Does anyone have further information on the Robert M. Grinnell who was a 1st Lt. in the 1st Special Battalion LA. Inf.(Wheat's)?

He was mustered into service June 26 1861 in Louisiana, captured Front Royal, VA May 29 1862, and exchanged at Aikens Landing, Va August 5 1862.

He is probably the same Robert M. Minturn who became a Major in Heth's Division.

I am trying to disover whether he was a member of the Grinnell family of New York City. This family were Union supporters and all the other sons fought for the Union.
However son, Robert Minturn Grinnell, born 1829, is missing from all family records for the period 1860-1873.

Any further information, or suggestions most gratefully received.

Sarah Batchelor