Page 39 - Petrology of Sedimentary Rocks

Split the total amount of sand to 30 to 70 grams and sieve as customary. Now, we
have sieved only a small fraction of the total amount of sand in the sample. To
compute the analysis, it is necessary then to multiply each sieved sand fraction by a
“splitting factor”, which is (I) the total weight of sand in the sample (obtained in the
first operation where gravel was separated out) divided by (2) the weight of the sand
split that was actually sieved.

Say we start with a sample of I500 grams, and 600 grams is gravel and 900 grams
is sand. Then we screen all of the gravel fraction, and a l05-gram split of the sand
fraction. The splitting factor is then 900/105, or 8.57 I. Each sand fraction is
multiplied by 8.571 to give the weight of that fraction in the entire original sample.
These weights are then cumulated with the gravel weights and percentages computed as
usual.

Held on: wt. X Splitting Cum. Wt. Cum. %
Factor (8.57 I)
Gravel: I6 mm 150 150 10.0%
8mm 230 15.3%
4mm 14”: 370 24.7%
2mm 230 600 40.0%

Sand: I 52 446 IO46 69.8%
0.5 :: 30 257 I303 87.0%
0.25 mm I8 150 I458 97.3%
0.125 mm 5 42 I500 100.0%

Size Analysis by Settling Tube

For rapid, generally less accurate work, grain size of sand may be measured by
settling the grains through a water column. Fancy devices are available to make sure all
the grains start settling together. Best results are obtained with water columns at least
IO cm. in diameter, and with a small number of grains. Results are recorded by
continuous weighing of grains accumulating at the bottom, or by automatic recording of
pressure differences. Both “sievers” and “settlers” are strongly opinionated as to which
is the best method of measuring grain size--one may just as well ask, “Which is the best
way of doing carpentry--a saw or a hammer?”

Pipette Analysis by Silt and Clay

There are several methods of analysis of silts and clays (finer than 4@ or 62
microns diameter) in common use. The most common method is by pipette. Hydro-
meter and decantation methods are more difficult and less accurate, and many
electronic sediment-analysis devices are now available (and expensive). All are based
on the settling velocity of the particles, usually computed on the bases of Stokes’ law:
in the pipette method, one sucks off a small volume of suspension, evaporates and
weighs the residue remaining suspended at the given time; in the hydrometer method
one measures the density of the suspension, which depends on the grains still suspended
after a given time, dries and weighs them. Electronic devices measure the density of
the suspension. No settling method is very apropos for particles finer than I I$ or l2$,
where Brownian motion interferes with settling.

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