Page 97 - Petrology of Sedimentary Rocks

pattern; this is termed “degraded illite” or “weathering montmorillonite.” Degraded
illites of course have K20 contents that continuously fill in the range between “good”

illite and montmorillonite as shown in the first table.

Origin and Significance. Clays form 25 - 35% of the terrigenous fraction of
sedimentary rocks. The clay minerals in a qiven sedimentary rock may have formed in
any one of ‘the following ways; ( I ) reworked without chemical change through simple
disaggregation of older clay-bearing rocks such as shales, clayey limestones, slates, or
phyllites; (2) by chemical weathering (soil formation) of minerals containing Al and Si,
chiefly feldspar but some from mafic minerals, micas and some from older clay
minerals; (3) by subaqueous weathering of volcanic ash; (4) by diagenetic changes taking
place on the sea floor after deposition; (5) by diagenetic changes taking place during
deep burial, migration of connate solutions, or incipient metamorphism; (6) by intense
metamorphism; and (7) by post-diagenetic weathering, taking place after the outcrop is
once more exposed to the surface and weathered-- thus running the cycle back to (2)
again.

(I) Simple disaggregation of older clay-bearing rocks yields chiefly illite, because
most shales are composed of illitic clay which is often reworked without much change
except for K stripping. Lesser amounts of sericite, chlorite, montmorillonite, and
kaolin come from this source. Disaggregation of slates and phyllites yields either illite
or sericite depending on the intensity of the metamorphism, with sericite indicating the
higher grade; considerable chlorite may also come from this source, but little kaolin and
almost no montmorillonite, as these are generally converted into other minerals (illite,
sericite or chlorite) on metamorphism. To ascertain whether this is the source
responsible for the clay minerals in a given sediment, look for other evidences of a slate
or shale source area, such as discrete shale or slate fragments in the associated sands.
This is a very important source of clay minerals in Recent sediments and those of
Tertiary or younger age, because the continents are now largely blanketed with older
clay-bearing sedimentary rocks. An abundance of detrital chlorite and biotite is often
associated with volcanism.

(2) Weathering (soil formation) is the ultimate source of most clay. The effect of
weathering on clay minerals may be understood if one thing is remembered: the clay
minerals because of their tremendous surface area are highly reactive, and will react so
as to attain equilibrium with the ions in their environment. Naturally the ions present
will depend on two things: (I) what ions are supplied in abundance by the source rock,
and (2) are those ions retained in the soil (arid climate) or rapidly removed by leaching
(warm, humid climate). Under incomplete leaching K and Mg remain in the soil; if basic
rocks are being weathered, abundant Mg ions are liberated and montomorillonite, the
Mg clay, forms (possibly sometimes chlorite and vermiculite); if acid rocks are being
incompletely leached, abundant K ions are freed and illite, the K clay, results. Under
complete leaching, both K and Mg are removed from the soil by percolating ground
water, and Kaolin (which lacks both K and Mg) forms. Temperature also affects this
reaction, cooler conditions inhibiting formation of kaolin. Under humid tropical
conditions and prolonged time, even the Si will be removed from kaolin and gibbsite,
purehydrous alumina, is left (but sometimes bauxite can be re-silicated to kaolin). In
the humid tropics, basalts give much amorphous clay (allophane). Note that not only
feldspars and ferromagnesians break down to clay; even illite, for example, if derived
from an older illitic shale and then weathered under warm humid conditions may first
break down into “degraded illite” (which is simply the illite structure stripped of most
its K ions) and ultimately form kaolin. “Degraded i Iii te” is a rather common product of
not-too intense weathering, and gives a mortmorillonite-like pattern on the X-ray
spectrometer. Weaver has found that this type of “montmorillonite” will take up K and

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