Page 22 - Petrology of Sedimentary Rocks

collect try to end up with the number of samples of each rock type in proportion with
the amount of that rock type in the exposure.

In detailed work on sedimentary units it is necessary to obtain samples showing
the complete range of grain size present in the unit--i.e. you should search for and
sample the coarsest and finest beds obtainable as well as getting samples typical of the
unit, for only in this way can the interrelation between size, sorting, shape, and mineral
composition be studied.

Preparation of Samples for Grain-Size Analysis

The purpose of grain size analysis is to obtain the grain size of the elastic
particles as they were deposited. This is often difficult because (I) elastic sandgrains
may acquire overgrowths, or be cemented into tough aggregates to form hard
sandstones; (2) chemically-precipitated materials may be introduced into the rock
which, if not removed, give erroneous size values because we are trying to measure size
of the elastic particles, not size of the cementing material; and (3) clay minerals
because of their flaky character and surface electrical charges tend to cluster in lumps.
The idea of disaggregation and dispersion is to separate all of the individual grains
without smashing any of them, and to remove all chemically-precipitated substances.
To avoid smashing grains as much as possible, one should start with gentle treatments
and gradually work up to the more severe treatments only if the gentle ones fail.

a. Disaggregation of Sands and Sandstones.

I. Unconsolidated Sediments. The sample should be dried, then placed on
a large sheet of glazed paper and crushed with the fingers. Spread the
sand out and examine with hand lens or binocular microscope to see
that all aggregates are crushed. Then gently rub these aggregates
with the fingers or run over the sand with a small rolling-pin.
Alternately, the sand may be poured into a mortar and gently pounded
with a rubber cork. If there are a good many aggregates, it is often
helpful to select a screen just larger than the size of most of the
individual grains of the sediment, then run the whole sample through
it; nearly all the grains passing through will then represent single
grains and most of the particles remaining on the screen will be
aggregates. In this way you can concentrate your efforts on crushing
the aggregates without wasting your energy on the single grains.

2. Weakly Consolidated Sediments. Place the sample in a mortar, and
place a large sheet of paper under the mortar. First, try crushing with
a rubber cork; if this doesn’t work, try gentle pounding with an iron or
porcelain mortar. Always use an up-and-down motion; never use a
grinding motion as you will break the individual grains. Examine with
binocular to check that aggregates are completely destroyed. Be
careful not to splatter any sand out of the mortar.

3. Carbonate-cemented Rocks. With a mortar and pestle crush the
sample to pea-size chunks or smaller. Place it in dilute hydrochloric
acid until effervescence ceases (be sure the acid is still potent when
effervescence ceases). If the sample is dolomite, it may be heated
gently to hasten solution. Pour off the acid and wash the sample. If
any fine clays are suspended in the acid after this treatment, pour the

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