Transformações de óxidos de ferro em rocha basáltica em situações de intemperismo induzido

Abstract

Powdered basaltic rocks were collected from three sites in Paraná State (Brazil), submitted to magnetic separation and then sieved. Further, the powder fraction smaller than 0,020 nm was chemically treated or thermally annealed aiming to reproduce the conditions of natural weathering in laboratory scale. The applied chemical treatment consisted of maintaining the powders permanently wet by a solution of HCL (pH 5) for 6 months. The thermal treatment was applied at the temperatures of 500 °C and 800 °C, for 4h, in free atmosphere. In each treatment, the powder samples were characterized by Xray diffraction, Mössbauer spectroscopy and electron microscopy (EDX). The results showed that the as-collected samples have mostly silicon, calcium, aluminum, magnesium, iron, titanium and potassium in their chemical composition associated to silicates and oxides. The magnetically separated fraction revealed a significant increase in the iron concentration and, consequently, in the silicates that contain iron and ironoxides. The samples chemically and 500°C thermally treated did not present any considerable alteration, chemical or structural, indicating that the treatment temperature and the chemical environment did not produce measurable weathering effects with the techniques employed. Conversely, the thermal annealing conducted at 800 °C deeply transformed the iron oxides, increasing the hematite fraction and forming pseudobrookita, in addition to other compounds without iron, as a result of the phase separation.