RESEARCH· ·1970·

B32
PETROLOGY AND PETROGRAPHY
place. Geochemical considerations further indicate that
the process occurred in a fresh-water environment
(Asquith, 1968)-.
The original materials from which the kaolinite
formed are difficult to identify, but they were probably
precursor clay minerals and (or) feldspar. After these
minerals were deposited as detritus, the alkali-metal
ions were selectively removed by hydrolysis in f.resh
\Yater that moved through the sediment. The action of
the water, however, was .not sufficient to lower the concentrations
of aluminum and silica in solution below
the activity product for kaolinite and illite ( ~) as is
shown by the presence of authigenic polycrystalline
quartz. Thus, silica and aluminum concentrations
·favorable to the subsequent formation of kaolinite
were maintained. The dissolution and reorganization
process~s probably occurred concomitantly with cation
removal. l{aolinite was probably derived from clay
minerals rather than from feldspar, ·because the preexisting
phyllosilicate structure of clay minerals could
be more readily utilized for nucleation.
CONCLUSIONS
1\.:aolinite in the thin sections studied represents precursor
feldspar and (or) cation-bearing clay minerals
that underwent diagenesis in a fresh-w.ater environment.
The evidence for nondetrital origin of the kaolinite
is large grain size and subhedral to euhedral vermicular
form. The vermicular masses of kaolinite
would probably not withstand much abrasion or disruption
·without destroying their structure. The dominant
minerals in the thin sections· are kaolinite and
quartz, indicating a high degree of selective removal of
the easily soluble cations of Ca, N a, and IC The postdepositional
environment was probably characterized
by large volumes of fresh water flushing through
porous sandstones.
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