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About: Chlorite is a(n) research topic. Over the lifetime, 3940 publication(s) have been published within this topic receiving 81000 citation(s). The topic is also known as: ClO2(-) & chlorite.
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Abstract: A detailed mineralogical and chemical investigation has been made of shale cuttings from a well (Case Western Reserve University Gulf Coast 6) in Oligocene-Miocene sediment of the Gulf Coast of the United States. The 10-µm fractions from the 1,250- to 5,500-m stratigraphic interval were analyzed by x-ray diffraction. Major mineralogic changes with depth take place over the interval 2,000 to 3,700 m, after which no significant changes are detectable. The most abundant mineral, illite/smectite, undergoes a conversion from less than 20 percent to about 80 percent illite layers over this interval, after which the proportion of illite layers remains constant. Over the same interval, calcite decreases from about 20 percent of the rock to almost zero, disappearing from progressively larger size fractions with increasing depth; potassium feldspar (but not albite) decreases to zero; and chlorite appears to increase in amount. Variations in the bulk chemical composition of the shale with depth show only minor changes, except for a marked decrease in CaO concomitant with the decrease in calcite. By contrast, the <0.1-µm fraction (virtually pure illite/smectite) shows a large increase in K2O and Al2O3 and a decrease in SiO2 The atomic proportions closely approximate the reaction smectite + Al+3 + K+ = illite + Si+4. The potassium and aluminum appear to be derived from the decomposition of potassium feldspar (and mica?), and the excess silicon probably forms quartz. We interpret all the major mineralogical and chemical changes as the response of the shale to burial metamorphism and conclude that the shale acted as a closed system for all components except H2O, CaO, Na2O, and CO2. Compositional changes in the shale as a function of metamorphic grade closely parallel compositional changes in shale as a function of geologic age.

1,270 citations

Journal ArticleDOI
Abstract: Sandstones and shales of the Wilcox Group (lower Eocene) in southwest Texas were examined by X-ray powder diffraction, electron microprobe, and petrographically to interpret their diagenetic history. Samples analyzed are from depths of 975 to 4650 m, representing a temperature range of 55°C to 210°C. No consistent trend of depositional environments is recognized with increasing depth, and mineralogic changes observed are interpreted as diagenetic. Major mineral distribution patterns are (1) disappearance of discrete smectite at temperatures >70°C, (2) gradation of mixed-layer illite/smectite to less expandable (more illitic) illite/smectite over the entire temperature range, (3) disappearance of kaolinite from 150-200°C accompanied by an increase in chlorite, and (4) replacement of calcite cement at about 117 120°C by ankerite. Calculations based on data of Hower and others (1976) indicate that the stability of smectite layers may be a function of composition. Smectites with high ratios of octahedral (Fe + Mg)/Al appear to resist conversion to illite until temperatures high enough to produce ordering are attained. A diagenetic model is proposed which involves the breakdown of detrital K-feldspar and of some smectite layers in illite/smectite to convert other smectite layers to illite. Silica and calcium released by the illitization of smectite is transferred from shales to sandstones to produce quartz overgrowths and calcite cements at temperatures as low as 60°C. Iron and magnesium released by the illitization reaction are transferred from shales to sandstones at temperatures >100°C and react with kaolinite to produce high-alumina chlorite and/or with calcite to produce ankerite.

793 citations

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