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U. S. Geological Survey Open-File Report 01-041
A Laboratory Manual for X-Ray Powder Diffraction

SEPARATION OF THE SILT AND CLAY FRACTIONS FOR X-RAY POWDER DIFFRACTION BY CENTRIFUGATION

Centrifugation is an important research technique with wide applications in the basic sciences. Examples of geological uses include the separation of silt and clay for X-ray powder diffraction analyses and recapture of the entire sample after treatment with acids or oxidizing agents. Decantation may not be an acceptable alternative to centrifugation because normal gravitational methods of particle sedimentation take an inordinate amount of time, and for particles finer than 0.5 micrometers Brownian motion interferes with settling (Folk, 1974).

Mathematical basis for the centrifugation program. We have written a program based on Stoke's Law that calculates centrifuge run times and speeds for particles of various diameter and density in media of different temperature, specific gravity, and viscosity (Poppe and others, 1988). The program, which was written in Unisoft C and implemented on UNIX system V, is fast, interactive, allows the user to select all particle- and medium-related parameters, and is not specific to any one piece of centrifugation equipment.

The program prompts the user for the particle radius in micrometers and the angular velocity in revolutions per minute and then converts these values into the proper units required by the formula. It is important to note that when analyzing non-spherical particles such as phyllosilicates, the value for the radius does not represent the actual radius, but the equivalent radius of a spherical particle of the same material which would settle at theDiagram showing initial and final distance from the axis of rotation. same rate as the actual particle. To obtain the correct results, the user must exercise care to ensure the the other values are in the proper units required by the program and when determining the initial (R1) and final (R2) distances from the axis of rotation. Because of mechanical differences in the designs of individual centrifuges, the user must measure these distances on the specific piece of equipment being used. The units of R1 and R2 are unimportant because they cancel out in the equation.

Program documentation can be viewed in either a text, Microsoft Word or HTML version. Program source code and include header file, program documentation, and a helpful README file are stored in the directory labeled software located at the top-level of this CD-ROM.

 
PROCEDURE FOR SEPARATION OF SILT AND CLAY
BY CENTRIFUGATION 
Materials Required for Centrifugation: 
  • marking pencil
  • labeling tape
  • plastic centrifuge tubes (~50 ml) with
  • caps
  • centrifuge tube rack
  • squeeze bottle with distilled water
  • plastic beakers
  • dispersant (sodium hexametaphosphate)
  • spatula
  • ultra-sonic probe
  • lab tissues
  • thermometer
  • timer
 Materials required for centrifugation.
If the supplied centrifugation program is to be used, the operator should first determine the necessary information (initial and final distances from the axis of rotation, and the acceleration and deceleration times of the centrifuge for various speeds (rev./min.) and construct a table of run times for common temperatures. Measuring the initial and final distances from the axis of rotation.
Apply labeling tape to the top edge of the centrifuge tube and label the tape with the marking pencil. Note: the bottom edge of the tape can act as a guide facilitating equal filling of the tubes that will keep the centrifuge in balance. Apply labeling tape to and label the plastic beakers. Labeling the centrifuge tube.
Pour enough sample into the centrifuge tubes to fill the rounded part of the bottom of the centrifuge tubes. Note: The powder from the randomly oriented aggregate mounts can be used. Pouring the sample into the centrifuge tubes.
Add a small amount (about 0.25 g) of dispersant to the centrifuge tubes. The dispersant will cover the tip of a narrow spatula as shown. Adding dispersant to the centrifuge tubes.
Add distilled water to the bottom edge of the labeling tape. Place a cap on the centrifuge tube and shake the tube to homogenize the suspension. Adding distilled water to the edge of the labeling tape.
Disperse the sample for 15-20 seconds with the ultrasonic probe. Rinse the tip of the ultrasonic probe with distilled water and wipe it dry with lab tissue between sonifications. Disperse the sample for 15-20 seconds with the ultrasonic probe.
Check the water temperature and find centrifugation times in minutes and seconds from the prepared table. Checking the water temperature.
Turn on the power switch and press Stop/Open. This releases the safety latch and allows the cover of the centrifuge to be opened. Place the tubes in the centrifuge. Be sure that the centrifuge is balanced by having opposite tubes filled equally; 2, 4, 6, or 8 tubes may be used. Close cover. Placing the tubes in the centrifuge.
Adjust the speed control, set the machine's timer to hold, adjust the brake, press start on the centrifuge, and start the timer. Note: the timers on most commercial centrifuges are not accurate enough to use for silt and clay fraction separations. When the total run time minus the deceleration time (T-td) has expired on the timer, press stop. The speed should read near zero when the calculated total time is reached. Adjust the speed and timer on the centrifuge.
Remove tubes from the centrifuge, pry off caps, and pour the supernatant liquid into the plastic beakers. Be careful that the sediment on the bottom of the tubes is not poured off. If the silt and clay fractions are to be completely separated, repeat the centrifugation procedure until the supernatant is reasonably clear (4-5 times). Pouring the supernatant liquid into plastic beakers.
 
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