ferr(o)- Also ferri-.
[Latin ferrum, iron.]
The more general of the prefixes is ferro-: ferroconcrete, concrete reinforced with steel; ferrocene, a crystalline compound containing iron between two flat layers of atoms; the adjective ferruginous describes a substance containing iron oxides or rust. The usual adjective is ferrous (but see below).
A few metals can be strongly and permanently magnetized; because this was first observed in iron, it is called ferromagnetism, though it is also shown by nickel and cobalt and their alloys.
In chemistry, ferro- refers specifically to iron compounds in which the iron is divalent, as in ferrocyanide, for which the adjective is ferrous; ferri- is used for chemical compounds that contain iron in its trivalent state, as in potassium ferricyanide, and here the adjective is ferric.