Wasn't looking for it, because it never crossed my mind that such documentation would exist. After all, Great-grandma Addie (More) Powers was born in Lapeer County, Michigan on 28 June 1864. Why would she need to file a Petition For Naturalization in the United States of America? However, such a petition is exactly what I found while looking through a Naturalization Index at the Archives of Michigan in Lansing. I did expect to find a naturalization record for Addie's husband, William S. Powers, who was born in Grimsby, Ontario, Canada on 27 August 1859. Therein lies the clue as to why Addie needed to be naturalized...
This site provided the answer: "An act passed in 1907 stated that a woman’s nationality depended entirely on her husband’s. This meant that if her husband gained citizenship, so did she. It also meant that if a woman who had been born a US citizen married an alien, she lost her citizenship. She could repatriate only if and when her husband naturalized." Addie married an alien - that's why.
Along with her husband, William S. Powers, Addie's children, Ward, Ralph (my grandfather), Mae, June and Marion, are also named on the Petition For Naturalization document (date of birth and current city and/or state of residence also given).
I was glad to find it -- the more documentation, the merrier!