The magnetic properties of ilmenite-haematite crystals

Abstract

The interpretation of both palaeomagnetic and geophysical prospecting data requires a better understanding of the magnetic properties of the iron oxide minerals found in rocks. The magnetic properties of ilmenite-haematite solid solutions have been investigated, the unusually large and pure haemo-ilmenite crystals from the Allard Lake region of Quebec being used. The magnetic component of these crystals is an ilmen-haematite phase, having a composition of about 10 mole % of ilmenite in haematite, and present in the form of exsolution lamellae that are roughly 5 µ long, 1 µ wide and 0.22 µ thick. The crystals have a very strong anisotropy causing magnetization in the basal plane and a weak anisotropy which produces an easy direction of magnetization within the basal plane. An improved ilmenite- haematite solvus curve has been produced by X-ray and Curie-point analysis of heat- treated crystals. Spontaneous reversal of magnetic polarity takes place with change in temperature in ilmen-haematite having between about 25 and 15% of ilmenite in haematite. This new reversing range of composition is quite different from that found to reverse by Uyeda. The reversal is due to a new antiparallel moment which grows as temperature falls. This has been interpreted as being due to an ordering of Fe 2+ ions on alternate cation layers b an electron transfer mechanism between trivalent and divalent iron atoms.

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

“…ap apatite; bd baddeleyite; hem hematite; ilm ilmenite; mt magnetite; plag plagioclase; sp hercynitic spinel; zr zircon. and hematite-ilmenite solid solutions (Carmichael, 1961). Ulvöspinel exsolutions in magnetite have been documented in various localities, e.g.…”

Self-reversed thermoremanent magnetization in a late Brunhes dacite pumice.

“…Self reversal was attributed to unresolved remnants of the Fe-rich lamellae. The properties of the lamellae remnants described by CARMICHAEL (1961) and the metastable phase inferred by ISHIKAWA and SYONO (1963) are similar: both are Fe-rich, disordered a-(Fe, Ti)203, are responsible for reversed TRM, and their persistence is governed by similar annealing conditions. The overall composition of the crystals studied by CARMICHAEL (1961), with x=0.70, indicates that disorder-order processes were operating in the natural environment.…”

“…Thermally-activated diffusion at 600C is significantly less than a-(Fe, Ti)203 solvus temperature of 800 to 950C (CARMICHAEL, 1961) for these compositions, and favors the presence of two phases. Annealing at higher temperatures approaches the solvus and favorss a single ordered phase.…”

“…Specimens of natural crystals (x=0.70), showing exsolved Fe-rich a-(Fe, Ti)203 lamellae, were annealed for periods of 34 and 100 days at various temperatures. CARMICHAEL (1961) observed that annealing temperatures of 600C or less are too low for resolution of lamellae to take place even after heat treatment for 100 days. Higher-temperature (>600C) annealing is much more effective, with 100-day specimens showing more complete resolution than the 34-day specimens.…”

“…(Hereafter, minerals of the ilmenite-hematite solid solution series will be referred to as a-(Fe, Ti)203, with the molar fraction of ilmenite given by x'). To this date selfreversal has been observed only on rare occasions in natural materials (CARMICHAEL, 1961;MINNIBAEV et al, 1966;SCHULT, 1968).…”