Why is denial of the primacy of the bishop of Rome considered a Schism, not a Heresy?

Why is denial of the primacy of the bishop of Rome considered a Schism, not a Heresy? **Disclaimer**: I'm interesting only in authoritative answer according to the Roman Catholic Doctrine. The Papal primacy is essentially dogma of Roman Catholic Church. So why are those who deny this dogma (say, Orthodox Church officials) not formally considered as *heretics*?