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.

Health, Safety and Environment

Clay separation requires the use of hazardous chemical reagents. Please read carefully the following before starting.

Reagents necessary for Clay Separation

• Ethylene glycol or ethanediol: used for clay glycolation
• 2 M HCl 16.4% v/v: used for carbonate dissolution
• Glacial acetic acid, 10% v/v solution: used for carbonate dissolution
• Distilled (dionized reagent) water (DI)
• 1% w/v Borax solution

Chemical Hazards

Ethylene Glycol (Ethanediol)

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

Hydrochloric Acid, Concentrated, or 2 M for Carbonate Dissolution

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.

Acetic Acid, Glacial, or 10% for Carbonate Dissolution

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

Borax

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

Nitric Acid, Concentrated, or 10%–15% for the Water Bath

Concentrated nitric acid (50%–70% HNO₃~ v/v) 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, nitric acid liberates large quantities of heat, so appropriate care should be used when diluting this compound. This compound is also a strong oxidizing agent, so nitric acid waste should not be mixed with any organic materials. Note that the nitric acid used in the water bath in the Chemistry Lab is still dangerous and should be treated with the appropriate care. Wear proper PPE.

Disposal of acid solution

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 XX). 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.

Figure XX. Black sink to dispose acid solution

Clay Separation Procedure

XRD analyses on clay separation requires several preparation steps:

1. Removing carbonates (to better identify the clay minerals)
2. Suspending material particles (to separate the < 2 μm clay size fraction from the rest of the particles)
3. Heating samples (to identify the presence kaolinite and chlorite)

Removing Carbonates before Clay Separation

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.

Acetic Acid Treatment

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 XX to XX.

1. Place ~2 cm³ (or 5 mL) of undried (preferred) sample into a centrifuge tube (Figure XX).
2. Add ~25 mL of 10 % Acetic Acid (Figure XX).
3. Mix and shake well (Figure XX). Let sit for at least 1 hour to decarbonate (until the reaction ceases). Close the centrifuge tube with its cap but not too tight to avoid unnecessary overpressure.
   
4. It helps to place the centrifuge tubes on the shaker in the cold room of the Chemistry Lab (Figure XX). Do not tight the tube too strong in the arm as it can break while vibrating. Set a shaking time of about 30 seconds (it is a good start) and a power value of 1. After using the shaker, shake the tube to ensure the reaction has stopped (i.e., no more bubbles).
   Note: Please note that sample with a large amount of carbonate (more than 50%) may require more than 1 treatment of Acetic Acid to reach complete decarbonation.
5. Next step is to spin the sample in the centrifuge. Make sure to choose the correct tube holders. It is very important to balance the centrifuge, and to evenly distribute weight. Put samples symmetrically opposite.  If you run a odd number of samples, keep the balance by filing up an additional centrifuge tube with DI to have an even number of tubes in the centrifuge. Turn on the centrifuge in the Chemistry Lab (Figure XXA). Press the "Speed" button (Figure XXB) and select a speed of 1500 rpm (rotation per min) by using the arrows to increase or decrease the value (Figure XXD). Press the "Time" button and select a time of 15 min by using the arrows to increase or decrease the value (Figure XXC). Press the "Start" button to start the centrifuge (Figure XXE).
6. Decant the acetic acid solution and dispose of the acid solution properly (Figure XX).
   
7. and wash with 25 mL of nanopure water (3X to remove all Acetic Acid)

Figure XX. Decarbonation with acid acetic.

Figure XX. Using the shaker to help decarbonation reaction.

Figure XX. Setting the centrifuge.

Hydrochloric Acid Treatment

HCl is the simplest method for removing carbonate from sediment but does have severe drawbacks. Strong acids damage the mineral structure, especially within trioactahedral minerals. Before proceeding, be aware that this treatment may affect clay crystallinity.

1. Place undried sample on a glass slide or quartz disk.
2. Using a Pasteur pipette, slowly drop 2M HCl on the sample until bubbling/fizzing stops.
3. Desiccate and transfer sample to sample holder for analysis.

Separating Clay

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.

Suspending Material

Get the sonic dismembrator case and probe power source. 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 22. The power source is connected to the probe. The probe fits into the case through a hole in the top. Put the probe into the top clamp and the sample tube into the bottom clamp. Adjust the positions so the probe goes into the tube. The probe should be about half-way into the sample material without touching the sides of the tube. The probe releases a lot of energy, and if it is touches the sides it will heat up the tube and the sample.

Figure 22: The dismembrator set up. Included is the soundproof box and the Probe with power source.

Figure 23: Dismembrator power source control Panel. (A) On/Off switch (B) Start button (C) Set button (D) Mode button

The probe power source is shown in Figure 23. Flip the "ON" Switch (Figure 23A). The settings are already set to our needs. To run as is, press the "Start" button (Figure 23B). If you need to change a setting, see the "Set" button (Figure 23C) and the "Mode" button (Figure 23D). From these two panels, you have access to the power, time, and displayed units. For more information, reference the Manufacturer Manual located in the side pocket of the dismembrator case.

