Counterions like magnesium are quite abundant. After all, this is how CO2 will be drawn down if we do nothing (but it will take hundreds of thousands of years.)
There has been much work on exploiting the more reactive silicates, like olivine, to fix CO2. The volumes of rock involved are large, but there's a lot of silicates out there.
Weathering rocks to absorb carbon makes a lot of sense. It doesn't require any new technology. The equipment can be powered by electricity and only run when there is excess power available. It should scale well, but does need large scale to make a difference.
It also can make sense to add limestone (or other carbonates) to the ocean. As long as you don't make the ocean too alkaline, you get the following net reaction:
There has been much work on exploiting the more reactive silicates, like olivine, to fix CO2. The volumes of rock involved are large, but there's a lot of silicates out there.