Page 25 - Petrology of Sedimentary Rocks

the cylinder up to exactly 1000 ml with water which has the proper
concentration of dispersant. Now stir the material in the cylinder vigor-
ously and let stand a day to check completeness of dispersion. Flocculation
can be recognized by a curdling and rapid settling of clumps of particles, or
by the presence of a thick soupy layer on the bottom of the cylinder passing
abruptly into relatively clear water above. Or you may suck out a drop and
place on a slide and examine with high power of petrographic microscope; if
the particles appear as individuals and show Brownian movement, then
dispersion is good; if they form clumps, chains or strings then dispersion is
unsatisfactory.

To find the best dispersing agent, try several concentrations of several
different dispersants with equal quantities of mud, and choose the one which
makes the mud stand up longest or best without signs of flocculation. A
frequent cause of flocculation is the use of too much sediment; try to use I5
gm or less.

IN ALL THESE PROCESSES, REMEMBER NEVER TO OVEN DRY THE MUD
IF YOU EVER EXPECT TO MAKE A GRAIN SIZE ANALYSIS. HAVE YOU
EVER TRIED TO DISAGGREGATE A BRICK OR A CHUNK OF ADOBE
INTO ITS INDIVIDUAL GRAINS? THIS IS WHAT YOU’LL BE DOING IF
YOU OVEN DRY THE MUD OR CLAY BEFORE YOU ANALYZE IT.

Separation of Sand From Mud

If the sample contains more than a few percent of material finer than 4$,
(0.0625 mm, the limit of silt and clay), it is usually necessary to separate the sediment
into two fractions at this diameter value. The coarser material (sand) is then analyzed
by sieving and the finer material (silt and clay) by pipette or hydrometer. The most
critical part of the operation is getting the weight of each fraction. If nearly all the
material finer than 44 consists of silt, then the entire sediment may be dried,
disaggregated and dry-sieved; then the material caught in the pan is weighed and
pipetted directly.

If there is much clay-size material, however, dry-sieving will not work because
the clay will gum up the screens; recourse must then be had to wet-sieving (some prefer
to use this decantation though). Most operators use wet-sieving although it is
admittedly slow and messy. It is probably the most accurate way.

I. Place the sample in a small bottle, fill approximately half full with water to
which the proper concentration of dispersant has been added, and muddle with a
rubber-gloved finger until all lumps are crushed (see under “2” on page 18). Screw
the cap on the bottle and shake vigorously. Be sure not to lose any of the
material.

2. Obtain a pan at least ten inches in width, and a wash bottle filled with no more
than one-half liter of water to which the proper concentration of dispersant has
been added (see pages 18, 19). (NOTE: If you are going to obtain the grain size
distribution of the fines, you must use a bottle with dispersant added; if you are
not going to pipette the fines, but simply weigh them to get the quantity present
7
In the sediment, then use a bottle of distilled water because when you evaporate
the fines to dryness you will be also weighing any dispersant you may have added).
Now take the 62 micron (230 mesh) screen that is reserved for wet-sieving, and

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