チタノマグネタイトの酸化と磁気的性質

Abstract

Titanomagneties, Fe3-xTixO4, are the most important carriers of magnetic remanence in terrestrial igneous rocks. In subaerial effusive rocks such as basalts, titanomagnetite often undergoes a high-temperature oxidation ; i. e., Ti-rich spinel → Ti-poor spinel + Ti-rich ilmenohematite, the latter being exsolved into (111) planes of the host spinel forming a lamella structure. Another form of oxidation, low-temperature oxidation, proceeds if a titanomagne-tite sample is heated to a moderately high temperature (<450°C). In this case, cation deficient spinels (titanomaghemites) are produced. Because titanomaghemites are the main ferromag-netic minerals in suboceanic basalts, study of magnetic properties of these phases is important in understanding the nature of the source region of the oceanic magnetic anomalies. There are four sets of reports dealing with the magnetic properties of the synthetic titanomaghemites (Ozima and Sakamoto, 1971; Readman and O'Reilly, 1972; Keefer and Shive, 1981; Nishi-tani and Kono, 1983). Unfortunately, these results are quite different. Some of the differ-ences can be attributed to the grain size of samples ; larger grains (>1μm) tend to deveolop rhombohedral phase due to oxidation even at low temperatures. However, significant variations remain even after the grain size effect is taken into account. One possibility is that the starting materials may be subtlely different because of the difference in sample preparation procedures.