We’re not excited about Peridotite?

Io9 did a post yesterday about peridotite and its ability to soak up carbon dioxide, and mentioned in the first paragraph that no one’s talking about it. And another article that says we’re not excited about it.

I imagine a lot of people haven’t even heard of peridotite, or don’t know what ultramafic rocks are, which is fair enough. Most people aren’t geologists, and have a hard time getting excited about rocks. I actually hadn’t heard of looking at ultramafic rocks for carbon sequestration until I took introduction to Geochemistry last year. After that, yes, I thought it was a pretty exciting concept.

Now, the reason we were talking about this in geochemistry is that the carbon sequestration comes down to a very basic chemical reaction that occurs every day – the chemical weathering of rocks. Most rocks in our lives are some form of silicate; their chemical formula is SiO₂ plus some other junk, and the crystalline structure is usually the silica tetrahedra arranged in different ways around the other junk. Most chemical weathering of these silicates comes from CO₂ dissolving in rain water to make carbonic acid, H₂CO₃. Rain is actually naturally a little acidic, since it’s made up of water plus a little carbonic acid. It falls, runs over rocks, and then you end up with something like this:

Mg₂SiO₄ + 4CO₂ + 4H₂O ⇌ 2Mg²⁺ + 4HCO₃^– + H₄SiO₄

Where the water and carbon dioxide are what make up the carbonic acid. In this particular equation, the rock in question is olivine, the main constituent of peridotite. So basically, it’s:

Olivine + water + carbon dioxide ⇌ magnesium ions + bicarbonate + silicic acid

So chemically, you can use this kind of reaction to get CO₂ out of the air. And peridotite is certainly a good candidate for this kind of reaction. Olivine has a mineral structure that’s basically individual silica tetrahedra jumbled together; it’s not really stable at surface conditions, and it’s easy for the tetrahedra to get picked off by whatever happens to come by. That’s why olivine weathers away much faster than something like quartz, which has a very organized framework and doesn’t allow a lot of room for party crashers. Once you’ve got the olivine broken down via this process, then you can separate out the ions and acid. The magnesium, you could make in to salts, or perhaps there’s a good industrial use for it. The bicarbonate just needs some calcium, and then you end up with limestone, which is the end result we want for getting the carbon chemically locked away. The silicic acid could be precipitated in to amorphous silicate if nothing else.

Honestly, I can’t say why people aren’t excited about this possible solution to getting carbon out of the air. It’s got its problems that need to be figured out for sure, though not necessarily more than any other proposed sequestration method. Off the top of my head:

The reaction is normally extremely slow, as noted in the articles, so you do have to find a way to speed it up. And in so doing, a way to speed it up that doesn’t involve producing more carbon via energy usage than what you’re taking out of the atmosphere.

Once you’ve got your schmutzed-up former olivine, you still have to put it somewhere. One suggestion my geochemistry teacher had was to just toss it in to old mines, which isn’t really that bad of an idea. But there’s still a question about hauling tons and tons of rock anywhere, to be honest.

You’d need to have a good, energy efficient way to get carbon out of the atmosphere and then dissolved in to your water.

And I’m sure there are more questions than that. But I also don’t think these are more difficult questions than the ones that come with any proposed carbon sequestration scheme. It even has its advantages; once your carbon is chemically locked in to limestone and you toss that limestone down an old mine, you don’t really have to worry about it again. The dissolution of limestone does release the carbon, but you’re not going to have to worry about that until millions of years in the future, when there’s been some uplift and the contents of the old mine are exposed to weathering. I’d say that’s easier to deal with than figure out how to keep CO₂ in gas form from escaping a reservoir you’ve injected it in to.

Most people I’ve explained this to have thought it was actually a very exciting idea, if one that’s so far just on paper. The big thing is that very few people have even heard about it, as is pointed out in the articles I’ve linked to. Maybe it’s because it’s difficult to get most media excited about talking rocks, unless we’re talking molten rocks that are poised to destroy a town, and then they’re all over it. Of course, one might argue that it’s more important to pump money in to research on finding energy sources that aren’t going to produce so much carbon dioxide. Fair enough, but until we get there it really wouldn’t hurt to figure out how to stuff at least some of that excess CO₂ back under the global couch cushions, so to speak. Or I suppose there are some that might say that none of this is a matter of concern, but I think I’ve already established that I wouldn’t want to sit next to them on the bus anyway.