An X-Ray Investigation of Oxidation and Reduction of Magnetite

Magnetite, when oxidized, becomes hematite of a different crystal structure, and when reduced, it becomes at first FeO and then Fe. In the present experiment, the orientations of these products as regard to the, parent Magnetite were studied by means of X-rays. Natural martite exists in an octahedral crystal form, the composition of which is chiefly Fe2O3. It was found that the orientation of the crystal of Fe2O3 contained in this substance is similar to that of magnetite heated in air. That is, the plane (111) of Fe2O3 is parallel to the plane (111) of the parent magnetite and the direction [112] in this plane is also parallel to the direction [110] of magnetite. Therefore, it may be concluded that natural martite is a product of oxidized magnetite. In the mechanism of oxidation which was formerly proposed by the author a few years ago, the plane (111) was found to be parallel to each other, but the direction [112] of hematite was not parallel to [110] of magnetite. Hence in the present communication, the former mechanism has been partly revised. The orientation of the crystals of FeO as obtained by the reduction of magnetite agrees with that of the parent magnetite; so it may be concluded that the mechanism of this change is the same as that which is reverse to the case of oxidation. Three kinds of the orientations of iron have been obtained from the reduction of FeO: the first agrees with the parent substance; the second is such that the plane (100) of Fe is parallel to the plane (100), and the direction [011] of Fe is also, parallel to [010] of the parent crystal. The last is that the plane (110) of Fe is parallel to the plane (111) of the original FeO and the direction [011] of FeO shows a rotation of 9° against the direction [211] of FeO in the same plane. From the above results the following mechanisms may be given: the first kind is produced by the contraction in the direction [111] and by the expansion in the directions perpendicular to each other in the plane (111), just as in the case of a solid body undergoing a compression. The second kind of orientations is explained just in the same way as the mechanism proposed by E. C. Bain and K. Honda and S. Sekito in the Ar3 transformation of steel. As for the third, it may be explained by the similar mechanisms to those proposed by G. Kurdjumow, G. Sachs and Z. Nishiyama for that transformation. That is, it is obtained by rotating as a whole the second kind of orientation by 9°44' about its axis of [100], and then by 9° about the axis of [011]. Hence, the mechanism of the lattice change of the second and third kinds of orientation, agrees with each other, if the rotation as a whole is not taken into account. In any of the three-kinds of orientation in FeO, those planes which are of the greatest atomic density for iron, becomes the planes (111) or (011) of Fe by the least relative displacement of atoms.