Page 23 - Petrology of Sedimentary Rocks

acid and the wash water through a filter paper that you previously
weighed, and place in an oven to dry. The weight of this dried
material then should be added to the “pan” fraction of the sieve
analysis. After the sand sample has been washed, dry it in the oven.

4. Ferruginous-cemented Rocks. Crush the sample to pea size and place
in 50% HC I, warmed over a hot plate. Continue heating until the sand
turns white. Pour off the acid (through a filter paper if “fines” are
suspended in the liquid). Dry in an oven, and if it is necessary to do
any more crushing, follow (I) or (2) above.

5. Siliceous-cemented Rocks. If the cement is opal or chalcedonic or
microcrystalline quartz (i.e., chert), then warm concentrated KOH
may work. If the cement is crystalline quartz there is no known
chemical way to remove it and still leave the quartz grains. For rocks
not too strongly cemented with quartz, try the pounding routine of (2)
above. For quartzites, there is no satisfactory mechanical method of
grain-size analysis, and it must be done in thin-section with a petro-
graphic microscope.

6. Sands Bonded With a Little Clay. Place the sample in a wide dish with
some water and rub with a cork until the clay is in suspension and
grains are separated. The clay may then be removed by decantation or
wet-sieving, and either weighed or analyzed by pipette or hydrometer.
NEVER WASH THE CLAY DOWN THE DRAIN--ALWAYS WEIGH IT.

(Note: after you have done your sieve analysis, if you find an abnormally high
percentage of aggregates (say over 25%) on any screen, that size fraction may be
removed, recrushed, and rescreened. Otherwise the grain-size analysis is value-
less).

If these methods fail, or for special types of cement, see Krumbein and
Pettijohn (I 9381, Carver (I 97 I ), or Royse (I 970).

b. Dispersion of Muds and Clays

Grain-size analysis of fine-grained materials is not very satisfactory
and there are a great many unsolved problems. In the first place we are
trying to measure the size of the individual particles, and to separate clay
lumps into individual grains is very difficult. As you may see, the grain-size
distribution we obtain on analysis may not really be a measure of the true
size of the particles, but only tells how efficient our disaggregation has
been. In general, one can rely on these analyses as giving a fairly true
picture of the size-distribution down to diameters of 6 or 7 4 t.016 to .008
mm) but for sizes finer than this the analyses are often invalid. Below this
size the analysis no longer measures true size of the particles, because the
settling velocity is now affected greatly by the flaky shapes of the particles,
degree of dispersion, electrical charges on the particles, etc. Two clay
flakes of the same size but different compositions (e.g. kaolinite vs.
montmorillonite) may settle at different rates because of these factors.

I. Dispersants. Clay flakes in distilled water are usually electrically charged.
Most clays have a negatively-charged ionic lattice, which to attain electric
neutrality must take up positively charged ions from the surrounding

I7