How do you personally define what is pure in heart and what is not? Do you consider innocence the same as purity and have you ever met anyone that declared themselves 'pure'?
"No one is more dangerous than one who imagines himself pure in heart; for his purity. by definition is unassailable"
-- James Baldwin (1924 - 1987) Notes of a native son, 1955
I think someone that is pure in heart has pure intentions when he does things. Even in the small things we have selfish reasons. Stopping to help someone, not because they need help, but because we are trying to earn `good` points or want others to see us, or because we hope that someone will help us if we help someone else. Those that are pure of heart have intentions that are not selfish in anyway. Their desires are not selfish or harmful towards anyone. I don't know how many people could be considered pure of heart today.
Innocence is different because that is a case of not knowing what evil is. Not knowing what wrong doing is only happens in children. When you get older, that attitude is considered naive and not innocence. Just my opinion, this subject is far from having an absolute answer.