Mechanism For The Magnetic Enhancement Of The Black Mudstone During Heating From The Nenjiang Formation In The Songliao Basin

The temperature-dependence of magnetic susceptibility??-T curves?can be used to determine the magnetic minerals and the phase transition during heating.Measurements of the ?-T curves have been employed on selected 30 specimens of the Nenjiang Formation?K2n1-K2n2: 972.46-1060.24m?of the SK-1 south borehole in the Songliao Basin?Li et al.,2017?.The results indicated that paramagnetic minerals dominantly contributed to the magnetic susceptibility of the mudstone samples.The cooling curves of most samples lied much above their heating curves.This suggested that some new ferrimagnetic minerals were produced during heating and revealed the Curie temperature of ⁵00-560?.But previous studies did not distinguish the new magnetic mineralogy and only speculated that the new magnetic mineral may come from the conversion of clay minerals at high temperature.Therefore,a new study on pure clay minerals,feldspar crystals and samples from the Nenjiang Formation has been carried out to reveal the mechanism of magnetic enhancement during their heating.The representative samples from the Nenjiang Formation and pure clay minerals and feldspar crystal were selected for the measurement of ?/T curves,X-ray powder diffraction?XRD?,the observation by scanning electron microscope?SEM?and the analyses of energy spectrum?EDS?.The main results are as follows:1.For clay minerals,chlorite and illite-smectite mixed layer?I/S?are thermally unstable and their magnetic susceptibility values increase due to the neo-formation of magnetic minerals during heating in argon.Another chlorite contained siderite with Mg²⁺ and Mn²⁺,is heated to form manganochromite with the ⁵20? Curie temperature.Montmorillonite and kaolinite are thermally stable and their ?/T curves are nearly reversible.2.Feldspar crystals are thermally stable and their ?/T curves are nearly reversible.But the feldspar with chloritization and the weak weathering K-feldspar have weak thermal stabilization.A little magnetite was formed during their heating in argon,which results in the magnetic enhancement.3.The ? of the black mudstone from the Nenjiang Formation increases when heating to over ³50 ? in argon.The magnetic enhancement results from the mineral transformation during heating.It is assumed that the minerals,containing iron and magnesium,clay minerals such as illite and kaolinite and quartz may change into mica,a little magnetite and some magnetite containing Mg/Mn during heating.The Curie temperature of magnetite containing Mg/Mn is ⁵20?.4.When heating to over ³50? in argon,pyrite in the black mudstone from the Nenjiang Formation transforms into antiferromagnetic hexagonal pyrrhotite.This does not contribute to any obviouse magnetic enhancement of the black mudstone during heating.