|MadSci Network: Earth Sciences|
G'day Lesley. The question you have asked is a really interesting one, and I like the way it arose. You saw something unusual, you investigated it with some tests, and you noticed an anomaly -- something that did not really fit in properly. That is all good science. The answer to your little problem is that steel is not the only thing that is attracted to a magnet -- just the most common one. The property of being attracted strongly to a magnet is known as ferromagnetism. The following metallic elements (as simple substances) are ferromagnetic: Iron, cobalt, nickel, and gadolinium. Many metallic alloys are ferromagnetic -- most steels, but not all; some copper/manganese alloys (interesting because neither copper nor manganese is itself ferromagnetic); certain alloys of rare earth elements. Certain salts and oxides of iron, cobalt, and nickel can be ferromagnetic. When iron rusts at ordinary temperatures, it forms red iron oxide, Fe2O3, the familiar form of rust. Red iron oxide forms a flaky, powdery coating on the underlying metal, which allows air and salt water to get at the underlying metal, and continue the rusting process. At higher temperatures, though, iron forms one of two different black oxides: Fe3O4, or, at really high temperatures, FeO. Fe3O4 is ferromagnetic. Either of these black oxides can adhere well to underlying metal and form a protective barrier to further rusting. So, too, can brown or black iron phosphate, but it is not easy to imagine how this might form over metallic iron in the natural environment. It is sometimes used in rust-proofing treatments for cars. I cannot be at all sure, but I suspect that your magnetic black sand is either the mineral magnetite, Fe3O4, or it is particles of iron coated with Fe3O4 and/or FeO. When I was about 10 or 11 I discovered that some black "sand" in our yard at home was magnetic. I put it in my rock collection and called it "magnetite". When I was a little older and wiser, I realised that it was just iron, which may or may not have been oxide coated; the previous occupant of the house where we were living had been an engineering contractor who did a lot of welding! It had not gone rusty, but then we were not living near the sea, and that part of Victoria is a fairly dry place by New Zealand standards!
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