Mössbauer study of Andean páramo soil from Ventaquemada, Boyacá, Colombia

Abstract

Samples of paramo soil for Mossbauer studies down to a temperature of 4.2 K were collected from a sampling site in the municipality of Ventaquemada, district of Montoya, sector of Matanegra. This location is close to Tunja, capital of Boyaca Province, Colombia. Paramo soils often result from weathering of volcanic ash and are rich in allophane and organic matter. The Mossbauer spectra show the iron in all samples to be present mainly as a ferric component that splits magnetically at temperatures below about 20 K into a sextet with a mean hyperfine field of about 44 T and broad resonance lines that indicate a distribution of hyperfine fields. Additionally, weak contributions of hematite, magnetite, goethite, and ferric and ferrous iron in clays, are present as well as a structure-less pattern (collapsed sextet) that appears at low temperatures. The broad magnetically split component represents up to 70% of the iron in the Paramo soils. It is quite stable towards heating the soils, converting to hematite mainly between 600 and 800 °C. This rules out that the component is ferrihydrite, which decomposes already below 400 °C. Presumably, it is attributable to the allophane component in the Paramo soils. Leaching by the DCB method removes this component as well as most of the goethite, but only part of the hematite, whereas oxalate leaching removes mainly the broad magnetically split component attributed to allophane, but leaves hematite, magnetite and goethite largely unchanged. Leaching with H2O2, which oxidises and removes the organic matter, has no effect on the Mossbauer spectra, showing that iron bound to organic substances is practically absent.