MINERALOGICAL CONTROL ON MINERAL SURFACE MEASUREMENTS AND ITS RELATIONSHIP TO ORGANIC CARBON CONCENTRATIONS IN SEDIMENTS

It has been argued that the interaction of organic compounds with clay mineral surfaces exerts an important control on the burial and preservation of organic carbon (OC) in the fine-grained facies like black shales. In particular, the abundant interlayer space/surfaces available in the 2:1 smectitic-clays have been proposed as sites for excess OM storage and preservation in such clay-rich rocks. We tested the validity of this hypothesis experimentally by measuring mineral surface area (MSA) on 3 types of standard clays (Kaolinite - KGa-1, Smectite group- SCa-3, Illite group- IMt-1) and quartz, along with appropriate mixtures of these mineral phases by the Ethylene Glycol Monoethyl Ether (EGME) method. Our results show that smectite EGME-MSA estimates are the largest (>800 m²/g), while Illite, Kaolinite, and Quartz are all below 100 m²/g; these values are comparable to those reported in the literature. The relative difference in MSA measurements recorded between the smectite and the other mineral phases suggest that the variability in EGME-MSA measured in natural samples will be strongly dependant on the relative concentrations of smectitic-clays present in them. We observed similar results when we compared EGME-MSA measurements of samples with variable proportions of clay types (estimated using X-ray diffraction techniques) from a sample set belonging to the Niobrara Formation in the Cretaceous Western Interior Basin. However, the total organic carbon concentrations of the same samples do not correlate well with their EGME-MSA values, suggesting that smectitic-clay interlayers might not be the sites that house OM in these samples. These observations have several implications: (i) interlayer sites in the 2:1 smectitic-clays may not be the sites for OM storage and preservation (ii) other factors like production, preservation, dilution, and compaction might be more dominant processes affecting the OC concentrations in these sediments, and (iii) The EGME-MSA method might not have provided accurate estimates of MSA in the samples.