Page 41 - Petrology of Sedimentary Rocks

NOTE: Some very careful analysts bake their clean beakers, keep them at
absolute dryness in a desiccator, and maintain a desiccating chemical (calcium chloride)
in the scales to keep everything as dry as possible. If it were possible to keep
everything perfectly dry, this would be the best method; but inactual fact there is
more
probably -- error in this procedure because of the initial rapid weight gain of the
samples when exposed, even for a few seconds, to ordinary room air. Obviously you
cannot weigh all the samples with the same speed, therefore some will gain more
moisture than others so you will never be sure of the real weights. On the other hand,
if you treat them all equally by letting them come to equilibrium with the room
atmosphere, you wi II get more consistent results. After all, in the natural sediment
there is considerable moisture absorbed in the clay particles. But remember: if you
weigh each beaker in the “room-moisture” state, you must do all your other weighings in
the same way.

Computation. Tabulate the data as shown on the next page. Subtract the weight
of the cleaned, air-dried beaker from the weight of the beaker plus sample. Now,
figure the weight of dispersant in the entire liter of water (computed by using
molecular weight and normality), and divide this value by 50; this gives the weight of
dispersant in each of the small beakers. This weight must be subtracted from the
weight of the sample.

The principle behind the computation is this: if the fine sediment is uniformly
distributed throughout the entire 1000 ml. column by stirring, and we draw off exactly
20 ml. at stated times, then the amount of mud in each withdrawal is equal to l/50 of
the total amount of mud remaining suspended in the column at that given time and at
that given depth (i.e., the amount of mud finer than the given diameter; all particles
coarser than the given diameter will have settled past the point of withdrawal). The
first withdrawal is made so quickly after stirring (20 seconds) and at such a depth that
particles of all sizes are present in suspension; therefore if we multiply the weight of
this first withdrawal by 50 (after subtracting dispersant weight), we will obtain the
weight of the entire amount of mud in the cylinder. Then if we withdraw a sample at a
settling time corresponding to a diameter of 64, and multiply it by 50, then we know
that the product represents the number of grams of mud still in suspension at this new
time, therefore the grams of mud finer than 69. Similarly we can compute the number
of grams present at any size, and obtain the cumulative percentages as shown below:

First, we wet-sieved the sample. Let the weight of sand caught on the 44 (62
micron) screen be called S. Then, 50 times the first pipette sample (the 20-second
sample) equals the total amount of mud in the cylinder, hence the amount of fines
passing through the screen; call this amount F. The percentage of sand in the sample is
then IOOS/(S+F). Let us now denote by “P” the quantity obtained by multiplying each
later pipette sample by 50; then to obtain the cumulative percentages of the total
sample directly from the pipetting data, we substitute the proper values in the
equation,
CUMULATIVE PERCENT COARSER = loo (‘++ F -‘)

and these values may be plotted directly on the cumulative curve.

An example of the computation is given on the following page.

Graphing. Results for gravel (if any), sand, and mud should be combined in a
smooth, continuous cumulative curve. Normally, the analysis stops at about IO@, and

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