how to sequestor carbon?

Well if we had to trap it using chemical weathering (the most stable way) it would probably weigh a lot more

Using this ideal case, one ton of serpentine can dispose of approximately one-half ton of CO2. Reaction 3 illustrates the transformation of forsterite, which is the end member of the common silicate mineral olivine. One ton of olivine can dispose of approximately two-thirds of a ton of CO2. Again, the reaction is exothermic and releases 90 KJ/mole of CO2

and there wouldn't be enough raw material available to do it....

Vast Capacity - Raw materials for binding the CO2 exist in vast quantities across the 

globe.  Readily accessible deposits exist in quantities that far exceed even the most 

optimistic estimate of coal reserves (~10,000 × 109 tons) (5). 

 (http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/6c1.pdf). 

not to mention mining all that rock would chop up the environment fairly badly (on the scale that coal is now)