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[SCG44-P08] The mineral occurrence of kaolinitic saprolite beneath the sedimentary kaolin deposit at Seto district
Keywords:kaolinite, Green saprolite, SEM-EDS, DCB method
Kaolin is the common clay minerals in the Earth’s surface, and important non-metallic mineral commodity used for ceramics, refractories and paper filler. The cluster of sedimentary kaolin deposits distributed in the Seto-Tono district is the largest kaolin field in Japan. Kibushi clay (ligneous clayey sediment) and Gerome clay (unsorted sediment consisting of coarse-grained quartz and fine-grained kaolin) in this district resulted from granitic clastics and have been mined for many years owing to its high plasticity, refractoriness and whiteness after baking. Recently, however, it is concerned that remained resource is being exhausted, and thus, utilization of kaolinitic saprolite which is the weathered crust of the basement granite underlying the kaolin deposit has been considered. Moreover, the close relationship between this kaolinitic saprolite and kaolin deposit is expected as a window to elucidate the formation condition for high quality kaolin deposit[1]. In this study, we investigated mineralogical characteristics of kaolinitic saprolite and chemical forms of iron which is undesirable impurity for industrial uses.
The samples used were obtained from the active mining area in the northern Seto-city, Aichi prefecture, where the kaolin deposits are embedded in the lower Tokai Group, Seto porcelain clay formation (Late Miocene to Pliocene). Several samples were collected from Kibushi, Gaerome clays and kaolinitic saprolite (hereafter “Green saprolite”, named after its greenish grey color). Bulk and the elutriated clay fraction of 0.2 – 2 μm size were analyzed by XRF, powder XRD and SEM-EDS to clarify the chemical and mineralogical compositions. All of three types of clays (Kibushi, Gaerome and Green saprolite) consist of quartz, feldspar and kaolinite, but SEM found the difference in kaolinite morphology: The kaolinite in Kibushi and Gaerome clays were fine particles, whereas Green saprolite contained both larger (~ several hundreds of μm), highly crystalline kaolinite grains and fine particles. Further analysis indicated that the fine kaolinite particles were derived from feldspar and the larger crystalline grains were formed by alteration of biotite. Dithionite-citrate-bicarbonate (DCB)[2] extractable Fe was measured to distinguish the free Fe from the structural Fe in the total Fe in both bulk and clay fraction of Green saprolite. More than the half of Fe in the clay fraction was extractable by the DCB method, while only ~ 25% of Fe in the bulk was extracted. This indicate that Fe in the clay fraction was distributed both in the structure of kaolinite and low crystalline, ferric iron hydroxides/oxides, and the bulk sample including the larger kaolinite grains contained structural Fe in the remnants of altered biotite. Further analysis using Mössbauer spectroscopy indicated that Fe valence ratio (Fe2+/Fe3+) in the fine kaolinite particles of the green saprolite was 7/5, and this Fe2+ is the possible cause of its greenish color. Our analysis suggests that the Green saprolite layer can be improved as a resource of kaolin commodity by separating clay fractions and removing free iron in hydroxides/oxides.
References
[1] Takagi, T., Shin, K. C., Jige, M., Hoshino, M., & Tsukimura, K. (2021). Microbial nitrification and acidification of lacustrine sediments deduced from the nature of a sedimentary kaolin deposit in central Japan. Scientific Reports, 1–17.
[2] Mehra, O. P., & Jackson, M. L. (1958). Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays and Clay Minerals, 7(1), 317–327.
The samples used were obtained from the active mining area in the northern Seto-city, Aichi prefecture, where the kaolin deposits are embedded in the lower Tokai Group, Seto porcelain clay formation (Late Miocene to Pliocene). Several samples were collected from Kibushi, Gaerome clays and kaolinitic saprolite (hereafter “Green saprolite”, named after its greenish grey color). Bulk and the elutriated clay fraction of 0.2 – 2 μm size were analyzed by XRF, powder XRD and SEM-EDS to clarify the chemical and mineralogical compositions. All of three types of clays (Kibushi, Gaerome and Green saprolite) consist of quartz, feldspar and kaolinite, but SEM found the difference in kaolinite morphology: The kaolinite in Kibushi and Gaerome clays were fine particles, whereas Green saprolite contained both larger (~ several hundreds of μm), highly crystalline kaolinite grains and fine particles. Further analysis indicated that the fine kaolinite particles were derived from feldspar and the larger crystalline grains were formed by alteration of biotite. Dithionite-citrate-bicarbonate (DCB)[2] extractable Fe was measured to distinguish the free Fe from the structural Fe in the total Fe in both bulk and clay fraction of Green saprolite. More than the half of Fe in the clay fraction was extractable by the DCB method, while only ~ 25% of Fe in the bulk was extracted. This indicate that Fe in the clay fraction was distributed both in the structure of kaolinite and low crystalline, ferric iron hydroxides/oxides, and the bulk sample including the larger kaolinite grains contained structural Fe in the remnants of altered biotite. Further analysis using Mössbauer spectroscopy indicated that Fe valence ratio (Fe2+/Fe3+) in the fine kaolinite particles of the green saprolite was 7/5, and this Fe2+ is the possible cause of its greenish color. Our analysis suggests that the Green saprolite layer can be improved as a resource of kaolin commodity by separating clay fractions and removing free iron in hydroxides/oxides.
References
[1] Takagi, T., Shin, K. C., Jige, M., Hoshino, M., & Tsukimura, K. (2021). Microbial nitrification and acidification of lacustrine sediments deduced from the nature of a sedimentary kaolin deposit in central Japan. Scientific Reports, 1–17.
[2] Mehra, O. P., & Jackson, M. L. (1958). Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays and Clay Minerals, 7(1), 317–327.