"Adamantite" is an AD&D-era name for adamantine
"Adamantite" is the name given to that metal in AD&D, while D&D 3e onward standardly used "adamantine".
"Adamantite" appears in the AD&D 1e Player's Handbook (1978) and Dungeon Master's Guide (1979), where it refers to a metal used to make exceptionally good magical swords and armour. It also appears in the adventure module S1 Tomb of Horrors, also written by Gygax, where a "1-foot-thick adamantite" door blocks progress (p.7), and likewise in S1-4 Realm of Horror (1987).
We see "adamantite" in AD&D 2e, such as the Planescape Planes of Conflict (1995), where adamantite dragons are said to inhabit Bytopia (Liber Benevolentiae, p.43).
However, by D&D 3e, we see "adamantine" replace "adamantite" to describe the same things. As JohnP noted, Daern's instant fortress is now made of "adamantine", where in AD&D 1e its material was called "adamantite". Bytopia is now inhabited by the "adamantine dragon" (Dragon #321, p. 44), and the Tomb of Horrors (3e web conversion) says the doors used to be made of adamantine (p.25):
The 1-foot-thick steel door (it's too expensive for the demons to keep replacing adamantine doors) is suffused with a globe of invulnerability effect.
Drow connection to "adamantite"
D3 Vault of the Drow features adamantite, a metal which the drow can craft (p.13):
These creatures dwell in a pool of molten lava at the far end of their smithy, and they make the adamantite alloy and draw the wire.
And:
Drow wear a fine mesh armor of exquisite workmanship. It is an alloy of steel containing adamantite [...]
The value of this alloy is that when it is exposed to the strange radiation in the Drow homeland (see MODULE D3, VAULT OF THE DROW) for a period of one month, its magical bonuses come to the fore.
The drow also use poison, but I cannot find any reference to adamantite working differently with poison, except that both drow adamantite armor and drow poison are impaired by being brought to the surface.