A Relationship Between Crystallographic Properties of Illite and Chemical Properties of Extractable Organic Matter in Pre-Phanerozoic and Phanerozoic Sediments

Abstract

Abstrael~Crystallographic properties of illite and chemical properties of organic matter in various mudstones ranging in age from Archean to Miocene were investigated. The breadth of the (001) X-ray powder diffraction peak of illite correlates significantly with the ratio of aliphatic to condensed.-aromatic components and the degree of condensation of aromatic rings of the polar ("humic') fraction of extractable bituminous organic matter. The broader the diffraction peak, the less highly condensed and aromatic (i.e. less highly humified) is the organic matter associated with the illite. Breadth of illite diffraction peak and degree of humification of polar organic matter vary in a complex and apparently systematic way through geologic time. All of the three different suites of mudstones investigated ("calcareous," "non-calcareous," and "glacial" mudstones) gave similar patterns of secular variation. In addition, the ratio of diffraction intensities for the (002) and (001) reflections (loo2/lool) of illite in calcareous mudstones and limestones showed a strong negative correlation with geologic age, indicating that the illites in the older rocks are enriched in magnesium with respect to aluminum.The correlation between crystal structure of illite and degree of humification of its associated organic matter could, by itself, be interpreted as an effect of post-depositional maturation processes, whereby "crystallinity" and degree of humification both increased as functions of heat and pressure. However, the patterns of secular variation suggest that the observed variations are primary, or were pre-determined by primary characteristics of the sample materials. The possibility that the original structure and composition of the organic matter influenced post-depositional changes in the crystallographic properties of sedimentary clay minerals, or post-depositional genesis of clay, would seem to merit consideration. The demonstrated relationship between crystallographic properties of iUite and the nature of the "humic" matter constitutes evidence that most of the extractable organic matter is truly indigenous to the rock in which it occurs. The results also imply that molecular structure of extractable organic matter can be used in place of the breadth of the (001) diffraction peak of illite as an index of incipient metamorphism within a single formation or stratigraphic sequence, provided the primary characteristics of the organic matter are nearly the same in all samples. This method would have the advantage of being applicable to all sediments, not just those containing illite (specifically, aluminum-rich illite).The secular variation of the Ioo2/Iool ratio of illite in calcareous sediments is provisionally ascribed to diagenetic processes whereby magnesium is abstracted from pore water and groundwater and is gradually assimilated into the crystal structure.

Variations in the abundance of photosynthetic oxygen through Precambrian and Paleozoic time in relation to biotic evolution and mass extinctions: evidence from Mn/Fe ratios

“…1 More detailed information about the rock formations is given elsewhere (Jackson, 1975;Jackson and Moore, 1976;Jackson et al, 1978). 2 The sample numbers are the same ones that were used in previous publications (Jackson, 1973(Jackson, , 1975(Jackson, , 1977Jackson and Moore, 1976;Jackson et al, 1978). 3 All chert samples contained carbonate minerals.…”

“…(Fine-grained detrital sediments (shales, mudstones, and argillites) were also analysed (Jackson, T.A., unpublished data) but were found to be unsuitable for this investigation, presumably because such sediments commonly have anoxic or reducing environments regardless of whether the overlying water and atmosphere are enriched in O 2 .) The samples had previously been used for research on major events and trends in biotic evolution -Precambrian evolution in particularas represented by variations in quantifiable properties of extractable and non-extractable organic constituents and illite through geologic time (Jackson, 1973(Jackson, , 1975(Jackson, , 1977Jackson and Moore, 1976;Jackson et al, 1978).…”

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

“…In fact, Jackson (1977) found that his sample set (comprised of a number of finegrained siliciclastics spanning the Proterozoic) displayed a positive correlation between illite crystallinity and the degree of OM humification. The original interpretation was that although burial diagenesis is known to increase OM condensation (leading to a decrease in aliphatic and polar groups), the variations were primary, or pre-determined by OM-clay interactions, perhaps leading to an interference of OM on illitization.…”

Sand and clay mineralogy of sal forest soils of the Doon Siwalik Himalayas

“…Among sesquioxides, Al 2 O 3 (6.78-18.62%) was more than Fe 2 O 3 (2.78-7.28%) and showed increasing trend with depth of the profile. The computation of I 002 /I 001 ratios for the illite, which is ≥3, showed that the illite is Alrich (Esquevin 1969;Jackson 1977). This is probably the reason for higher quantities of Al 2 O 3 in the soil.…”