Curie temperature and lattice constant reference contours for synthetic titanomaghemites

Abstract

Curie point and lattice parameter measurements were made on 57 titanomaghemite samples produced from sintered titanomagnetites with x (mole fraction ulvospinel) values of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0. Oxidation was carried out in various atmospheres at temperatures as low as 113°C for durations of about 24 hours. The oxidation process was successful in producing homogeneous titanomaghemites with z (oxidation parameter) values between 0.0 and 1.0. Spinels with z < 0.6 are generally single phase, but the necessity of using temperatures greater than 300°C to produce strongly oxidized phases resulted in spinel oxidation in the presence of a rhombohedral phase. Curie temperature and lattice constant data are significantly different from results presented by Ozima and Sakamoto (1971), Readman and O'Reilly (1972), and Nishitani (1979). Although some of these differences may be due to unmixing (inversion), most of them apparently result from differences in techniques of titanomaghemite preparation and oxidation.

Moessbauer spectra of titanomagnetite: A reappraisal.

“…NISHITANI and KONO (1983) suggested that the differences in cation distribution of original titanomagnetite samples might be one of the causes for the inconsistencies within the results of the low temperature oxidization experiments reported by four groups: OZIMA and SAKAMOTO (1971), READMAN and O'REILLY (1972), KEEFER andSHIVE (1981), andKONO (1983), These discrepancies cause a severe problem when oxidized oceanic basalts are examined (MosKowlTz and BANERJEE, 1981). NISHITANI and KONO (1982) indicated that the main reason for the difference between the data of OzIMA and SAKAMOTO (1971) and those of READMAN and O'REILLY (1972) was in the average size of the ferromagnetic grains, although no significant grain size differences were reported between the samples of other authors.…”

Lattice constants of titanomagnetites at high temperatures.

“…As noted by KEEFER and SHIVE (1981), most of the divergence of the above results come from the differences in Curie temperatures. Titanomaghemites are thermally unstable, and some chemical changes occur during the Curie temperature measurements, even when the heating is made in vacuum.…”

“…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.…”

“…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. Difference in the grain size, the technique for synthesizing the starting material, the inhomogeneous oxidation, and the method for estimating the oxidation stage have been noted to explain the difference (KEEFER and SHIVE, 1981;NISHITANI and KONO, 1983). But, these do not totally clarify the reason for the discrepancy.…”

Preliminary results on the use of magnetic techniques on the study of the granite aureoles.

“…The Curie point technique is based on the idea that each magnetic mineral of specific composition has a unique Curie temperature (OzIMA and OzIMA, 1964;BANERJEE and O'REILLY,1967;HEDLEY,1968;MERRILL,1970;MuLLIN5, 1977) so that the Curie point can be used to determine the precise composition within the titanomagnetites (OZDEMIR and O'REILLY, 1981;KEEFER and SHIVE, 1981) and 544A.…”