High-temperature thermomagnetic properties of vivianite nodules, Lake El'gygytgyn, Northeast Russia

Vivianite, a hydrated iron phosphate, is abundant in sediments of Lake El'gygytgyn, located in the Anadyr Mountains of central Chukotka, northeastern Russia (67°30′ N, 172°05′ E). Magnetic measurements, including mass-specific low-field AC magnetic susceptibility, field-dependent magnetic susceptibility, hysteresis parameters, temperature dependence of the induced magnetization, as well as susceptibility in different heating media, provide ample information on vivianite nodules. Electron microprobe analyses, electron microscopy and energy dispersive spectroscopy were used to identify diagnostic minerals. Vivianite nodules are abundant in both sediments of cold (anoxic) and warm (oxic) stages. Magnetic susceptibility of the nodules varies from 0.78 × 10−6 m3 kg−1 to 1.72 × 10−6 m3 kg−1 (average = 1.05 × 10−6 m3 kg−1) and is higher than the susceptibility of sediments from the cold intervals. Magnetic properties of vivianite are due to the respective product of oxidation as well as sediment and mineral inclusions. Three types of curves for high-temperature dependent susceptibility of vivianite indicate different degrees of oxidation and inclusions in the nodules. Vivianite acts as a reductant and reduces hematite to magnetite and masks the goethite–hematite transition during heating. Heating vivianite and sulfur mixtures stimulates the formation of monoclinic pyrrhotite. An additive of arsenic inhibits the formation of magnetite prior to its Curie temperature. Heating selective vivianite and pyrite mixtures leads to formation of several different minerals – magnetite, monoclinic pyrrhotite, and hexagonal pyrrhotite, and makes it difficult to interpret the thermomagnetic curves