Low Temperature Magnetic Properties of Variably Oxidized Natural and Synthetic Siderite

Siderite (FeCO₃) is an important ferrous iron carbonate in the geochemical cycling of iron, as it is a sink for iron under reducing conditions. However, its detection is not straightforward with classical analytical approaches because in natural samples it is often fine-grained and/or occurs in low concentrations. In this study, we explore the analytical potential of low-temperature magnetometry. Synthetic siderites with a limited amount of associated ferric iron of up to 5 mol% and some natural siderites were subjected to investigation. Maxima in the cooling curves in a 5 T magnetic field shows that the Néel temperature of siderite is at 37 K in agreement with literature data. Those maxima appear at a higher temperature in the synthetic siderites with associated/sorbed ferric iron; it is 45 K for the 5 mol% Fe³⁺ synthesis. With the increasing amount of ferric iron, the synthetic siderites show an increasingly prominent remanence tail beyond the nominal Néel temperature in field-cooled (FC) and zero-field-cooled (ZFC) warming curves of the remanent magnetization acquired in 5 T at 5 K. Fine-grained siderite alters in air on laboratory time scales which is manifested by more pronounced remanence tails up to higher temperatures. Siderite's presence is best diagnosed by evaluating a combination of FC warming curves and a FC/ZFC remanence ratio >3 at 5 K. Standard addition experiments of FC warming curves enable the determination of siderite down to 0.1 wt%.