Dr. Schuiling’s idea is one of several intended to reduce levels of CO2, the main greenhouse gas, so the atmosphere will trap less heat.
No, carbon dioxide is not the main "greenhouse" gas -- that would be water vapor by an huge margin. Carbon dioxide has increased, allegedly, from about 280 parts per million to around 400. It is projected to increase to around 600 PPM. That is still a miniscule amount, though some people seem to think it might contribute to acidification of the oceans.
The amount of CO2 in the atmosphere has been elevated above the 280 PPM level in the past due to natural events. In fact, increased solar activity and heating may cause CO2 to increase as there is some evidence that atmospheric carbon dioxide may lag warming rather than cause it.
While something like olivine seems relatively benign in terms of modifying the environment, we really do not know how the environment adapts and balances itself out. How do we know that our human activity is not part of that balancing process?
For someone like me who believes in a Creator and human free will, it makes sense to talk about stewardship and being responsible for our actions. I'm pretty sure that switching to liquid fluoride thorium reactors would be far more beneficial to humanity, the global economy, and the environment than using olivine in sandboxes.