Magnetic Mineralogy of Unheated and Heated Red Sediments by Coercivity Spectrum Analysis

A method of deriving the remanent coercivity spectrum from the isothermal remanence vs. applied field characteristic is described and applied to a series of Triassic sandstones and late Cretaceous—early Tertiary clays and limestones from France. Low- (0–1 kOe), intermediate- (1–3 kOe), and high- (3–18 kOe) coercivity remanence fractions are identified with magnetite, specularite, and haematite pigment respectively. In the limestones, specularite is the only important magnetic phase; the sandstones contain in addition considerable pigment; and the clays contain all three phases. The relative pigment content indicated magnetically agrees with the colour of each rock type. When the sandstones are heated in air, enough magnetite is produced below 675°C to account for 20–25 per cent of the saturation remanence. Secondary magnetite production may therefore be serious when red beds are thermally demagnetized, even though the heating is done in air. The source of this secondary magnetite seems to be a non-magnetic mineral, rather than haematite. Secondary haematite production is insignificant below 675°C, but the pigment coercivity spectrum becomes harder with annealing, as observed previously with synthetic fine-particle haematites. Either annealing-out of defect ferromagnetism or impurities entering the haematite lattice could explain the results. Either coercivity spectra or rotational hysteresis curves can be used to estimate pigment/specularite ratios in red beds and to monitor mineralogical changes resulting from heating, but the coercivity spectrum method is much simpler to use.