Hmm, are you sure? The scale at bottom right is 200nm, I didn't think chromium oxide was available in that small of a particle size.
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I don't know the particulars of that sample but the individual xtls are one thing. Often times the actual media is made up of xtls that are fused together to make the actual media so that clump you see could be one "particle of media or a part of one particle" As you hone or strop those particle break down.