Developments in oxidation of the (Mg,Fe)SiO3 pyroxene series and use of thermogravimetry as a quantitative analytical method

Interest in the pyroxenes has increased since the realization that their alteration products could carry a stable remnant magnetization which is possibly related to the earth's magnetic field and would therefore be of great geophysical importance.In the present work, the oxidation of artificial samples of the (Mg,Fe)SiO₃ pyroxene series was studied thermogravimetrically at the constant oxygen pressure of air and temperatures up to 1350°C. The resulting alteration products were determined after complete oxidation by a specific technique which is here reported as a new quantitative analytical method. This method could be applied to many other silicate groups, provided that iron is the only transitional element. Using natural samples, the accurate measure of the ratio $\text{Fe}{}^{\mathrm{3+}}\text{Fe}{}^{\mathrm{2+}}$ provides a basis for a possible quantitative magnetic analytical method and for a geothermometer. Factors affecting the results, i.e., (a) the volatiles and (b) the contamination of transitional elements other than iron, were recognized but could be eliminated by using specific techniques.A large region of metastable solid solution of hematite (Fe₂O₃) in the spinel phase was found. The resulting oxides, after complete oxidation and precipitation of the Fe₂O₃ phase from the metastable spinel phase, were studied by thermogravimetric analysis and X-ray diffraction.