This image was taken at Fort Pickering, Memphis, Tennessee. The officer appears to be a major of artillery. Majors are uncommon since captain was usually the highest available in light artillery volunteer units. There were U.S. colored troops heavy artillery units stationed at Fort Pickering. Unlike light artillery heavy artillery units had a regimental structure and would have had officers holding the rank of major. It is possible this man may be an officer of one of those heavy artillery units despite his wearing shoulder knots, which the regulations state were for light artillery officers. Period images often show that regulations dress distinctions between foot (heavy) and light artillery were not closely followed.
The branch insignia is worn on the leading edge of the top of the kepi. It is possible that the regimental number was worn in a circle at the intersection of the cannon tubes. The crossed cannon insignia is embroidered on a velvet oval with a Jaceron wire border. Insignia might be worn on the top of kepi or in other cases on its front at the wim of the wearer or his tailor.
The uniform coat has a fairly high collar, typical of the period. Standing collars became lower was the 19th Century progressed. It is double breasted, which tells us the wearer is a field officer. The shoulder knot does not appear to have any grade devices on it, which is consistent with that of a major. Second lieutenants wore knots of three cords without grade devices while majors wore ones of four cords.
The Hurlburt/Rawlins Collection is interested in estalishing the identity of the above unidentifed officer. If you are able to do so please contact them..