Purification means removing the coverings and coatings of ignorance so that one may find the true soul hidden inside. It is like extracting the grain of gold (soul) from the surrounding dust (lower nature) by washing it with streams of devotion and rain of Grace plus putting it through the fire of aspiration to burn down the impurities.
Transformation is turning the entire nature into gold (soul stuff and eventually divine nature). It is done by opening and exposing our present human nature to the alchemist Energy of the Divine Mother.
Affectionately,
Alok Da