Titanomagneties, Fe_3-xTi_xO₄, 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.

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A single crystal of Mn-Zn ferrite and polycrystalline Ni-Zn or Mn-Zn ferrite areused as head cores in VTR and computer disk systems. These ferrite are produced by various methods, i, e., Bridgman's method, vacuum sintering, HP sintering, HIP sintering etc. Spinel ferrite has mechanical and magnetic anisotropies : the former affects head wear resistance and residual stress after precise machining, and the latter does signal out-put of head. Grain size of polycrystal ferrite also affects the scattering of head out-put and mechanical workability. A dense Mn-Zn polycrystal ferrite with grain size as small as 12 μm, having the properties of resistance to grain pull-out, chipping, and high permeability at high frequency, is produced. Furthermore, a Mn-Zn polycrystal ferrite will be desired to have small grain size, about 1 μm, high saturation flux density Bm ∼6600(G), and high permeability μ, ∼1000 at 10 MHz, to correspond with high recording density and high reliability of the magnetic head.

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A formation mechanism of a rectorite-type mixed-layer mineral from mica is proposed on the basis of experimental study. Dehydroxylated sericites were treated with various solutions containing salts specify. By the above procedure, 2M₁ sericite was easily changed into a regularly interstratified mineral of mica/smectite. A random mixed-layer mineral of mica/ smectite was formed from 1M sericite. In order to change the 2M₁ sericite into a regularly interstratified mineral, it should be heated up to the temperature range of dehydroxylation. The formation of a rectorite-type mixed-layer mineral from 2M₁ sericite may be explained on the orientations of the (OH) bonds after release of the potassium ions.

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We review the studies of spectral reflectances of meteorites and the mineral assemblages of asteroidal surfaces. We have measured spectral reflectances (0.25-2.5 μm) of mixtures of olivine and pyroxene separated from the ALH-769 ordinary chondrite (L6). The results were compared with those of olivine-pyroxene mixtures from the Hawaii olivine sand. We have examined the spectral behaviours of the mineral mixtures by calculation of the spectral reflectances of the mixtures assuming that the samples consist of multi-layer.

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