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Bulk sediment samples are prepared for X-ray diffraction by grinding, which depending on the sample compaction, i.e. soft versus compactedlithified, can be accomplished by two different methods.
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The freeze dryer is comprised of a sample "bell" chamber and a Labconco freeze-dryer. On the bell are two valves, as shown in Figure 1. Each valve has an "Open" and "Closed" position. The top valve controls the vacuum inside of the bell, and the bottom valve controls the air flow between the cooling coil and bell. A valve parallel with the tube is open and allows air flow; a valve perpendicular with the tube is closed. In Figure 1, configuration A (closed) will hold a vacuum, but configuration B (open) will not.
Figure 1. (A) Freeze dryer bell valves in closed position. (B) Freeze dryer bell valves in open position.
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- Cut open the sealed sample bags and fold the top edge over to keep the bag open.
- Take the top of the bell off of the dryer and arrange samples in the bell, making sure no sample bags are pinched closed.
- Open the vacuum valves slowly so that you do not cause a large rush of air to blow the samples around. Figure 1A shows the configuration to dry samples and Figure 1B shows the configuration for loading and unloading samples.
- Flip the "On" switch located on the right side of the Freeze Dryer.
- Press the "Auto Refrigeration" button (Figure 2A) and then the vacuum button (Figure 2B). The temperature will start to drop and the vacuum pump will turn on.
- When the temperature drops to –40°C, the vacuum is created in the bell and the pressure drops.
- Expect the temperature to be between –42° and –52°C and the pressure to be ~0.350 mBar.
- The indicator lights (Figure 2D) show how the cooling and pressure reduction are progressing. When all indicator are lights are on the freeze dryer is at its peak performance.
- If there is an error the red "Alarm" light will turn on. Press the "Menu" button (Figure 2C) to view it and clear it if necessary.
- After samples are dry, slowly open the valves to release the vacuum in the bell. Remove the samples from the bell and store them inside the desiccator until they are ready to be ground to prevent reabsorption of moisture.
Figure 2. Freeze dryer control panel.
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Mortar and Pestle
Choose the appropriate mortar and pestle size (large or small) and place it on the counter. Obtain a glass slide, a sample holder, and a scoopula (Figure 3). Clean all items after each use with isopropyl alcohol and a Kimwipe.
Figure 3. Mortar and pestle sample preparation set up to grind soft sediment.
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The follow instructions are for two mixer mills, Spex 8000 and the Newer Retsch MM400. Currently the Retsch MM400 is the recommended mixer mill to use and it is located in X-Ray Prep or MBIO area of Chem lab.
Figure 8: Spex 8000 mixer mill. Featured are the safety latch, timer dial, and start/stop button in the middle of the dial.
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There are three types of grinding vessels available for the 8000 mixer mill: alumina ceramic, tungsten carbide, and hardened steel (Figure 9). Tungsten carbide and steel vessels are better for more robust grinding, and alumina ceramic is better for minimizing contamination. Check with the Science Party to see which vessel type is preferred.
Figure 9: Three types of mixer mill grinding vessels.
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Below are the components for each type of vessel. The steel (Figure 10) and tungsten carbide (Figure 11) both have a vessel body with attached lid and one separate lid that is screwed on. Cross-threading is very easy with these containers, so be very careful when screwing on the lid. Also note that the steel container has an O-ring, whereas the tungsten carbide does not. The alumina ceramic vessel (Figure 12) is assembled differently than the other two vessels: two cork rings are placed inside each lid, and the lids slip onto either side of the vessel body. 
Figure 10: Steel vessel components. (A) Lid (B) Container (C) Steel Grinding Ball (D) O-Ring
Figure 11: Tungsten carbide vessel. (A) Vessel container (B) Lid (C) Tungsten carbide grinding ball
Figure 12: Alumina ceramic vessel components. (A) 2 cork rings (B) Vessel body (C) Alumina ceramic grinding ball (D) 2 lids
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Put your sample inside the vessel. The material should be approximately the size of a pea to prevent any jamming and to ensure all pieces are ground up. Place 1 to 2 grinding balls inside the container. Tungsten carbide and steel vessels can take up to 2 balls. The alumina ceramic vessel is more brittle and 1 ball is recommended. Finish assembling the grinding vessel and open the lid to the mixer mill.
Figure 13: Inside the 8000M mixer mill. Highlighted is the sample holder clamp that holds the grinding vessels, the primary lock, and the secondary lock.
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The manufacturer's manual for the Retsch MM400 can be found here (in Retsch MM400 XRD Lab Notebook). Figure 14 shows the Retsch mixer mill.
Figure 14: Retsch MM400
Figure 15. Agate grinding jars and ball.
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Each grinding jar for the Retsch mill (Figure 15) has a main jar, a lid, a ball, and a white teflon spacer. Use the spacer to avoid damaging the agate (Figure 16). The inside of the jar and lid are lined with agate and the ball is of made of agate. Poor the freeze dried soft sediment (not hard rock) grains into the jar. Fill the jar half way to three-fourths with sample. Place one grinding ball in the jar. Clean off any of the sediment sample with a kimwipe that has gotten onto the jars threads and screw the lid on. Be mindful not to cross thread the lid to the jar.
Figure 16. Agate grinding jars with teflon spacer and the bent metal spatula for removing the teflon spacer.
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Place closed grinding jar horizontally in the clamps. Verify the pin is in closed position (Figure 17) and tighten the clamps by turning the handle until the jar is securely tighten (hand tight is good). Close lid and select desired program to run.
Figure 17. Retsch jar clamp
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After the grinding jars are secured in the clamps, the pin is in the closed position and the clear cover is closed it is time to select grinding settings (Figure 19). Program 1 (A) is currently set for 12 min at 25 1/s. This setting is good for clay rich material. Softer sediments such as CaCO3 typically need less time. It is important to only grind soft sediments in the agate jars and hard material should be ground in the shatter box. After Grinding cycle has completed turn knob counter clockwise and pull pin up and into its open position (Figure 19A). Unscrew clamp and open jar to verify grind size. If sediment is still too coarse regrind.
Figure 18 A) Preset programs. B) The shaker frequency C) The time grinder is set for D) The start and stop.
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