Page 95 - Petrology of Sedimentary Rocks

amounts indicate brief transport. The abundance of limestone fragments (CRF’S) may
be used as a climatic indicator. Some sands (e.g., Oakville) are made up largely of
limestone grains, as are the Tertiary (molasse) sandstones and conglomerates of the
calcareous Alps, Carpathians, etc. Such CRF-rich rocks are known as calclithites.

Clay Minerals

For detailed material on clay minerals, see Grim, Millot, Thorez, Weaver, Velde
or other standard works. This is just the merest outline of a few significant facts.

There are three definitions of clay: (I) based simply on size, including anything
finer than 4 microns be it true clay minerals, quartz, calcite, pyrite or any other
substance; this is the “clay” of the grain-size analysis workers, of the sedimentologists
and oceanographers; (2) based on composition, defined as one of the hydrous aluminum
silicates belonging to the kaolin, montmorillonite or illite groups and also including
fine-grained chlorite and vermiculite; (3) the petrographic definition, which includes
under the general term “clay”, the true clay minerals listed in (21, plus sericite and fine-
grained muscovi te, biotite and chlorite if finer than about 20 microns, and even the
hydrous aluminum oxides “bauxi te” and gi bbsi te.

Clays are sheet-structure silicates closely allied to the micas; nearly all clays are
crystalline (allophane is the only common amorphous one), and all are biaxial negative,
with small 2V, length-slow, and if colored are pleochroic with the darker direction N-S.
Because they are so similar in these properties, they are difficult to identify
microscopically. Study is hindered by their fine grain size (most true clay minerals are
finer than 2 microns), by the fact that most naturally-occurring clayey rocks contain
intimate mixtures of several different clay minerals, and by the abundance of
impurities and stains. Furthermore, the appearance in thin section may be changed by
thickness of the slide, by saturation with various materials which may change the
refractive index, or by difference in orientation.

Because of the difficulty of microscopic study, clays are commonly identified by
X-Ray techniques. The best technique is to get the composition by such instruments,
then go back to the thin section armed with this information and study the genetic
relationships and associations of the minerals as they occur in the rock (for example, X-
Ray might show that the rock contained 20% quartz, 60% illite and 20% chlorite, but it
would take a thin section to tell whether (I) the quartz occurred as silt laminae, the
chlorite was partially replacing the quartz, and the illite formed the bulk of the shale;
or (2) illite in the form of pellets formed the bulk of the rock, chert cement (showing up
as quartz on X-Ray) occurred between the pellets, and several chlorite-filled veinlets
traversed the rock.

Typical chemical composition of the common clays is shown in the table below,
adapted from Grim. It wi II be seen that in passing from muscovite (sericite) through
illite to montmorillonite, the K20 content drops in discrete steps from 12% to 7% to
less than I%, and that kaolinite and chlorite also lack K20; also that illite is high in Fe

while most montmorillonite is Mg-rich; and that chlorite is very rich in Mg and Fe and
quite low in Si. Water content is omitted in this table.

Some “clays” are very rich in iron. Most “glauconi te” is iron-rich il Ii te,
“nontronite” is iron-rich montmorillonite, and “chamosite” is iron-rich kaolinite. These
three minerals often form as pellets, or even sometimes oolites.

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