Both clay and nonclay authigenic minerals are common in the pores of early Paleozoic sandstones of the upper Mississippi Valley. The paragenetic and stability relations among these minerals provide clues to the diagenetic history, especially to the variations in pore-fluid geochemistry. The chemical compositions of authigenic mineral phases indicate ionic content of pore fluids. Paragenetic relations show the changes in the ionic content through time. In the early Paleozoic sandstones studied, five stages of authigenic mineral formation are evident. From oldest to youngest they are: (1) K-feldspar with some quartz, (2) illite-smectite-chlorite, sometimes with calcite or dolomite, (3) quartz (overgrowths), (4) pyrite, and (5) kaolinite. This paragenetic sequence indicates hat pore fluids initially had a high Ph and K content, and that K concentration relative to Si and Al, as well as Ph, decreased through time. Kaolinite, for example, has formed only where pore fluids are presently fresh. Reversals in the paragenetic sequence, that is, some illite formation after quartz or some quartz formation after kaolinite, document slight fluctuations in pore-fluid chemistry.

Stability relations are useful for interpretation of diagenetic history and pore-fluid geochemistry only if disequilibrium exists between authigenic mineral species. Disequilibrium is common because solution is retarded by the slow movement of pore fluids. In early Paleozoic sandstones authigenic kaolinite may be precipitated before complete solution of K-feldspar or illite has occurred.

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