1853 R. Hunt Man. Photography 55That peculiar intermediate oxide to which the name of Ferroso-ferric has been given by Berzelius. 
1870 J. T. Sprague in Eng. Mech. 11 Mar. 621/3A natural substance, Ferrosoferric Oxide Fe3 O4..known as the loadstone. 
ferroso-
combining form 
Etymology: New Latin ferrosus ferrous — more at ferrous 
 : ferrous and 
< ferrosoferric >
< ferrosoferric >
ferroso-
Prefix
- chemistry ferrous