Hugh Garland, of Island Bay, asks :-

I collected some black sand from the beach and did some experiments with it, with bags and magnets. I noticed it was magnetic. Why doesn't magnetic sand rust?

Andy Edgar, a physicist at Victoria University of Wellington, responded.

The black magnetic sand that Hugh collected has already “rusted”, but it’s not the ordinary rust we are familiar with. The red-brown rust we see on bits of iron or steel exposed to the elements is an oxide of iron called ferric oxide (though often it contains some water as well). This oxide has two atoms of iron combined with three of oxygen. It is not magnetic in the usual sense. Rather, the black sand is most likely a rarer oxide of iron called magnetite which has three atoms of iron combined with four of oxygen.

Magnetite, as the name suggests, is magnetic, and was used in the very first magnetic ships’ compasses when it was known as “Lodestone”. It is readily attracted to a standard bar magnet just like iron filings, but is not itself a very strong magnet. Applications needing strong magnets such as electric motors or loudspeakers traditionally use an alloy (a mixture) of iron metal with cobalt or nickel. But in the last twenty years, new very powerful magnets have become available based on so-called “rare-earth” metals such as Neodymium and Samarium alloyed with iron or cobalt. These rare earth metals are not actually as rare as the name might suggest, being more common on the earth’s surface than gold, but they are now a major mining and economic resource for countries such as China and Australia because of their widespread use in many modern appliances. Your cellphone probably has a miniature loudspeaker which uses “rare-earth” metal magnets.

The sand at Oriental Parade was brought from Golden Bay in Nelson, and it is known that in some parts of Golden Bay the sand does contain some magnetite, so that is the probable origin. There used to be an ironworks at Onekaka near Takaka using local limonite ore (an iron-containing compound), lime from nearby Tarakohe, and coking coal from the West Coast. It closed down in 1935, but the old hydro-electricity station which powered it has been restored by a group of enthusiasts and is delivering power to the national grid.