Magnetic properties of synthesized titanomaghemite

Abstract

Three titanomagnetite samples, of composition x = 0.7, 0.9, and 1.0 (where x is the ulvSspinel molecular fraction), were oxidized according to the method of Sakamoto et al., namely, wetgrinding followed by heating to 200-300øC. Like the samples of Sakamoto et al., our samples showed reversal of saturation magnetization during alteration of titanomagnetite to titanomaghemite. Our experimental results seem to be compatible neither with the ionic model of Verhoogen nor with the model of O'Reilly and Banerjee. The Curie point gradually increased and the lattice parameter decreased during low-temperature oxidation (titanomaghemitization). On heating above 300øC, the •-titanomaghemites underwent high-temperature oxidation (unmixing into magnetite, pseudobrookite, which is formed only during heating to 600øC, and rutile phases). The unmixing results in a sudden increase in Curie point and decrease in lattice parameter and saturation magnetization. On further heating, the saturation magnetization reverses for a second time. Therefore, double self-reversal of remanent magnetization in naturally oxidized rocks is considered a real possibility.Titanomaghemite is a major ferromagnetic constituent in some rocks, for example, submarine basalt. Therefore, it is important to study its magnetic properties.

Magnetic properties of submarine basalts and the implications on the structure of the oceanic crust.

“…Experiments on synthesized titanomaghemites show that the Curie temperature of the titanomaghemites increases systematically with the increasing degree (OZIMA and SAKAMOTO, 1971) and by dotted lines (READMAN and OREILLY, 1972). of oxidation (OZIMA and SAKAMOTO, 1971;READMAN and OREILLY, 1972).…”

“…However, recent studies on magnetization of many submarine basalts appear to show that ferromagnetic minerals in submarine basalts undergo progressive oxidation with time (SHAEFFER and SCHWARZ, 1970;OZIMA and OZIMA, 1971;LOWRIE et al 1973). Since such oxidation may affect the primary remanent magnetization resulting in a violation of the assumption, it is important to know the nature and the extent of the oxidation in the oceanic crust.…”

“…Lately much efforts have been made to understand the magnetic properties of titanomaghemites by two groups (OZIMA and SAKAMOTO, 1971;READMAN and OREILLY, 1972), where the studies were made on synthesized titanomaghemites. However, for titanomaghemites of high Ti content, there is a considerable difference in the Curie temperatures and the lattice parameters between the two groups.…”

Lattice constants of titanomagnetites at high temperatures.

“…To identify the composition and the oxidation stage of titanomagnetites and titanomaghemites in rocks, measurements of Curie temperature and lattice parameter have been commonly used. These parameters (Tc and d) for synthesized titanomaghemites were investigated by OZIMA and SAKAMOTO (1971), READMAN and O'REILLY (1972), KEEFER and SHIVE (1981) and NISHITANI and KONG (1983), which all give significantly different results. This situation is very unfortunate, since we must choose one of them to determine the composition and the oxidation stage of titanomaghemites in natural rocks.…”

Effect of low-temperature oxidation on the remanence properties of titanomagnetites.

“…The parameter x is defined as the molar fraction of ulvospinel; where x=0.0 corresponds to magnetite and x=1.0 to ulvospinel. The oxidation parameter z is defined as where z=0.0 and 1.0 indicate the unoxidized and fully oxidized states, respectively (OZIMA and SAKAMOTO, 1971). MARSHALL and Cox (1971) investigated the change of NRM and CRM in pillow basalts when they are heated in air.…”