Ferrous (Fe2+\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}^{2+}$$\end{document}) and ferric (Fe3+\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}^{3+}$$\end{document}) iron content in mineral samples was determined from total iron (as obtained, for instance, from X-ray fluorescence analysis) and mass increase upon switching the purge gas from inert to oxidizing, at high temperature, in a thermogravimetric analysis. The method was validated using analytical-grade ilmenite (FeTiO3\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {FeTiO}_{3}$$\end{document}, or Fe2+O·TiO2\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}^{2+}\hbox {O}\cdot \hbox {TiO}_{2}$$\end{document}) and magnetite (Fe3O4\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}_{3}\hbox {O}_{4}$$\end{document}, or Fe2+ · Fe23+O3). The estimated mass percentages of FeO are less than the expected values, according to chemical composition, by 1.1 and 1.2 percentage points for ilmenite and magnetite, respectively. The application of this method is further illustrated by determining the Fe2+\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}^{2+}$$\end{document} and Fe3+\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}^{3+}$$\end{document} content, expressed as FeO and Fe2O3\documentclass[12pt]{minimal}
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\begin{document}$$\hbox {Fe}_{2}\hbox {O}_{3}$$\end{document}, in three samples of igneous rocks.
