Abstract
The study was conducted to characterize Fe/Mn, layer silicate minerals in the concretions of Okinawan and Brazilian soils and investigated their micromorphological features. Concretions and soil samples of Okinawa (Luvisol) and Miyako (Cambisol) Islands were developed from Ryukyus limestone, while Brazilian samples of Cambisol and Ferralsol were originated from sandstone rock. Fe/Mn minerals in both experimental sites were characterized using the successive selective dissolution procedure followed by the treatments of sodium hydroxide, hydroxylamine hydrochloride (HAHC) at 25°C and 60°C and dithionite-citrate-bicarbonate (DCB) in combination with X-ray diffraction technique (XRD). Elemental composition and micromorphological information on Fe/Mn minerals and their associated layer silicates were investigated using inductive coupled plasma atomic emission spectrometry (ICP-AES) and scanning electron microscopic (SEM) techniques respectively. XRD and SEM techniques were successfully able to investigate and distinguish the major minerals in Okinawan and Brazilianl samples were kaolinite (Ka), gibbsite (Gb), illite, quartz, and goethite (Ge), while the rear natural minerals of birnessite and smectite were investigated from the samples of Cambisol in Miyako and Luvisol in Okinawa respectively. Elemental analysis also confirmed that Brazilian samples did not contain any Mn oxide minerals. Detailed micromorphology of minerals exhibited that poorly crystalline birnessite has a blade or plate-like habit, which forms globular aggregates inside veins and cavities and widely spread smectite mineral in Okinawan samples showed a honeycombed polygonal pattern. Well-developed hexagonal kaolinite and gibbsite crystallites were also observed in Okinawan samples, while the book-like structure of kaolinite crystals with beveled edges in Brazilian samples imbedded inside concretions. Goethite crystals in Brazil samples are acicular and consist of several domains within the crystals along the needle axis and these acicular crystals arranged into stars, while cloudy shape of hematite crystals aggregated with kaolinite and imbedded inside silicate minerals to produce hard iron concretions.
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