Using Iron, Cobalt, and Nickel Contents in Coal Seam to Calculate the Palaeo-pH Value and Palaeo-Eh Value of Coal-Forming Swamp

In endogenic deposits, Fe, Co, and Ni are closely paragenetic at the valences of two. But in the process of they will deposit or migrate separately according to different pH and Eh conditions of the water-medium. And. in a certain pH and Eh condition, the relative amount of depositiion of these three elements are quite different. Thus, the paleo-pH value and the paleo-Eh value of the water-medium during sedimentation could be indicated by Fe, Co, and Ni contents in the sediment. From eastern Yunnan Province to western Guizhou Province in southwestern China, the sedimentaary environments of the Late Permian Coal-bearing Formation change gradually from a continental environment to a marine one. Therefore, these are right sites for the study of sedimentary geochemistry of coal-bearing formation. And, coal seams formed in these different sedimentary environments are the important targets for the study of this topic. Six representative coal seams of the coal-bearing formation were sampled separately from roof down to floor. Each division sample was analyzed for Fe. Co. and Ni with an atomic absorption spectrometer. Then. 1520 ratios of 10 kinds (Fe/Co/Ni. Fe/(Co+Ni), Fe/Co, Fe / Ni, Co / Ni, InFe/ InCo / InNi, InFe / (InCo+lnNi), InFe / InCo, InFe / InNi, and InCo / InNi) were computed. The results showed that the ratio of Fe. Co, and Ni contents in a coal seam, especially the ratios after taking the natural logarithm, might serve as the paleo-pH indicator of the coal-forming swamp. Among these ratios, InFe / (InCo+lnNi) is the best. A series of studies, inferences, and calculations arrived at three empirical formulae which are recommended for calculating the paleo-pH value of the water-medium of continental facies, transitional facies, and marine facies, respectively. According to formula (1). the pH values of Minnie's Lake site and Chesser Prairie site, the two major peatforming environments in Okefenokee Swamp, Georgia. USA, were calculated, which are 4.47 and 3.68, respectively. This is basically coincident with the description that "the freshwater peat (Okefenokee Swamp) is at the pH 4" by Casagrande et al. (1977). And it is then proved from one aspect that the above mentioned empirical formulae are close to practically useful. Finally, since there is a certain relationship between Eh adn pH: Eh = E -0.059pH. such as in an open-system (Po2 = 1 atmospheric pressure) of 25@@ . the oxygen in water solution has the Eh= 1.229-0.059pH, the paleo-Eh value of the coal-forming swamp may then similarly be determined by the Fe, Co, and Ni contents in the coal seam. And the paleo-Eh value of oxygen in a coal-forming swamp could be calculated according to other formula.