Size distribution of quartz in mudrocks

Sixteen penecontemporaneously deposited mudrock-sandstone pairs ranging in age from Devonian to Cretaceous were treated chemically to isolate the quartz and chert fraction from the other constituents of the rocks. The amount of crystalline silica was determined from the chemical treatments; its size distribution was determined using a combination of normal and Micro Mesh sieving and settling tube analysis; the crystalline silica coarser than 10 μu was examined petrographically to determine the amount of chert. Percentages of crystalline silica in the mudrocks range from 6·7 to 46·7 % and average 27·6 (σ= 10·7). Mean grain size ranges from 4·4 φ to 7·3 φ and averages 6·1 φ. The crystalline silica fraction is a poorly sorted medium to fine silt consisting of one-eighth sand, six-eighths silt, and one-eighth clay size sediment. Percentage of crystalline silica and mean size of crystalline silica in the thirty-two mudrocks and sandstones are positively correlated; r= 0·685, which is significant at the 99 % level. The best fit linear regression line is: Y= 102·7–11·3X. Extrapolation of the regression line indicates that, on the average, crystalline silica is lacking in mudrocks in grain sizes finer than 9·1 φ (1·μm), a result consistent with observations by clay mineralogists. The crystalline silica fraction of both the mudrocks and associated sandstones averages 4% chert in the >10 μm portion. There is no correlation between the mean grain size of the crystalline silica and the percentage of chert in it.