1. Add ~25 mL of a 1 % borax solution into the centrifuge tube. Borax prevents the sample from flocculating. Too much Borax however will increase flocculation. 
2. Put the tube into the bottom clamp inside the dismembrator case. When satisfied with the positioning, press the "Start" button on the probe power supply (Figure 20B). If the material does not appear to be circulating throughout the whole tube or the tube is heating up, adjust the probe position and start the dismembration over.

Standard IODP Clay Separation Method: Not for Semi-quantitative Analysis

1. In centrifuge tube, mix a small amount of bulk sample (~5 mL; fresh, not dried) with 1% borax solution. Use agitator or dismembrator, if necessary to suspend particles.
2. Centrifuge the borax solution/sample mix at 750 rpm for 4 minutes to remove the >2 µm size fraction
3. Decant the supernatant liquid (containing suspended clay) into a new centrifuge tube 
4. Centrifuge the <2 µm fraction for 15 mins at 1500 rpm to remove the Borax solution
5. Decant the borax solution and add 25 mL of nanopure water to wash the clay
6. Repeat Steps 4 and 5 as necessary to remove the borax (USGS recommends repeating up to 4-5 times)

Alternative method from Exp. 379

1. Add 25 mL of 1% Borax solution to the clay plug 
2. Dismembrate the sample (machine is auto set on time), to remove the >2 µm clay fraction
3. Centrifuge for 4 mins at 750 rpm, decant the supernatant liquid into a separate centrifuge tube (you should end up with a ~full centrifuge tube of suspended clay)
4. Repeat steps 1–3 on the remaining >2 µm fraction
5. Centrifuge the <2 µm fraction for 15 mins at 1500 rpm to remove the Borax solution 
6. Decant and add 25 mL of nanopure water
7. Centrifuged for 60 mins at 3000 rpm, the liquid decanted before loading onto a zero background silica disk.

Preparing the Mount

Clay mounts are put onto a zero-background silicon disk that fits into a 2 mm steel sample holder (Figure 21). Only put the disks into sample holders that have a hole drilled in the bottom. 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 disks are at different depths, so a quartz insert disk can also be put in the bottom of a sample holder with the silicon disk on top. Do not force the zero background disks into the holder, it may break.

Figure 21: Steel sample holder with silicon disk

1. Remove the <2 µm size fraction by collecting the uppermost 1 cm of solution with an eye dropper (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. 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. Once spread, let the sample dry in the desiccator. 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 rescan. 
4. Once the sample is dry, you are ready to run it through the D4. If there are additional treatments requested, continue to the sections below.

Additional Clay Treatments before Scanning

Treating with Ethylene Glycol

The following techniques are modified from the U.S. Geological Survey Open-File Report 01-041, A Laboratory Manual for X-Ray Powder Diffraction. 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:

1. Vapor
2. Quick

 Vapor Treatment

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.

Figure 22: Glycolator

1. Find the "glycolator" container (Figure 22) stored in the ICP preparation sink cupboard.
2. Pour ethylene glycol to a depth of ~1 cm in the bottom of the container.
3. Take off the lid and place the samples on the rack inside the glycolator.
4. Place glycolator in an oven (60°–70°C) overnight (~12 hours).
5. Figure 19: Glycolator.Keep samples in the glycolator until ready to run through the D4. Glycolation only lasts for 4 hours after the samples are removed from the glycol atmosphere.

Before placing samples in the D4, 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.

Quick Treatment

1. Using a glass rod or eye dropper, apply a drop of ethylene glycol directly to the surface of the sample mount.
2. Samples are ready to be analyzed as soon as the glycol is uniformly absorbed on the sample mount. Excess ethylene glycol may be gently mopped up with a Kim Wipe.

Heating Samples

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 the D4, 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.

1. Prepare an oriented clay mount by adding several drops onto a zero-background Si disk in a sample holder and spreading it out evenly with a glass rod.
2. Put the clay mount in the desiccator until it dries.
3. Put the sample in the muffle furnace in the Chemistry Laboratory.

Different temperatures and heating times are used when trying to identify different minerals. Confer with your scientist to determine the parameters.

1. 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. 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 D4.

Clay Separation Method for removal of Chlorite

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.

1. Take the <2 µm clay fraction, this should be rinsed and free of any treatments, in centrifuge tube add ~20 mL of 1N HCl
2. Vortex the sample to completely dislodge all material before pouring sample into a 100 mL beaker, add stir bar once sample is in place
3. Set hotplate to 300°C (boiling point of HCl is ~101, but in order to maintain a continuous boil the hotplate needs to be much hotter)
4. Set the stir RPM below 100, this is only to keep the material suspended within the HCl
5. Start time once the samples have come to a complete boil, leave for 2 hours — add more HCl as necessary (DO NOT, let the sample go dry and burn)
6. Once the sample has boiled for 2 hours, turn off hot plate and allow sample to return to room temperature before pouring beaker contents back into a clean centrifuge tube, rinse beaker with DI water to collect all material
7. Centrifuge down for 15 mins at 1500 rpm, decant acid and rinse with 25 mL nanopure (3X)
8. After samples has been washed 3X and is free of HCl, centrifuge for 60 mins at 3000 rpm

References

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).

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. 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.