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Origin and Significance of Glauconite in the Geologic Sequence: Abstract
Glauconite is defined as any sand-sized, earthy, greenish pellet found in sedimentary rocks. This definition has no specific implication regarding chemical or mineralogical composition. However, most glauconite pellets are composed primarily of a randomly interlayered 10-Angstrom non-expandable (illitic) material and expandable (mont-morillonitic) material. Much of the variation in glauconite properties is related to variation in the amount of expandable layers present.
Glauconite pellets reveal much variety in external appearance (morphology) and internal structure (as seen in thin section). These characteristics can be used to interpret the origin and/or subsequent history of pellet types. Suggested origins include (1) chemical precipitation, (2) expansion and alteration of detrital mica, (3) alteration of fecal pellets,(4) alteration of clay fillings of fossil tests, (5) mechanical aggregation, and (6) chemical replacement. Original morphologies may be obscured by abrasion (reworking) and internal structures changed by recrystallization.
Glauconitization apparently requires four essential factors: (1) parent material (generally an expandable layer lattice silicate), (2) a source of iron and potassium (sea water), (3) local reducing conditions, and (4) time. The last factor emphasizes the progressive nature of glauconitization, which may be terminated at any stage (most likely by burial).
The progress of glauconitization results in certain interrelated changes in glauconite pellets:
(1) An increase in iron and potassium, (2) a decrease in the amount of expandable material, (3) an increase in crystallinity (degree of ordering), (4) a change from light green to dark green color, and (5) an increase in rounding and sorting of pellets. There are only general trends and exceptions may be common.
Glauconite is a reasonably safe criterion for a marine, shallow water environment and slow rates of deposition. It is most abundant at unconformities; e.g., at the base of marine transgressive sequences. Re-deposition in terrestrial environments is unlikely. Transportation of glauconite after its formation inhibits its use as a more specific environmental indicator on a simple presence or absence basis. However, since glauconite occurrences differ in kind of and variety of pellets, recognition of pellet types and their distribution is potentially useful for stratigraphic correlation or environmental determinations.
Glauconite is the only clay material occurring in sedimentary rocks which is known to be authigenic in origin, is abundant, and is relatively free from impurities. Thus the geochemistry of glauconite should be a fruitful area of study. At present, most of the emphasis has been restricted to age-dating aspects, but there are numerous other possibilities.
Acknowledgments and Associated Footnotes
1 Sinclair Research, Tulsa
Copyright © 2006 by the Tulsa Geological Society