All you have to do is take the list of spells for an elementalist and bend them to the element of your choice. For the purpose of example, let's use chlorine.
First of all, at least in Legend Quest, every elementalist can summon their element. This is fairly self explanatory. You cast the spell, and a small bit of your element appears. For Chlorine, that would cause a cloud of the gas to appear.
Secondly, there are elementals. In Legend Quest, each element has a certain creature established as its default elemental.Sylphs, Gnomes, Salamanders, and Nereids for Air, Earth, Fire, and Water (respectively). For Chlorine, this would be some form of diseased or rotten creature due to the poisonous qualities of Chlorine. I'm thinking a zombified sea creature of some kind. We can get back to that later.
Next, there are spells that are keyed specifically to each element. Fire mages shoot fire balls, earth users can summon walls of dirt, water casters can breathe underwater, etc. These would be a bit complex, but are easier than one would originally think. For Chlorine, there would likely be variations of necromantic spells. Acid splash, evil eye, paralyze, rot, and maybe life drain. Not too many spells, but enough that you won't cast the exact same thing every single fight.
So, if you want to do this with other elements, you can use the same basic model. For iron, you would summon the element, and might summon some kind of golem. For someone with a metal based element like that, there would likely be some kind of weapon and armor creation or modification. Stone skin and strength boosts would also make sense.
In conclusion, I'm totally putting these guys in my game world, and encourage you to do the same. If you want any brainstorming about different elements, feel free to comment. I'm sure that I and anyone else who reads the sight would love to help. Final note: chlorine should have an undead flying piranha.