Page 24 - Petrology of Sedimentary Rocks

solution (usually H+, but sometimes Na+, Ca++ or others). This leaves the
surrounding solution swarming with unsatisfied negative ions (OH-, etc.) in
the vicinity of the clay flakes. Thus when a clay flake with its surrounding
“fog” of negative ions approaches another clay flake, also with a negative
fog of ions around it, the two flakes repel each other. This is the state we
try to maintain, because if the flakes repel each other then they will not
aggregate into clumps, and we can then make a grain-size analysis on the
individual grains. If however we add a strong electrolyte like HC I or NaC I
to the solution, then the solution becomes a conductor and the negative
swarm of ions leaves the clay crystal, and left unprotected the clay flakes
attract each other and flocculate into chains and large clumps. Grain-size
analysis is thereby made impossible. Therefore we must always use distilled
water, never tap water, in these analyses, because tap water always
contains small amounts of ions which cause flocculation. The best way to
insure a good analysis is to add a small amount of certain dispersing
chemicals (called peptisers) which prevent flocculation. These chemicals
either build up the charges on the clay particles so that they repel each
other more strongly, or else “plate’ the particles with a protective and
repulsive coating. The quantity of peptiser or dispersant used must be
pretty exact because too little peptser can result in flocculation, and too
much may also cause flocculation. To a large extent the grain size obtained
by analysis depends on the type of dispersant, efficiency of stirring, etc.
Some disperants are the following: (I) a few drops of cont. NH40H per
liter of suspension; (2) .02N Na2C03; (3) 0.01 N sodium oxalate; (4) 2.55gm/I
sodium hexametaphosphate. The latter (Calgon) is best for most purposes.
In all your operations using dispersants, keep the water you are using to
mash or wash or suspend the particles in, always at the exact concentration
of dispersant. This can be done by using a solution of the proper strength in
your wash bottle and never using any water unless it has the right
concentration of dispersant in it. You must always know exactly how much
dispersant is in the water because it is an important factor in computation
of the results. Keep in mind that you should always try several dispersants
and use the one that works best; one of them may work on one sediment, and
fail completely on another one. This is just a matter of trial and error.
2. Procedure for Dispersion. This technique is usable only if the sample is an
unconsolidated or semi-consolidated sediment. Grain size analysis of hard
shales is pointless as one can never break the material into individual grains;
these can only be analyzed in thin-section. UI trasonic treatment can
disaggregate softer shales.

Place the material in a wide mouthed evaporating dish, add a small amount
of water to which the proper concentration of dispersant has been added,
and crush to pea-size lumps. Don a rubber glove and muddle the sample
with the rubber-gloved finger until all the lumps, even the tiniest ones, have
been crushed. Be careful not to lose any of the material that clings to the
glove. Pour into a small bottle, again MAKING SURE THAT NONE IS LOST,
screw a cap on the bottle, shake vigorously for a few minutes, and let stand
over-night. If it is necessary to remove sand by wet-sieving, see page 20.
Pour the suspension from the bottle into the dispersing cup (simply a drink
mixer) and fill up about halfway with water which has the proper concentra-
tion of dispersant. Stir with the mixer for about 5 minutes. Rinse it out
carefully into a liter cylinder, MAKING SURE THAT NONE IS LOST, and fill

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