Submicroscopic Structure of Fe-Coatings on Quartz Grains in Tropical Environments
Formation of Fe-coatings or segregation at the expense of quartz grains is a common process in the tropical environment. Limited information is available about their internal structure at the submicroscopic level. The transmission electron microscope (TEM) and energy dispersive X-ray microprobe analyses (EDXRMA) techniques were used to identify the nature and arrangement of fundamental mineral particles within the Fe-coatings. Chemical and mineralogical studies showed that the coatings were composed of well-crystallized Fe-oxides, quartz and kaolinite. The EDXRMA analyses revealed the presence of linear concentrations (laminae) of nearly pure Fe oxide along the edges and the contact zone with quartz and within the coatings. Similar atomic proportions of Al and Si in several areas within the interior regions of coatings and the XRD pattern of the crushed coatings are supportive evidence for the presence of kaolinite. Under the TEM, the dense laminae (< 10 μm thick) consisted of elongated Fe-oxide particles (< 1.5 μm long and 0.2 μm thick) accommodated in subparallel arrangement. The interior areas had very high porosity and, in addition to Fe-oxides, contained other minerals: mainly kaolinite, quartz and isolated areas of Al-oxides. High amounts of ultramicroscopic pores (<0.5 μm) in the interior region suggested that dissolution of Fe-oxides occurred under reduced conditions, with subsequent reprecipitation of pure Fe-oxides in the laminae. Very low porosity and parallel arrangement of Fe-oxide particles (laminae) provided new surfaces (barrier) for Fe-accumulation when soil solutions provided new influxes of iron, thereby creating a thicker Fe-coating. The size and geometry of the ultramicroporosity were shown to play a significant role in the dissolution and precipitation of soil minerals, especially those involved in redox reactions.
Key WordsElectron Microscopy Fe-coating Fe-nodules Fe-oxide Forms Submicroscopic Structure Ultrathin Sections X-ray Microprobe Analysis
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- Brewer R. 1976. Fabric and mineral analysis of soils. Melbourne, FL: Robert E. Kriger. 482 p.Google Scholar
- Bullock P, Fedoroff N, Jongerius A, Stoops G, Tursina T, Babel U. 1985. Handbook for soil thin section description. Wolverhampton, UK: Waine Research. 152 p.Google Scholar
- Eswaran H, Lim CH, Sooryanarayana V, Nordin Daud. 1978. Scanning electron microscopy of secondary minerals in Fe-Mn glaebules. In: Delgado M, editor. Soil micromorphology. Proceedings of the 5th international working meeting on soil micromorphology; Grenada, Spain. Grenada: Departmento de Edafologia, Univ de Granada, p 589–609.Google Scholar
- Eswaran H, Sys C, Sousa EC. 1975. Plasma infusion—a pedological process of significance in the humid tropics. Anales de Edafologia y Agrobiologia 34:665–673.Google Scholar
- Jackson ML. 1973. Soil chemical analysis—advanced course. 2nd ed, 8th printing. Madison, WI: ML Jackson, Univ of Wisconsin. 895 p.Google Scholar
- Krauskopf KB. 1967. Introduction to geochemistry.New York: McGraw-Hill. 721 pGoogle Scholar
- Milnes AR, Fitzpatrick RW. 1989. Titanium and zirconium minerals. In: Dixon JB, Weed SB, editors. Minerals in soil environments. 2nd ed. Madison, WI: Soil Science Society of America, p 1132–1205.Google Scholar
- Mohr ECJ, van Baren FA, van Schuylenbourgh J. 1972. Tropical soils: a comprehensive study of their genesis. The Netherlands: Geuze Dordrecht. 481 p.Google Scholar
- Schwertmann U. 1985. Occurrence and formation of iron oxides in various pedoenvironments. In: Stucki JW, Goodman BA, Schwertmann U, editors. Iron in soils and clay minerals. NATO ASI Series C: Mathematical and Physical Sciences, vol 217. Boston: D. Reidel. p 267–308.Google Scholar
- Schwertmann U, Taylor RM. 1989. Iron oxides. In: Dixon JB, Weed SB, editors. Minerals in soil environments. 2nd ed. Madison, WI: Soil Science Society of America, p 379–438.Google Scholar
- Sposito G, Reginato RJ, editors. 1992. Opportunities in basic soil science research. Madison, WI: Soil Science Society of America. 109 p.Google Scholar