The following letter combinations share an unusual property. Can you determine what it is?

DMNT
LMNT
NTAL
DRUS
NSAL

HINT:
It has to do with vowels.

Answer:

Each group of four letters becomes a seven-letter word when three of the same vowels - in respective A, E, I, O, U order -  are added as the first, third and fifth letters:

AdAmAnt   (ADAMANT)
ElEmEnt     (ELEMENT)
InItIal          (INITIAL)
OdOrOus    (ODOROUS)
UnUsUal      (UNUSUAL)