|MadSci Network: Earth Sciences|
Congratulations, you have already correctly identified your black powder. It is magnetite.
Most magnetite doesn't have a strong enough magnetic field to be attracted to paper clips, but it will be attracted to a magnet. Some magnetite does have a strong magnetic field. If its magnetism is strong enough to attract metal it is called lodestone. Coal dust would not be attracted by a magnet.
Magnetite won't rust (oxidize) because it is already iron oxide. Its chemical formula is Fe3O4, with its basic crystal "unit cell" formed of 3 iron and 4 oxygen atoms. Not all iron oxide is red. The color of a mineral is the result of its crystal structure. Iron rusting on the Earth's surface produces a red iron oxide, but magnetite is formed deep in the crust at higher temperature and pressure than exist at the surface of the Earth, so it has a different crystal structure and a different color (black). We find magnetite on the surface after it weathers out of igneous and metamorphic rocks. Sometimes it can be found as black sand grains in sedimentary rocks too.
As for the best way to identify rock dust in general, there are many laboratories that identify samples of unknown dust, sand or powder. These can be identified by various chemical tests, or more commonly today, by x-ray diffraction and fluorescence.
You used a good approach to solving the problem of your unknown substance. You came up with some possible answers, then did more tests to try and narrow down the possibilities. You are using the scientific method, just like a research scientist would do. Keep up the good work.
---- Sean Sherlock, Geologist Morgantown, West Virginia, USA
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