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The nature of clay-mineral diagenesis in shales has received much attention because of its possible role in providing material to interbedded sandstones. In particular, the smectite to illite transformation has been cited by many authors as a source of silica during mudrock diagenesis. We suggest that illite is not an end-member itself but a transition between smectite and muscovite. This is supported by chemical and physical differences between the two minerals from the literature.

Mudrocks from the Stanley Formation (Mississippian) of the Ouachita Mountains of Oklahoma and Arkansas were investigated both chemically and petrographically to test the hypothesis that illite undergoes a continuous transformation to muscovite. Results were related to thermal maturity as determined from vitrinite reflectance data of Houseknecht and Matthews (1985) on the same samples. Within the phyllosilicate fraction, the concentration of silica was found to decrease with increasing thermal maturity. Loss of water from interlayer positions also showed a linear decrease across the same interval. This is consistent with the change of illite into muscovite mica.

Coincident with the observed changes in the phyllosilicate fraction, a corresponding increase in the amount, grain-size and percentage of composite grains was found in the non-phyllosilicate fraction. This is interpreted as the result of authigenic growth of solid silica released during clay-mineral transformations. This is consistent with reported increases in the amount of polycrystalljne quartz in metapelites compared to their precursors. Because of the increase in the percentage of quartz in metapelites, they are a likely source of abundant silt-size quartz. The approximate mass-balance of silica between the phyllosilicate and non-phyllosilicate fractions also suggests that the Stanley mudrocks behaved as closed systems during diagenesis.

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