Page 99 - Petrology of Sedimentary Rocks

Mg, chlorite develops in their place, and if the environment is rich in K, illite forms.
Thus kaolin and montmorillonite are uncommon in pre-Devonian rocks. As true
metamorphism begins, both illite and chlorite grow larger, then sericite begins to
replace illite in the higher-rank slates, takes over almost completely as illite disappears
in the phyllites, and then sericite in turn gives way to muscovite as schists develop.
Finally the muscovite abdicates in favor of feldspar and the rock becomes a gneiss or
perhaps even a granite. At the same time, if the rock contains much Fe and Mg,
chlorite is transformed into biotite in the low-rank schist stage, then may be upgraded
to hornblende in the higher-rank schist. In determining if diagenetic processes have
happened in a sedimentary rock, look for evidence of recrystallization (i.e., larger-than-
normal flakes), or replacement of detrital quartz, feldspar, or calcite by illite or
chlorite.

In some cases pore space in permeable sandstones is filled with vermicular
hexagonal stacks of water-clear kaolin, encrusting fringes of chlorite or montmoril-
lonite, or large flaky sericite crystals, all probably precipitated from solution just as
salt in a beaker in one of the many manifestations of diagenesis. These changes can
take place in geologically young sediments that have never been folded or deeply
buried, therefore require no extreme heat or pressure. Kaolinite is the most common
clay mineral encountered as authigenic pore-fillings in sandstones, but authigenic
chlorite is common in marine sandstones with basic volcanic rock fragments.

(7) Post-diagenetic weathering-- the same processes occur that operated in (2).
The main caution here is to be careful when samples are collected as the clay mineral
may be changed on the outcrop of weathering.

To summarize:

Gibbsite is a product of very prolonged humid tropical weathering, as are the
other bauxitic minerals.

Kaolin forms on intense weathering where K is removed; a little may be reworked.
Possibly diminished by marine diagenesis and destroyed on deep burial or
incipient metamorphism. Another school holds that kaolinite does not
change on marine diagenesis, and the differences in kaolinite distribution
are due to source area changes or to differential sorting.

lllite has less K than muscovite and more K than kaolin, but is a distinct mineral
group not gradational to either one. Its structure is less ordered than that
of muscovite, and it is responsible for the color of most green shales. It
forms in a high-K environment, such as is present in temperate to semi-arid
weathering, and develops from other clay minerals on marine diagenesis,
deep burial, and slight metamorphism. Much of it is derived from older
illi tic shales or slates.

Serici te is fi ne-grained impure muscovi te. In sediments the name is virtually
abandoned because it is almost impossible to tell from illite. The name is
still the one used by hard-rock petrographers to describe the fine, flaky
mineral so abundant in phyllites and hydrothermally-altered zones. Most
sericite in sediments is derived from erosion of these rocks.

Mixed-layer illite-montmorillonite is a transitional product. It can form when
illite undergoes temperate weathering, so that much of the K+ is removed
from the structure; and it can also be formed when almost completely

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