Hi Laine, what a wonderful photo...judging by the hair and dress I would guess at about early to mid 1880s...the hairstyle still has a severe center part, but the fussy 1870s look doesn't seem to be there...and the puffy shoulder seam of 1889 isn't there either. So maybe 1883-1885 or so. Just my opinion, you may get others.
You're right about the dating of photography...the first photo was 1829 and the exposure took several hours, and commercial photography didn't come about until ten years later and was mostly on glass. The quality of this photo is so good it has to be much later. Could you scan the entire photo including the mount, including the corners, and if there is a design on the back that would be helpful in dating as well. Lovely photo!
Cheers,
China