User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-77.194.131.213-20150119141826/@comment-63.135.255.62-20150119143306

Maybe Mirajane absorbed the demon particles inside Minerva's body. After all she said she used them for her demon take over and since Minerva is on friendly terms with Erza, it wouldn't be strange for her to ask Mira for a favor on Minerva's behalf.