Mercurial Longsword (400gp), 1d10, x3 crit weighs 6 lbs.
Mercurial Greatsword (600gp), 2d8, x4 crit, weighs 17 lbs.
I know that the Greatsword has been errated somewhere, but I don't remember what it went to...
I haven't seen a mercurial scythe anywhere, and I don't think it would work anyway.
The Mercury is inside the sword, and it travels to the end of the sword during the swing to add more weight to the end, giving the sword more momentum.