As I’ve said before, I often hear about the interesting things my colleagues are doing that I want to share with you. The latest was the need to cut a piece off a too-heavy rock for display in our new Earth Gallery—by blasting it off.
Research Scientist and geochemist Scott Ercit, Ph.D., was the one for the job, having both the know-how and microblaster equipment to tackle it. As you’ll see in this video of the event, he’s a natural educator, helpfully explaining to his small audience of curious colleagues what he’s doing.
Perhaps you wondered why Scott started out by saying he wasn’t sure if the blast would work. That’s because the atomic bonds in the minerals that make up the rock are weak: he was concerned that the force of the blast would dissipate with a useless “pfft”, a possibility if there wasn’t enough resistance for the blast to push against. Fortunately, the atomic bonds were strong enough, and the rock fractured.
The force of the blast was generated by rapidly expanding gas. The red cartridges contain a compressed azide gas. When the metal firing pin strikes the small, brass, gunpowder-filled cap at the top of the cartridge, the spark that it makes ignites the gunpowder, which in turn ignites the azide gas, and then BANG the gas expands.
The rock is a chunk of copper-nickel ore from the Sudbury Basin, in Ontario, and its origin tells a dramatic story: a giant meteorite from outer space.
About 1.85 billion years ago, a 10 km-wide meteorite struck the area at eight times the speed of sound. The impact generated so much heat that the meteorite and that area of the Earth’s crust melted. The materials mixed, and cooled, and the deposit is now mined for its copper and nickel. Vale, the gallery’s original sponsor, mined our rock from this deposit. (And then they donated it to the museum for use in the gallery).
The copper in the rock is the mineral chalcopyrite, and it was likely present in the crust before the meteorite hit. The nickel is the mineral pentlandite, and it was probably provided by the meteorite. Until I started working at the museum, I hadn’t really put it together that metals are minerals, just like diamonds and salt.
It’s the metals that made the original chunk of rock too heavy to go into the gallery: it weighed about 1800 lb. (810 kg). As we were hoping, the blast broke off a piece that we can use—its 800 lb. (360 kg) weight is more than the gallery team had in mind, but a convenient girder in the gallery floor will support it.
Surprised that a rock of that size could weigh so much? It’s because metals are dense, and there is a large proportion of copper and nickel in the rock. Its density is 7 or 8, meaning that the rock is 7–8 times denser than water. (By comparison, granite has a density of a bit less than 3). While the original rock was in storage, its weight broke slats in the pallet.
Look for the copper-nickel ore specimen on display in the magmatic-rock section of the gallery. The new Earth Gallery opens on November 30, 2013, at the Canadian Museum of Nature.