The mind is almost always impure, and it almost always brings in unaspiring thoughts. Even when it is not doing this, the mind is still a victim to doubt, jealousy, hypocrisy, fear and other undivine qualities. All negative things first attack the mind. The mind may reject them for a minute, but again they knock at the mind's door. This is the nature of the mind. The heart is much, much purer. Affection, love, devotion, surrender and other divine qualities are already there in the heart. That is why the heart is much purer than the mind. Even if you have fear or jealousy in the heart, the good qualities of the heart will still come forward.
But again, the heart may not be totally pure because the vital being is near the heart. The lower vital, which is situated near the navel, tends to come up and touch the heart centre. It makes the heart impure by its influence and proximity. But at least the heart is not like the mind, which deliberately opens its door to impure ideas. The heart is far better than the mind. And best is the soul. The soul is all purity, light, bliss and divinity.