What is this black ionic compound?

[Justonium]

I was heating magnesium metal and suddenly became really deformed looking and turned black. When I continued to heat this, it then caught fire and left white magnesium oxide, with a little of the black stuff in the center. It can't be mangesium nitride, I tried burning magnesium in pure nitrogen and the resulting powder was yellow. I really have no idea what else magnesium could do, unless there is another metal alloyed with it? It was labeled as lightweight magnesium, but it could have had something else added to improve its strength or other properties. If the formation of this black substance indicates another metal present, any ideas on what this metal is? And when I heated it to ignition, why did almost all of the black stuff turn white like the magnesium oxide, or if it is a magnesium compound, oxidize to magnesium oxide?

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I burned magnesium just now in air but with a limited supply and the result was a mixture of black and white powder, but some nitride must have been formed because the mixture smelled strongly of ammonia. No yellow was visible, but before, when I burned magnesium in completely pure nitrogen gas, the resulting substance was dark yellow.

[UC]

Have you considered carbon powder from the reduction of CO2? This is especially relevant with a flame of some kind because the Mg will be bathed in CO2 as it reaches the ignition temperature.

 

Try adding some of the material to HCl. If the black does not dissolve, there's your answer.

[Justonium]

You were right on, it's carbon! Thanks a bunch!

All of the magnesium I used was lit with a bunsen burner, and all of the blackening occured WHILE IT WAS UNDER THE FLAME. When it was allowed to burn alone, the reason most of the black stuff vanished is probably because the high temperature created by the burning magnesium burned it. The reason black only remained in the center is probably because it has less oxygen to burn there, and magnesium takes priority over carbon to grab oxygen.

I don't know why I didn't consider it before; I threw away the idea because the air is only less than half a percent carbon dioxide, not even considering the gasses from my burner.

Edited June 15, 2009 by Justonium