Iron - Carbonyl Iron

I am borderline anemic, so my doctor told me to start taking an iron supplement. I purchased a supplement and the ingredients indicates "Iron (as Carbonyl iron)". When I looked up Carbonyl Iron online, there we're mixed results about whether or not this is safe to take while breast-feeding, as it says it passes through breastmilk. Am I taking the wrong type of iron? Anybody know anything about this?