A note on the micro-texture of the Arltunga Meteorite
Abstract
The Arltunga meteoritic iron is of interest because its micro-texture, as might be expected from its composition (10.22% Ni), illustrates a stage in the development of the Widmanstätten structure, which is intermediate between the incipient stage, as shown in the Tawallah Valley iron (16.90% Ni) (Hodge-Smith and Edwards, 1941), and the fully developed structure of the octahedrites proper (6%–8% Ni). Mawson (1934), who described the Arltunga iron, and classed it as an ataxite or micro-octahedrite, gained the impression at the magnification at which he worked (up to 500 diameters) that the fine octahedral structure of the iron arose from the alignment in octahedral fashion of "taenite rods ... embedded in a directionless kamacite nickel-iron alloy low in nickel content" (1934, p. 2, and description of Plate i, Fig. 4). Such a structure is inconsistent with the explanation of the origin of the Widmanstätten structure put forward to explain the micro-texture of the Tawallah Valley iron (Hodge-Smith and Edwards, 1941, p. 6), which called for the development of ex-solution bodies of a-nickel-iron (kamacite) in the octahedral directions of the original gamma-nickel-iron (taenite). In meteorites with a lower Ni content than the Tawallah Valley iron, the transformation to alpha-iron is more complete, and the ex-solution bodies develop into parallel plates, the residual high nickel gamma-iron being trapped as thin lamellae between the plates of alpha-iron. The structure of the Arltunga iron as interpreted by Mawson would be the reverse of this.
