This is a special post for quick takes by Thomas Kwa. Only they can create top-level comments. Comments here also appear on the Quick Takes page and All Posts page.
One of the obvious first problems is that pretty much every mountain and most of the hills in the world will experience increasingly frequent landslides as much of their structural strength is eaten, releasing huge plumes of dust that blot out the sun and stay in the atmosphere. Continental shelves collapse into the oceans, causing tsunamis and the oceans fill with the suspended nanobot dust. Biological photosynthesis pretty much ceases, and the mean surface temperature drops below freezing as most of the sunlight power is intercepted in the atmosphere and... (read more)
It's hard to imagine these constraints actually holding up well, nor the unstated constraint that the ability to make nanobots is limited to this one type.
My actual prediction depends a whole lot on timeframes - how fast do they replicate, how long to dust-ify all the feldspar. If it's slow enough (millenia), probably no real harm - the dust re-solidifies into something else, or gets into an equilibrium where it's settling and compressing as fast as the nanos can dustify it. Also, humans have plenty of time to adapt and engineer workarounds to any climate or other changes.
If they replicate fast, over the course of weeks, it's probably an extinction event for all of earth life. Dust shuts out the sun, all surface features are undermined and collapse, everything is dead and even the things that survive don't have enough of a cycle to continue very long.