XRD Sample Preparation

Sample Preparation

Sample preparation techniques include the following:

• Drying and powdering solid samples

• Mounting smear slides for small samples

• Removing carbonate from high-carbonate samples

• Separating the clay fraction from larger grain size particles

• Expanding swelling clays with ethylene glycol treatment

Preparing Solid Samples

Solid samples are prepared for X-ray diffraction by grinding, which can be accomplished by several different methods, depending on the sample matrix, the size of the sample, and/or quantity of prepared material needed:

Grinding Solid Samples

1. Freeze-dry sample(s) for at least 12 hr before grinding.

2. Grind solid samples to a fine talc-like powder using one of the following methods:

* Agate mortar and pestle

* Spex Shatterbox

* Spex Mixer Mill (tungsten carbide, hardened steel, agate, or alumina ceramic)

* Rock Labs hardened steel “masher”

3. Transfer the powdered sample to an appropriate labeled glass bottle.

4. Select the steel or plastic sample holder unless the sample material is very small (see Sample Slurry/Smear Slide Mounting for Small Sample Amounts).

5. Place enough powder from the labeled sample bottle to fill an empty sample holder. Gently press the powder flush with the sample holder using a glass slide. The surface of the powder must be smooth. Remove excess powder from the sample holder edges and carefully place in the appropriate XRD slot.

Sample Slurry/Smear Slide Mounting for Small Sample Amounts.

The following technique is modified from the U.S. Geological Survey Open-File Report 01-041, A Laboratory Manual for X-Ray Powder Diffraction. For a very small amount of sample material (i.e., end of a tooth pick), samples may be ground to a fine talc-like powder and smeared onto one of the quartz disk inserts. Although not useful for semi- quantitative analysis, this method is useful for rapidly determining bulk mineralogy.

Preparing Smear Slury/Smear Slide

1. Grind the sample to a talc-like powder (<0.062 mm).

2. Place a small amount of sample in the center of a quartz or single crystal quartz disk.

3. Add 2–3 drops of acetone or distilled water to the sample. Note: Acetone dries faster than water will.

4. Spread the sample to a thin layer with a glass rod (rolling it over the sample works well).

5. Place sample in dessicator to dry before running in the XRD.

Removing Carbonates before Clay Separation

It may be necessary to dissolve the carbonates in some sediments before the clay minerals can be identified. The

goal is to remove as much carbonate as possible to isolate the material contained within the carbonate for analysis.

Hydrochloric Acid Treatment

The simplest method for removing carbonate from the sample is treatment with HCl. However, treatment with

strong acids to remove carbonate can attack the structure of the clay minerals, particularly trioctahedral minerals.

Be aware that this treatment may affect clay crystallinity.

1. Place undried sample on a glass slide or quartz disk.

2. Using a pastuer pipette, slowly drop 2M HCl on the sample until bubbling/fizzing stops.

3. Dessicate and transfer sample to sample holder for analysis.

Acetic Acid Treatment

A slightly more involved method that removes carbonates (from Kitty Milliken, UT-Austin) is as follows:

1. Place ~2 cm3 of undried sample into a centrifuge tube with 25 mL of acetic acid (10% solution). Mix well, and

let sit until the reaction ceases.

2. Shake well again to ensure the reaction has stopped (i.e., no more bubbles).

3. Spin sample in the centrifuge (15 min at 1500 rpm)

4. Decant the acetic acid solution and dispose of the acid solution properly.

5. Add 25 mL of DI to the centrifuge tube and centrifuge again for 15 min at 1500 rpm.

6. Decant the clear water.

7. Repeat the “wash cycle” (Steps 5 and 6) with DI.

8. Place washed sample in a large beaker with some distilled water and a little 1% Calgon.

9. Suspend the clay material by placing the beaker in a sonic dismembrator for ~1 min.

10. Do not let the sample heat up.

11. Transfer the sample to a clean centrifuge tube and spin for 5 min at 1000 rpm to remove the >2 μm size frac- tion from suspension.

12. Remove the <2 μm size fraction by collecting the top 1 cm of solution with an eye dropper. If it is necessary to

resuspend flocculated clay particles using the dismembrator and more Calgon solution, 15 min at 1500 rpm in

the centrifuge may not settle the <2 μm particle size. Increase the speed to as much as 5000 rpm to remove

the clay from the Calgon solution.

13. Make an oriented clay mount by placing 2–3 drops (enough to cover the quartz disk) of clay suspension onto a

glass slide and let dry in a dessicator.

Separating Clay

There are various methods for separating clay from coarser material. Those listed below are methods used

onboard. Discuss with the scientist(s) if other methods should be used.

“Quick and Dirty” 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% Calgon solution. Use

an ultrasonic bath or dismembrator, if necessary.

2. Centrifuge the Calgon solution/sample mix at 1000 rpm for 5 min to remove the >2 μm size fraction.

3. Decant the Calgon solution (containing suspended clays) into a new centrifuge tube, and spin it at 1500 rpm

for 15 min to remove the remaining <2 μm clay-size fraction.

4. Decant the Calgon solution and wash the clay residue with distilled water.

5. Spin-down again at 1500 rpm for 15 min. Repeat steps 4 and 5 as necessary to remove the Calgon.

6. Make an oriented clay mount by placing 2–3 drops (or enough to cover the quartz disk) of solution onto the

quartz disk and let dry in the dessicator.

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, mont- morillonite, nontronite and beidellite), some mixed-layer clays, and vermiculite as an aid to mineral identification.

There are two ethylene glycol treatment methods:

*Vapor

*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.

1. Find the “Glyconator” container stored in the ICP prep sink cupboard.

2. Pour ethylene glycol to a depth of ~1 cm in the bottom of the container.

3. Place the samples to be gylconated onto the shelf.

4. Place container (glyconator) in an oven (60°–70°C) overnight.

5. Keep samples in glyconator until ready to analyze. Glyconation may only last for 4 h after the samples are re- moved from the glyconation container.

Quick Method 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 lab tissue.

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).