MadSci Network: Earth Sciences

Re: black sand why does it not go rusty from the salt sea water

Date: Thu Nov 4 19:34:11 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Earth Sciences
ID: 1099537823.Es

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 
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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