Optical Microscopy of Meteoritic Metal

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NWA6259 (Ni-rich Ataxite)

A mass of 1805 grams was found in northwest Africa in February of 2010.


This iron meteorite is of special interest because it contains ~42 wt.% Ni, one of the highest Ni contents of all irons. This sample is strongly magnetized, and will actually attract ferrous materials. The high Ni contents and magnetic properties suggest that tetrataenite is a major mineral, and this inference is supported by the strong optical anisotropy of the metal.



Photos by Mirko Graul, used with permission.




Apparent length versus orientation for intragranular needles (likely phosphides) in NWA6259, digitally measured from five photographs each collected at a different region of the sample.  The photos are color-coded (red dots from one photo, blue dots from a different photo of a new area, etc).  Angles were measured relative to the photograph's horizontal.  The apparent length of a needle depends on its orientation relative to the cut surface.  A needle cut perpendicular to its major axis appears as a point (zero length), whereas a needle cut parallel to its major axis has maximum length.  In spite of the apparent polycrystallinity of NWA6259, the intragranular needles tend to form clusters, with fairly well-defined gaps between the clusters.