Mordant Earth vs Black Iron Oxide

How close are they?

ΔE 19.8, loose: different enough that you should pick on purpose. Black Iron Oxide is noticeably darker, much more muted than Mordant Earth.

Can I use Black Iron Oxide instead of Mordant Earth?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Black Iron Oxide is noticeably darker, much more muted than Mordant Earth.

Can I use Mordant Earth instead of Black Iron Oxide?

The same difference applies the other way round: Mordant Earth is noticeably lighter, much more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.