Atributos químicos, físicos e mineralógicos de magnetitas magnesianas e suas alterações aos diferentes estresses físicos

Abstract

The studies involving iron oxides have become important because of their several uses. Besides being part of natural systems, these minerals have been used in the areas of civil engineering, chemistry and health, most recently in the areas of biology and medical sciences. The iron oxides and hydroxides are also important because they are abundant in soils of the tropics and subtropics regions. This work aims to synthesize magnetite and magnesium magnetites with different degrees of isomorphic substitution and submit them to different physical and chemical environments. For this synthesis ferrous sulfate and magnesium sulfate were used in molar fractions ranging from 0.00 mol% to 33.0 mol% magnesium replacing iron in the presence of potassium hydroxide and potassium nitrate. The analyses of the synthesized minerals were based on methodologies described in the literature, having also carried out the selective dissolution of undesirable products using Mehlich-1 extractor. It was also made heat treatment of all synthesized fractions for the conversion to hematite and maghemite. The results showed that besides the synthesis of magnetite, magnesioferrite and other minerals were precipitated during the synthesis procedure proportional to the amount of magnesium sulphate added. The other mineral precipitates were arcanite, yavapaiita and an unprecedented muskoxite, coexisting with magnetite and magnesioferrite. The muskoxite was formed from increased levels of iron and magnesium and have greater solubility than magnesioferrite, competing with magnesioferrite by the iron and magnesium in solution. Other results showed that not all magnesium was added to the crystalline structure of magnetite, and it was removed by the acid extractor. Yavapaiite and muskoxite minerals were not dissolved unlikely what happened with arcanite. The results of the heat treatment show that increasing magnesium substitution within the synthetic minerals increased the temperature of magnesioferrite formation and reduced, in the same proportions, the formation of hematite at 800oC. It was used scanning electron and transmission microscopies to obtain high-quality images, and the Rietveld method for refining of the crystallographic structures, achieving satisfactory adjustments.