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Pacific Northwest Geologic Mapping and Urban Hazards

United States Geological Survey

P.O. Box 25046

Denver, CO

80225

Contact: Shannon Mahan (smahan@usgs.gov)

Initial sediment preparation

1.      From initial sample, measure in approximately 50 g of sample, to ensure that the 4-11 and 90-125 micron sizes will recover about 3 to 5 g of sample before heavy liquids (less will be needed, approximately 25g, if the sample is comprised primarily of sand-sized particles, more if the sample is silty).

2.      Place sample in litre beaker. Add 4N HCl until entire sample is covered for carbonate removal. Let stand for 1 hour. Continue to add small amounts of 4N HCl to completely dissolve carbonates until no reaction is seen. Pour out liquid and replace with 500 ml deionized water and let stand for 2 hours. Repeat washing with deionised water.

3.      Add 20 ml of 30% H2O2 to sample. Stir thoroughly then cover and watch. If there is no foaming after 10 minutes or foaming subsides immediately add an additional 20 ml of H2O2. Repeat process until 100 ml H2O2 has been added to sample. Allow to sit for 2 hours, then add a second 100 ml of 30% H2O2 and let stand overnight.

4.      Pour out H2O2 and add 500 ml of dionized water. Let stand for 2 hours. Repeat washing with deionised water.

5.      Add 100 ml Na pyrophosphate (50g/l) as decoagulant. Break up and separate sediment by vigorous stirring. An ultrasonic bath may be used in place of stirring.

6.      To separate out size fraction during wet sieving, stack screens in following manner: 60-80 mesh (250-180 micron) while throwing out material >250 micron; 120-170 mesh (125-90 micron); >230 mesh (>63 micron) and place over bucket. The bucket will catch 4-11 micron and clay-sized particles. Wet sieve entire stack of screens then rinse each individual screen mesh into separate beaker.

Fine grains

1.      Follow steps 1-6 of initial sediment preparation. Place fine grain sediments retained in bucket (step 6) into graduated cylinder and fill with 1000 ml deionized water. Cover and shake vigorously completely mixing sample. Let stand for 50-55 minutes. Decant suspension (<11 micron) into beaker and dispense remaining sediments.

2.      Centrifuge suspension at high setting for 3 seconds then 15 seconds at low setting. Dispense liquid and retain silt sediment in bottom of tube. Wash in methanol. when enough silt has accumulated from the settling and centrifuge refining process.

3.      Dispense suspended fine grain sediments in methanol solution (1.5-2.0 mg of sediment /ml) on discs. Allow sediment to dry.

Feldspar grains (90-125 micron)

1.      Follow steps 1-6 initial sediment preparation. Place 2.5 g (or less) of sediment (90-125 micron) in test tubes. Fill tube full with heavy liquid (i.e. Na polytungstate or Lithium Sodium Tungstate) density of 2.57-2.62 g/cm3. Stir vigorously then centrifuge 10 minutes at high setting. Remove test tube from centrifuge. A clear liquid middle should result with feldspars on top and quartz plus heavy minerals on bottom. If middle liquid is not clear too much clay is present or too much sample was placed in tube.

2.      Immerse bottom of test tube in liquid nitrogen until bottom sediments are frozen. Rinse feldspar grains out with deionized water into clean beaker then wash continually to remove residual heavy liquid. Dry sample. Once dry the sediments should flow freely in beaker, any clumping means residual heavy liquid is adhering to the grains, rewash if this is encountered.

Quartz grains (90-125 micron)

1.      Follow steps 1-6 initial sediment preparation.

2.      Place 2.5 g of sediment (90-125 micron) in boiling tubes. Fill tube full with heavy liquid (i.e. Na polytungstate or Lithium Sodium Tungstate) density of 2.67-2.68 g/cm3.

3.      Follow feldspar procedure for separating minerals as discussed in steps 1-2. The liquid in the boiling tube should have a clear middle with quartz on top and heavy minerals on bottom.

4.      Rinse quartz from tube into clean beaker with deionized water. Dispose of remaining sediments in bottom of tube.

5.      Place sediments in Teflon beaker. Add 50% HF solution covering the grains. Stir every 5 minutes for 30 minutes (use sonicator if possible). Let sit for 10 additional minutes. Total etch time will be 40 minutes. Dilute with deionized water 5 times allowing grains to settle each time.

6.      Add 4N HCl until entire sample is covered and let sit for 5 minutes to remove any fluorocarbons that may have formed. Rinse 3 times and dry at 50C overnight.

This site is maintained by the Pacific Northwest Urban Corridor Geologic Mapping Project, part of the Geology, Minerals, Energy and Geophysics Science Center

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