Now I am curious why 3M was using monazite specifically as a tape dispenser ballast.

So it is sand encased in resin. My initial thought was it is heavier(denser) than plain silica sand, And while it probably is, It feels weird that 3m specifically searched it out to use for that reason alone, I bet they had quite a bit of the stuff on hand for other products, and so might as well use the waste as a tape dispenser ballast.

A tangent on heavy rock ballast, I once saw a documentary on an offshore oil platform, and it was towed to the site, sunk and then filled with rocks to anchor it. the rocks used were specifically iron ore as that is significantly denser than most rocks.