How to tell an atheist mother I'm Wiccan?

I don't really see much of a conflict here, she should be ok with it. Atheists have pretty much worked out all the kinks in their personality, in my opinion anyway. So you should be entitled to the same search for spirituality, or lack there-of, that she has been on in her own life. I mean people don't become Atheist without some level of spiritual quest.