Page 28 - Petrology of Sedimentary Rocks

C. It contains more than 40% mud: Disperse, wet-sieve, use pipette, wet-
sieve replicate or moisture-replicate method to get percent of mud,
then pipette the mud.

Decantation. Another way to get rid of small quantities of mud from the
sediment sample is by decantation. Decantation is not recommended if the sediment
contains more than 20-30% mud. Essentially, the sediment is stirred with water in a
beaker, and after allowing a certain time for the coarser particles to settle to the
bottom, the finer material remaining in suspension is siphoned off. This process is
repeated until the water is clear. The advantage of this method is that it is somewhat
easier than wet-sieving where only small quantities of mud are involved; also it can be
used when no wet-sieve is available. The disadvantages are the tremendous volumes of
water required and the necessity to evaporate all this water to dryness.
I. Decide what size material is to be decanted. If decantation is done in connection
with sieving, then either 44 or 4.54 (62.5 or 44 microns) is chosen. Determine the
settling time of particles of this size and density at the given temperature by
means of Stokes’ Law. For particles of 4.54 diameter (44 microns), the settling
velocity at 20°C is 0.174 cm/set.

2. Set a 1000 ml beaker or cylinder on a small stand about 6” above the table. Fill it
about one-third of the way with distilled water. Introduce the sample into the
cylinder and stir vigorously, distributing all the sediment evenly throughout the
column. Now, when the time comes for siphoning, you are to insert the sucking
end of the siphon to a distance about one inch above the layer of sediment that
has settled to the bottom of the cylinder. Measure the distance from the top of
the water to the point where the siphon will be. Let us say that this is 50 cm.
Then, particles of 4.5@ diameter will take 50/0.174 seconds, or 287 seconds, to
settle that distance. In practice this means that at the end of 287 seconds all
particles coarser than 4.5$ will have settled to the bottom and that only particles
finer than this size remain suspended. After computing the desired time, stir
vigorously and start timing the moment stirring stops. Fifteen seconds before
withdrawal time, insert the siphon into the bottom of the cylinder, keeping the
siphon I” above the deposited sludge on the bottom. At the proper time, start
siphoning the material off into an extra 1000 ml beaker. DO NOT SUCK ANY
SEDIMENT OFF THE BOTTOM OF THE CYLINDER.

Now, place the beaker in the oven and start evaporating it (this assumes that you
are not going to analyze the size-distribution of the fines themselves, but only
weigh them-- remember never to oven dry the fines if you ever expect to obtain
their grain size). Fill the cylinder with water, stir again, wait the same length of
time, and siphon off another liter of muddy water into another beaker. Repeat
this process until the water in the cylinder stands clear (you see this takes a great
deal of water, and you cannot use this method for obtaining fines for pipetting
because you end up with some 3 to 5 liters of water, and you can only have I liter
for pipetting).

Evaporate all the beakers to dryness, and add their weights (subtracting the
weight of any dispersant you may have used). The sand in the bottom of the
cylinder is also dried and weighed. Then the sand is sieved, and THE AMOUNT OF
MATERIAL THAT PASSES THE FINEST SCREEN (say 4.5@) -MUST BE ADDED TO
THE WEIGHT OF THE MUD IN THE EVAPORATED BEAKERS. The percentages
of each can then be computed.

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