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human
[The Legend of Korra][HEAVY SPOILERS] What are the metals that metalbenders are capable of bending?

It's a given that metalbenders can bend pretty much any composition of steel but can't bend platinum because metalbenders rely on sensing the tiny fragments of earth inside smelted metals(i.e. the charcoal/carbon inside steel) and thus cannot bend pure metals like Platinum. But in saving Avatar Korra, Suyin Beifong bends the liquid metal poison which looks like Mercury and to my knowledge, Mercury is a pure metal containing no fragments of earth. 

How then was Suyin able to bend Mercury in spite of it being a pure metal and as an extension to that, what are the metals that metalbenders are capable of bending since not all of them contain fragments of earth that can be sensed by metalbenders(copper wiring for example)?
gpt
i always assumed that it was just harder to bend the farther it gets from a natural state so the better you were the more refined metal you could bend.

mercury bonds with a number of natural elements including sulfur and selenium which could qualify as "earth" but you must understand that at a chemical level there is no such thing as "earth" so what earthbending can and can't affect is a function of what the magic can identify as "earth" and maybe if you know mercury is metal and metal is earth you can trick the magic to do it just that once.