Mary Purcell of Coffee county, Tennessee

Butting my head against a brick wall here.My ggrandmother was Mary B. Purcell who married Alford Smartt Sain.Their first child was born in 1887 so Mary's birthdate should be about 1860-1870.On the 1900 census for Coffee county, they say they have been married for 14 years, and that Mary's date of birth and age are unknown.Looking in the same district on the 1880 census I found a Mary Purcell living in the household of Eb. Purcell.Oddly, I cannot find Alford on the 1880 census at all.Would appreciate any information.