My ancestor Patrick Fountain Alexander was born 24 Dec 1826 in Blount Co, AL. I don't have his first wife's name in the partial records I have with me right now.One of his children from the first marriage was Martha J. Alexander according to the 1880 Census of Smith Co, TX. That Martha was 28 at the time so she would have been born c. 1855.That doesn't really fit with the dates you have given - do you have any more information about your Alexanders?My Alexanders were in Smith Co, TX & Cherokee Co, TX which is just south of Smith Co.
I haven't researched the children of PF Alexander's first wife since I descend from PF & his second wife Martha C. McKinley (she married first RR Walker).
I don't know if any of this helps but I will try to dig up my Alexander file and see if I come across anyone else who may be your Martha.Let me know what other information you have that I might be able to correlate.