New Grain Size Limits for Palaeomagnetic Stability in Haematite

Abstract

HAEMATITE (α−Fe₂O₃) is an important carrier of natural remanence magnetization (NRM) in sedimentary rocks (such as red sandstones and shales, oceanic red clays) as well as in some volcanic rocks (such as highly oxidized basalts, ignim-brites), and monomineralic rocks (cassiterite, for example). To be useful to the palaeomagnetist, however, such haematite particles should preserve the integrity of the intensity and direction of their NRM over geological time-scales. One of the primary requirements, therefore, is that the grain size should fall between two size thresholds; the lower of these two is the superparamagnetic threshold (d_s.p.) and the higher size corresponds to the multi-domain threshold (d_m.p.). The importance of the superparamagnetic threshold in palaeomagnetism was illustrated by Creer¹ in his study of the Keuper marl. He showed that the principal reason for the instability of NRM in this red rock was due to the haematite grains being smaller than d_s.p. ; thus the thermal energy, kT, at room temperature was larger than the stabilizing magnetic anisotropy energy, vK, where volume v=πd³/6 and anisotropy energy per unit volume = K. Creer studied² the variation of magnetization with grain size using synthetic haematite derived from dehydrated goethite. One concludes from his data that d_s.p.,the superparamagnetic threshold in haematite, is less than 100 Å since even at d=100 Å, haematite exhibited field-dependent susceptibility. Strangway et al.³ used dehydrated natural goethite to establish the value of d_s.p., and found it to be 5000 Å, vastly different from the value of <100 Å, concluded by us from Creer's work². Kündig et al.⁴ prepared synthetic haematite in commercially available silica gel matrix and studied the onset of superparamagnetism using Mössbauer effect. Since these samples are not likely to be contaminated by goethite (goethite, in spite of being an antiferromagnetic compound, can possess weak ferromagnetism due to internal defects)^3,5, the Möss-bauer-derived d_s.p. of 150 Å ought to be more reliable. Kündig's preparation technique was to soak silica gel samples with an inorganic iron salt solution and then heat the samples to prepare the haematite grains. This process leads to “clumping” and a large distribution of grain sizes, and Dunlop (in the press) has pointed out the fact that Mössbauer effect measurements lead to the determination of the product (vK)_s.p. and not v_s.p. uniquely. Thus one should regard the Mössbauer derived d_s.p. of 150 Å as tentative, until purely magnetic measurements can establish uniquely the superparamagnetic threshold. In what follows we present a new d_s.p. value using magnetic measurements on synthetic haematite particles, prepared by a modification of the technique of Kündig et al. leading to a much narrower size distribution.