How to differentiate magnetite from metallic iron?

Both metallic iron and magnetite, $\ce{Fe3O4}$, are magnetic. Now, iron is grey in color, whereas hematite is black, so that they can be distinguished visually. Now, I have a powdery mixture of iron/magnetite and lighter grains so that the mixture appears grey, making visual differentiating a bit tricky. I assumed it was iron, but someone else claimed it could also be magnetite, and now I'm not sure.

Is there a really easy way to be sure what I am dealing with? I don't want to do an XRD just for this.

• Hematite is red. – blacksmith37 Sep 19 '17 at 14:49
• oh my goodness, I meant magnetite! changing it now – Phanz Sep 19 '17 at 16:08
• Do the XRD. Unless you have to pay for it or something. – Karl Sep 19 '17 at 21:55

These are the methods that come to mind:

1. LIBS
2. XRF
3. Streak color. When scratching on unglazed porcelain, magnetite leaves charcoal-black traces, whereas iron leaves shiny steel-gray streaks. Having a powdery mixture this test can be tricky, unless there are reasonably big shards left. Alternatively, one can try to assemble a bulky piece by gluing the powder together with silicate adhesive or cyanoacrylate (as paracetamol suggested in the comments, "rubbing some of the mysterious powder between two shards of porcelain" can also do).
4. Compare density of your sample with $\rho(\ce{Fe}) = \pu{7.874 g cm-3}$, $\rho(\ce{Fe3O4}) = \pu{4.9 .. 5.2 g cm-3}$ and, assuming this mechanical mixture consists of iron and magnetite exclusively, calculate the percentage of each phase.

Magnetite in its mineral form often contains $\ce{(Fe,Mg)O}$, $\ce{TiO2}$ and isomorphous impurities of $\ce{Mn}$, rarely $\ce{Cr}$, $\ce{V}$; therefore physical methods of analysis (1, 2) are more preferable. Due to the same reason I'm not suggesting treatment with $\ce{HCl}$ and other methods based on chemical transformations as primary for this analysis.

• "Alternatively, one can try to assemble a bulky piece by gluing the powder together with silicate adhesive or cyanoacrylate." That's a very novel method :D – paracetamol Sep 19 '17 at 13:15
• @paracetamol Well, when means are limited, one must improvise. Colorless adhesive that doesn't change composition of phases shouldn't influence the outcome, and if a single phase is dominating, then it will become clear quite fast. But again, it depends on the grain size -- maybe this gluing thing isn't necessary. – andselisk Sep 19 '17 at 13:18
• How about rubbing some of the mysterious powder between two shards of porcelain? A little messier, maybe... – paracetamol Sep 19 '17 at 13:20
• @paracetamol Actually, this is a very good point, thank you! I added this idea to the answer. With the hardness of hematite this should work even with the fine fraction. – andselisk Sep 19 '17 at 13:21
• @paracetamol Thank you very much, I edited accordingly. I noticed that I already used wrong formula for magnetite, so no big deal. I also screwed up:( (free flags!) – andselisk Sep 19 '17 at 16:49