One of my classmates is writing her blog about rocks. That made me remember the geology paper I had as an undergrad. Friends and I kept saying the word ‘stone’ instead of rocks. That made our teacher pretty upset more than once. I can still hear her saying: “Girls, those are not simple stones. They are rocks. Minerals, …”. And a few minutes after, I was back talking about ‘stone’. I have to say I have never been really interested in them. I like animals, plants, and interactions between them. I am particularly interested in their evolution to adapt to our changing world. But what do rocks do? They are just there. Static. Right?
False!
Once again I was wrong. Rocks are not that boring. They actually do stuff. Today I am going to talk about one of the things some rocks do. Well, don’t be too excited. You won’t actually see anything.
Let’s have a closer look at a specific type of rock, the peridotite. This rock is mostly made of two minerals, olivine and pyroxene. But what are minerals? Briefly: Minerals are substances naturally occurring, solid at room temperature and with an organised structure. Another characteristic of minerals is that they have a specific chemical formula (e.g.: olivine is (Mg, Fe)2SiO4) contrary to rocks. Rocks are made of those minerals and other non-minerals constituents and can’t be characterized by a specific chemical formula.
Minerals halite (pink) with nahcolite.
Credit: Jolyon Ralph/Flickr
Back to our peridotite. What’s special with this rock? Like I told you before, the peridotite is mainly composed of olivine and pyroxene. What is actually interesting is that these two minerals are reacting with CO2 from the atmosphere. Yes, you read it! There is a rock able to capture CO2 and transform this gas into stable minerals, such as calcite (yes, I already talked about calcite).
How is that possible? Chemistry!
There is a reaction between the rock’s components and the atmospheric CO2. This mineral carbonation (name of the reaction) occurs between CO2 and alkaline-earth oxides. These alkaline-earth oxides are simply the elements (metals) from the second columns of the periodic table. Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). They are characterized by their solid state even at very high temperatures (700°C to 1290°C).
The reaction transforms the gas into minerals like magnesite and calcite.
Equations of Carbonation reaction.
Credit: Kelemen and Matter, 2008
Periodic table made of biscuits.
Yeah, it seems much easier to learn it by eating it, don’t you think?
Credit: Prato Fundo/Flickr
Of course, that’s not all, there is also conditions of temperature and pressure needed for the carbonation to take place. But I won’t go further into those parameters today. But be aware that they are also very important for the mineral carbonation to take place.
Where can we find this precious rock?
The peridotite is the main constituent of the Earth’s upper mantle (see figure below). That means it is all but unreachable. However some is actually exposed in some locations. You can find them in Oman (Southwest Asia), Papua New Guinea, New Caledonia and along the coast of the Adriatic Sea.
Credit: Manon Knapen, based on My Science Box
With all the problems about the increase of CO2‘s concentration in the atmosphere, it is good to know that we get some help from the rocks. So, what have we learnt here? Rocks can do things! And the peridotite actually absorbs and mineralizes the CO2 atmospheric.
Knowing that, different companies are studying the cost of injecting CO2 in rocks. But that’s not a technology. That’s us, once again, wanting more from what Nature has to offer.
Where is the real technology? In Calera. Fonded in 2007, this company is actually using atmospheric CO2 and the process of carbonation to produce calcium carbonate. And finally they use it to make cement! Isn’t that great?
Even with this good news, be careful. Even if rocks can help absorb CO2 and some companies can succeed in doing the same, we still have the to limit our personal CO2 production. There are lots of simple things you can follow to reduce your carbon and you overall global ecological footprint!
Credit: Calera
What have we learned? 