Page 27 - Petrology of Sedimentary Rocks

Methods 2 and 3 split the sample into two portions with the mud-sand ratio
being determined on one portion, and the analysis of the individual fractions being
performed on the other portion. Both of these assume that both sub-samples have
exactly equal proportion of constituents. This can be approached by very careful
mi xi ng. In Method 2 (wet-sieve replicate method) the sediment sample is spread
out and thoroughly mixed. Then by repeated quartering or simly scooping portions
from every part of the pile, one divides it into two subsamples. The first
subsample is then wet-sieved, and the amount of sand weighed; the mud passing
through is evaporated to dryness, allowed to come to equilibrium with room
temperature and moisture content, and weighed. The percentage of sand versus
mud is then computed for the first subsample and assumed to hold true for the
second subsample. The second subsample is also wet-sieved, and the amount of
sand weighed. Knowing the percentage of sand (from the first subsample), and the
weight of sand in the second subsample, it is then possible to compute what the
weight of mud in the second subsample should be. Then the mud in the second
subsample can be dispersed and pipetted to obtain its size-distribution, because it
has not been oven-dried.

If the sample contains over 40% mud, Method 3 (moisture-replicate method)
may be used. Again the sediment is spread out and carefully divided into two
subsamples. In this procedure the sediment is regarded as being made of three
constituents: sand, mud, and water, and it is assumed that each subsample
contains an equal proportion of these. Both samples are weighed moist. Then the
first subsample is evaporated to dryness in an oven and weighed again. The loss in
weight represents the water content, and is regarded as equalling the water
content of the second subsample. The second subsample is then wet-sieved and
the weight of sand obtained. Now, the mmtal weight of the second subsarnple
is known; the weight of sand in the second subsample is known; and the weight of
water in the second subsample can be computed from the known percent of water
in the first subsample. Subtraction thus gives the weight of mud in the second
subsample, and the mud may then be pipetted to obtain its size-distribution. THIS
PROCEDURE CANNOT BE USED IF THE SAMPLE CONTAINS LESS THAN 30-
50% MUD, as the error is too great. The advantage of this method is speed.

These various methods may be summarized as follows:

I. The sample is dry

A. It contains little or no clay: Dry sieve, weigh the sand fraction,
weigh the pan fraction, and pipette the pan fraction.

B. It contains considerable clay: Disperse and treat as a moist
sample.

II. The sample is moist, or must be moistened to disperse it

A. It contains little or no clay: Disperse, wet-sieve; use pipette or
set-sieve replicate method to obtain percent of mud, then
pipette the mud (or the sample may be air-dried, then treat as
IA).

B. It contains under 40% mud: Disperse, wet-sieve, use pipette or
wet-sieve replicate method to obtain percent of mud; then
pipette the mud.

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