary.
If the dismembrator does not start the program after pressing the button "Start", i.e., if the screen still displays 'P 0' (Power=0) and not 'rdy' (Ready), press the "Reset" button, then press "Test" and finally "Start". The dismembrator should clear the error and start the program normally. Power on the display screen should be 'P 11' when a sample is vibrating. The cause of this error is unknown and happens when the dismembrator is turned on.
Jackson, M.L., 1956. Soil Chemical- Analysis Advanced Course by Hsueh-Wen Yeh, Hawaii Institute of Geophysics, 1980.
Moore, D.M., and Reynolds, R.C., Jr., 1989. X-ray Diffraction and the Identification and Analysis of Clay Minerals: New York (Oxford University Press).
XRD Sample Preparation Clay Separations - September 27, 2022