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Taking the carbon from the atmosphere will always be more expensive than from some carbon-rich rocks that form a huge fraction of the Earth's crust.

In fact, our most effective ways to take carbon out of the atmosphere today all involve a step of letting some mineral turn into one of this carbon-rich ones, and extracting it from there.

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It'll always be more expensive to extract carbon from the atmosphere, but if it produces a valuable byproduct and the power to run the process is very cheap (excess solar) it might still pencil out.

I fail to see anything that doesn't apply equally to the rocks.

When you have surplus photovoltaic capacity — an increasingly common situation — you could turn it to electrolysis. You still have to pay for the equipment, but it sounds like it could be pretty cheap.

It would cut out the cost of mining, and possibly a lot of transportation costs, if you can use the products nearby.

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