Page 26 - Petrology of Sedimentary Rocks

dip it in distilled water or dispersant to get the mesh thoroughly wet on both
sides, otherwise the sludge will not run through easily. Pour the sample from the
bottle onto the screen while holding it over the large pan, and rinse out every
grain from the bottle. Now rock the sediment in the screen back and forth while
playing a fine jet of water on it, to wash the mud through the screen. Periodically
wash down the under side of the screen with your water jet. Continue washing the
sediment back and forth over the screen until the water runs through clear (this
takes a little time). Use as little water as possible, because you should end up
with a volume of water small enough to fit into the dispersing cup (page 20); by all
means end up with less than a liter or else you will have too much to fit into the
pipetting cylinder.

3. Pour the muddy water collected in the pan, into the dispersing cup, stir for 5
minutes, then pour into a liter cylinder and make it up to exactly 1000 ml.
Continue as under “2” on page I 9.

4 Take the sand remaining on the screen, dry it, disaggregate it, and sieve according
to the standard method. Any material that passes the 4$ (230 mesh) screen on this
sieving, retain and dump into the liter cylinder of mud, as it is material that
should have gone through when you wet-sieved it.

5. Pipette the fine fraction according to instructions starting page 34.

In working with samples that contain more than a few percent of clay (i.e.,
those that have to be dispersed and then wet-sieved), it is impossible to get the
total weight of “fines” (silt plus clay, material finer than 4$1 or 0.0625 mm)
directly; yet you must somehow obtain this figure in order to determine the
percentage of fines in the sample. Here’s the difficulty. Let us say you have a
moist sample of sandy mud (or else a dry sample which you have to moisten in
order to disperse it). It might appear easy to weigh the moist sample, then wet-
sieve it and weigh the amount of sand retained on the sieve, which would be a
certain percent of the total sample. But this is not true, since the total sample
weight we determined was that of the sample plus a large amount of water. A
second method would be to wet-sieve the sample and weigh the amount of sand;
then evaporate the muddy water passed through the screen to dryness, and weigh
that. This would be satisfactory were it not that further size analysis of the fine
fraction is made impossible by over-drying. You can never analyze the fines once
they have been baked.

There are three ways to solve this dilemma, and all involve an indirect
determination of the mud content. In Method I (the pipette method), the sample
is wet-sieved and the amount of sand weighed. The mud fraction, after
dispersion, is placed in a cylinder and diluted up to exactly I liter, and stirred
thoroughly. Twenty seconds after stirring, a pipette is inserted to 20cm depth and
exactly 20 ml. of suspension withdrawn and put in a beaker. The suspension is
evaporated, and the dried mud is weighed. The weight of mud multiplied by 50
gives the weight of mud in the total sample (since you have withdrawn exactly
l/50 of the muddy water in the cylinder). This assumes (I) that there has been no
loss of fines in the various operations, (2) that during stirring all the particles
have been perfectly uniformly suspended throughout the column of water, and
that exactly l/50 of a liter has been withdrawn, and (3) that there is negligible
error in the weighing operation, especially in regard to absorption of moisture
during weighing. Despite its drawbacks, this is probably the most accurate
method and the one we will use. Results obtained by this method may be checked
by using either of the two methods below.

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