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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
The Occurrence of Polycrystallinity and Undulatory Extinction in Quartz in Sandstones
John R. Conolly
Petrographic analyses of 208 sandstones from New South Wales are used to evaluate the mode of occurrence of polycrystallinity and undulatory extinction in quartz grains in sandstones. The amount of polycrystalline quartz present in a sandstone is a function of grain size and increases with increase in grain size. Percentages of polycrystalline quartz present in sandstones can be used for determination of provenance only when corrections are made for differences in grain size. The texture and mineralogy of the rock fragments and other minerals present should also be known, to determine whether the variation in polycrystalline quartz is due to changes in source rock mineralogy or due to changes in transportation or dispersal history.
The amount of undulatory quartz in a sandstone is affected by post-depositional diagenesis, folding and faulting. Close proximity of faulting frequently induces undulatory extinction in non-undulatory quartz grains. The amount of non-undulatory or undulatory quartz in a sandstone may be important for provenance reconstruction but detailed studies of sandstone formations in vertical profiles and in lateral extent are required before these amounts can be used effectively. The percentage of undulatory quartz in a sandstone is dependent on the modal sand-size of the sandstone and increases with increase in grain size. There is no correlation between the total amount of quartz in a sandstone in the samples studied and the percentage of non-undulatory quartz if all the sandstones are consid red impartially. Sandstones with greater than 75 percent of non-undulatory quartz are rare in the geological column. Sandstones with low percentages of undulatory quartz will also tend to have low percentages of polycrystalline quartz but the reverse is not true.
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