Magnetic Properties of Synthetic Analogues of the Altered Olivines of Igneous Rocks

Abstract

SUMMARY A laboratory simulation has been made of the high temperature deuteric alteration of olivine such as occurs in slowly cooled basic intrusions. Three compositions in the Forsterite/ Fayalite solid solution series were synthesized by a three-firing method using self buffering and controlled atmosphere to obtain material containing no detectable magnetite (Fe,O,) as a contaminant (<0.005 per cent). The prepared olivines were oxidized between 850 and 1280°C at oxygen fugacities of between lo-”.’ and lop5.’ atmospheres. The composition and concentration of the multiphase oxidation products were studied by magnetic, optical, electron optical and X-ray methods. The magnetic constituent of the intergrowth was magnetite. Below about 1150 “C the microstructure is similar to the ‘dendritic’ intergrowths found, for example, in the altered olivine of the Central Layered Intrusion of the Isle of Rhum. Above about 1150 “C the form may be similar to ‘symplectite’ intergrowths. The magnetization process parameters (coercive force, etc.) vary systematically with temperature of oxidation ( To,) which evidently controls the microstructure of the magnetite. The magnetite produced is magnetically ‘hard’, samples altered at 900 “C and 950 “C having higher coercive force (Hc) than any magnetite reported in the literature. The room temperature properties appear to bridge the published trends found in ‘grown crystal’ magnetites and ‘crushed grains’. Inferred effective grain sizes of the complex intergrowths range from 0.1 to 3 pm as To, increases from 900 to 1200 “C. The temperature dependence of magnetization process parameters (principally H,) measured between room temperature and the Curie point are modelled in terms of (i) monodomain grains subject to thermal fluctuations, (ii) monodomain grains not subject to thermal fluctuations, and (iii) as multidomain grains. Model (i) yields plausible values for effective grain dimensions and length/diameter ratios, higher To, producing larger, more equidimensional particles. Model (ii) suggests that the relative importance of cubic and uniaxial anisotropies may be influenced by a microstructure-dependent uniaxial particle-particle interaction term. Model (iii) implies that, if domain walls are pinned by both surface and volume defects, magnetostriction plays a greater role in volume pinning than in surface pinning. Plausible values of demagnetizing factors appropriate to grains with few domains are produced by model (iii).