Differential Thermal Analysis Using High Temperature Susceptibility Instruments

Abstract

Measurements of susceptibility as a function of temperature are an important, standard method of quantifying a sample’s magnetic mineralogy. These measurements are used to identify the Curie/Néel temperatures of constituent minerals and show evidence of thermally-induced oxidation and formation of magnetic minerals. For these reasons, the High Temperature Susceptibility Bridge (HTSB) is a common instrument in paleomagnetism laboratories around the world. Here, we argue that HTSBs can be used to perform simple differential thermal analysis (DTA), a technique used to measure phase transitions and exo-/endothermic reactions by measuring deviations in temperature from a steady heating rate. Because HTSBs are designed to heat samples at a relatively constant rate, the standard susceptibility measurement can be used to obtain similar information, albeit crude, about the phase transitions taking place. For standard samples of vivianite, goethite, and maghemite, we compare the heating curves of the sample and a blank run, to identify DTA-analogous information. In the case of goethite and vivianite, the reactions identified do not have a corresponding expression in susceptibility, demonstrating that this approach expands the utility of this common piece of equipment. We estimate the minimum enthalpy of reactions detectable and their mass dependence. Additionally, we show that this approach is successful in detecting the dehydration of vivianite in a mixed mineralogy natural sample.