John --
If this man died during the Atlanta Campaign, it's highly unlikely that a permanent marker exists for him. Because the armies were involved in almost daily marching and fighting, there was little opportunity to give the dead a proper burial. Also, Federals occupied ground given up by Confederates as the Army of Tennessee retreated towards Atlanta. That means that Federals buried many Confederates whose bodies were left behind, so no identification was possible.
During June 1864, the 12th Mississippi Cavalry was assigned to Brig. Gen. Samuel W. Ferguson's Brigade. This command screened the left flank of the army near Marietta, so the soldier in question may be buried somewhere in the Powder Springs-Marietta area. Most Confederates were laid in shallow graves that may have been marked by a narrow pine slat at one time.
There may be further clues about location in this man's service file. Do you have that as well as the widow's pension application?