TRANSMISSION ELECTRON MICROSCOPY (TEM) INVESTIGATIONS REVEAL MICROTEXTURE AND NANOTEXTURE COMPLEXITY IN NATIVE COPPER FROM NORTHERN MICHIGAN'S KEWEENAW PENINSULA
Imaging indicates a variety of complex polycrystalline textures prevalent at the scale of tens of nanometers or smaller We observe a texture of slightly irregular circles or ovals commonly 3 to 20 nm in diameter but ranging as large as 150 nm in diameter. These circles or ovals appear to be enriched in Cu relative to the matrix, which has more Al, and occasionally displaylinear features which are not congruent to the Al-enriched matrix. Selected area electron diffraction (SAED) indicates diffraction rings with d-spacings of approximately 0.25, 0.21 and 0.14 nm. Spot EDS analysis reveals the presence of Au, Ag, Al, and Zr in apparent solid solution. Discrete Mn-rich minerals are also present. Preliminary SAED shows the Mn-rich crystals have d-spacings of approximately 0.22 nm and have orthogonal – or nearly orthogonal – nets. STEM investigation at magnifications between 800,000x and 1,200,000x establishes that Al is evenly distributed spatially. The Cu is less evenly distributed and occurs in localized concentrations, particularly in the circular and oval nanotextures. The presence of large amounts of Al signifies that there may be unrecognized phases in native copper. This investigation demonstrates that this native copper sample is a complex nanostructured material and not simply elemental copper with solid solution.
Although more than 340 TEM papers have been published on synthetic copper alloys, to our knowledge this is the first TEM study on native copper. Our overall preliminary investigation indicates that native copper from this area is much more complex than previously perceived and serves as a foundation for TEM investigation of other native copper samples and perhaps native metals globally.