Hello Sarenid,
Since I posted, also found a family tree with Mary Rankin listed. Goes under Calhoun family (username Houner)  Unfortunately the last log in was over 6 months ago but could be worth a try.
I found a Mary Rankin on the 
1870 Census St Louis Ward 9, St Louis, MissouriAnn Murphy/Rankin was aged 30 years on the 1850 Census but (apart from the age being out) the thought crosses the mind - could that be Ann remarried in 1870  

Charles Williamson 30 Born NY. Salesman in paperhouse.
Ann Williamson 42 Born England 
Fanny Rankin 16 Born Louisiana
Mary Rankin 15 Born Louisiana
Charles Williamson 18 Born Louisiana 
Regards
Sandra