OVERVIEWS OF BIOGENIC SMECTITE-TO-ILLITE REACTION
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概要
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The smectite-illite (SI) reaction is a ubiquitous process in siliciclastic sedimentary environments. For the last 4 decades the importance of smectite to illite (S-I) reaction was described in research papers and reports, as the degree of the (S-I) reaction, termed "smectite illitization", is linked to the exploration of hydrocarbons, and geochemical/petrophysical indicators. The S-I transformation has been thought that the reaction, explained either by layer-by-layer mechanism in the solid state or dissolution/reprecipitation process, was entirely abiotic and to require burial, heat, and time to proceed, however few studies have taken into account the bacterial activity. Recent laboratory studies showed evidence suggesting that the structural ferric iron (Fe(III)) in clay minerals can be reduced by microbial activity and the role of microorganisms is to link organic matter oxidation to metal reduction, resulting in the S-I transformation. In abiotic systems, elevated temperatures are typically used in laboratory experiments to accelerate the smectite to illite reaction in order to compensate for a long geological time in nature. However, in biotic systems, bacteria may catalyze the reaction and elevated temperature or prolonged time may not be necessary. Microbes and clay minerals coexist in shales, siltstones, and sandstones under diagenetic conditions. Metal reducing microbes have been discovered in sedimentary rocks from great depths (2,700 meters below the land surface) and in various hot environments (up to 90-100℃). These are similar conditions under which clay minerals undergo transformations. Under these conditions, thermophilic and hyperthermophilic bacteria can "witness" and actively participate in clay diagenesis. Abundant and viable microbes (10^4-10^5 cells/g) have been found inorganic-rich shales. Microbes may be more abundant and active in shales at the onset of clay diagenesis and in shales under the "overpressure" condition, where there exists significant porosity and permeability. Microbially promoted S-I conversion represents a previously unidentified mechanism. The role of microbes, therefore, should be considered in the study of clay mineral diagenesis. Despite the important role of microbe in S-I reaction, factors that control the reaction mechanism are not clearly addressed yet. This paper, therefore, overviews the current status of microbially mediated smectite-to-illite reaction studies and characterization techniques.