Oxides and Hydroxides

Abstract

Oxidic minerals can only be recognized differential thermal analytically by the appearance of structural or magnetic changes. The high-low inversion of quartz (~573° C) has been for many years a means of calibrating the temperature, as well as for calorimetric measurements (following a proposal of Faust), but neither the inversion temperature (= t_i) nor the heat of reaction (3.1 cal/g after Faust) can be used for calibration, since the t_i can vary by a great temperature interval (Keith and Tuttle; Smykatz-Kloss, 1970, see III-8), and the heat of reaction is dependent on the degree of disorder of the crystals. Grim and Rowland have measured 565° for the t_i of authigenically formed quartz crystals of soils. The t_i-variation of the SiO₂ minerals cristobalite and tridymite is still much greater than the t_i-variation of quartz crystals (see III-7). Magnetites oxidize in two steps to Fe₂O₃, reflecting two very strong exothermic effects with peaks at 380 and 580° C. Shape and temperatures of these peaks are dependent on the grain size (Egger), and on some other factors (Kirsch). A weak endothermic effect at 680° C points out the Curie point of Fe₂O₃ (Egger); the temperature of this peak varies with different contents of titanium and manganese (compare with III-3). In DTA curves of hematite Peters obtained a broad endothermic deflection between 400 and nearly 700° C with a small endothermic peak at 675° C, which probably points out the Curie point of the Fe₂O₃.