Characterization and Micromorphological Observations of Fe/Mn and Silicate Minerals in Okinawa and Brazilian Soils.
Alexandre Pereira De Bakker, Instituto Nacional de Pesquisas da Amazonia (INPA), Avenida Andre Araujo, 2936, Manaus-Amazonas, Brazil
The study was conducted to characterize the Fe/Mn and layer silicate minerals in the concretions of Okinawan and Brazilian soils and investigate their micromorphological features. Concretions and soils of Okinawa (Luvisol) and Miyako (Cambisol) Islands were developed from the Ryukyus limestone while Brazilian samples of Cambisol and Ferralsol have originated from sandstones. The Fe/Mn minerals in both experimental sites were determined 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 the X-ray diffraction technique (XRD). The elemental compositions and micromorphological features of the Fe/Mn minerals and their associated layer silicate minerals were investigated using the inductive coupled plasma atomic emission spectrometry (ICP-AES) and scanning electron microscopic (SEM) techniques respectively. XRD and SEM were successful in determining the major minerals in the samples. The major minerals observed in Okinawan and Brazilian samples were kaolinite, gibbsite, illite, quartz, and goethite, while the rear minerals of birnessite and smectite were investigated from 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 the minerals revealed, that the poorly crystalline blade or plate-like birnessite, which forms globular aggregates inside veins and cavities with widely spread smectite in Okinawa samples showed a honeycomb- polygonal pattern. Well-developed hexagonal kaolinite crystals of book-like structure and gibbsite crystals were also observed in Okinawan soils, and with beveled edges in Brazilian samples embedded inside concretions. Goethite crystals in the Brazilian soils were acicular developing to star-like domains, while cloudy hematite crystals were aggregated with kaolinite and embedded inside silicate minerals to form iron concretions.