Thermal alteration of the magnetic mineralogy in ferruginous rocks

Abstract

The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite. The magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane). This inverse fabric is attributed to the dominating presence of siderite. The rocks were incrementally heated to 600°C, and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step; cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric. After heating the samples above 250°C, the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane. The low‐temperature magnetic fabric remains inverse until the samples are heated to about 500°C. After heating to higher temperatures a normal fabric was observed at both room temperature and low temperature. Rock magnetic evidence shows that magnetite is created during the heating. The start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy, powder X ray diffraction, and acquisition of isothermal remanence. Berthierine starts to break down at approximately 250°C and is totally oxidized by 500°C. The creation of magnetite in the heating experiments corresponds with this mineralogical change. The change in the room temperature magnetic fabric is associated with the decomposition of berthierine, part of which alters to magnetite. The new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite.