I have stuff in my notes about adamantine's durability and ability to autocrit against structures and objects (5e), reduce critical hits to normal ones (5e) and act as a virtually indestructible material in general across editions.

I also have something in my notes about adamantite, a material developed by the drow. Poison doesn't dry when applied to its surface and it has all other properties of adamantine, but crumbles to dust in the sun.

I've gone back to check on some things, and apparently the sun-disintegration comes from drow-forged weapons in general, or at very least their adamantine weapons. To make matters worse, now I can't seem to find any instance where adamantite actually exists as anything other than an incorrectly-used reference to adamantine.

Am I going crazy, or does lore about adamantite as a drow-forged variant actually exist somewhere?

I'm tagging this question under multiple system editions to try and get a more thorough, and perhaps wider-reaching, knowledge base for this.

Adamantine metal was known as "adamantite" in earlier D&D products.

In AD&D 1e, Gygax refers to the metal as "adamantite". Later writers, during AD&D 2e, would correct this nomenclature, since the English suffix -ite normally denotes a mineral, and the metal was renamed to adamantine.

Drow of the Underdark (1991) retconned that "adamantite" was a mineral from which the metal alloy adamantine could be made (credit to illustro's answer for this, which deserves upvotes). Pure adamantite worked into a metal is called "adamantium", which Volo's Guide to All Things Magical (1996), p.55, carefully renames to "adamant", defining adamatine as a durable alloy of adamant, silver and electrum.

However, this is subsequently contradicted by the D&D 3.5 Dungeon Master's Guide (2000), p. 283, where adamantine is simply a unique meteoric metal. Magic of Faerûn (2001) concurred with this and wrote off the "alloy of silver and electrum" theory described in Volo's Guide as a fanciful myth.

The connection between "adamantite" and the drow is that this was the nomenclature used by Gygax when he wrote D3 Vault of the Drow (1978). Even in the specific context of the drow, we see this corrected to "adamantine" in later books (e.g. Drow of the Underdark (2007) p.199). From 3e onward, the older term "adamantite" was almost never used.

Adamantite

"Adamantite" appears briefly in the AD&D 1e Player's Handbook (1978), and again in the Dungeon Masters Guide (1979), where it is a material certain items are made from: Daern's Instant Fortress (p. 152) has "adamantite walls", The Saw of Mighty Cutting (p. 153) is an "adamantite blade", and the Talisman of the Sphere (p. 155) is an "adamantite loop". +5 armor is adamantite alloyed steel (p. 154).

An "adamantite door" appears in Gygax's S1 Tomb of Horrors. Even after Gygax's departure fron TSR, the use of "adamantite" continued into many products of AD&D 2e, such as the Planescape Planes of Conflict (1995), where adamantite dragons are said to inhabit Bytopia (Liber Benevolentiae, p.43).

Adamantine

Some AD&D 2e books referred to "adamantine". After Drow of the Underdark (1991), Book of Artifacts (1993) refers to the metal "adamantine" (credit to illustro for this reference). This is not used entirely consistently throughout AD&D. The AD&D 2e revised Dungeon Master Guide (1996) still used "adamantite" throughout.

However, the use of "adamantine" to refer to the metal was standardized with D&D 3e (2000), where it appeared in the Dungeon Master's Guide, and has remained so ever since. References to "adamantite", "adamantium" and "adamant" have been discontinued.

D&D 3e routinely converts specific instances of "adamantite" to adamantine. Daern's Instant Fortress is now made of adamantine. Bytopia is now inhabited by adamantine dragons (Dragon #321, p.44). The 3e Tomb of Horrors web conversion even describes the 1e-era doors as having been 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 described the drow working an alloy of "adamantite" and steel, which they draw into a wire used to make a fine mesh for armor. (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.

I cannot find any reference to adamantite working differently with poison, except that both drow adamantite armor and drow poison lose their efficacy when taken from the Underdark to the sunlit surface.

In fact, the drow specifically have to re-apply their poison over time:

Drow sleep poison decays instantly in sunlight. Its power is lost after about 60 days in any event, and the coating on the small bolts and javelins must be periodically renewed with fresh applications of the fungoid substance. The Dark Elves will often have small barrels filled with several packets of this poison, each sealed to insure the poisonous substance remains fresh for about 1 year.

Summary

• 
  AD&D 1e: Adamantine is a metal which is virtually indestructable. It can be made into armor or items. The drow alloy it with steel.


• 
  AD&D 2e: Technically, adamantite is the ore from which the metal adamant (also called adamantium) is made. This metal is alloyed with silver and electrum to make the alloy adamantine. However, the term "adamantite" is still widely used to refer to the metal itself.


• 
  D&D 3e onward: Adamantine is a rare meteoric metal. It's not made by alloying with silver and electrum; that's just a myth. All previous references referring to the metal as "adamantite" are updated to the new nomenclature.

There is a difference between the two, elucidated in the AD&D (2e) Volo's Guide to all things Magical

On page 55 of AD&D 2e Volo's Guide to all Things Magical (1996) [in the section on Raw Materials: Metals] the following paragraphs speak about the differences between the substances:

Adamant: This is the pure metal form of the hard, jet-black ferromagnetic ore known as adamantite, from which the famous alloy adamantine is made. Adamant is rarely found in nature, but when it is, it is always be in large spherical pockets in hardened volcanic flows.

[...]

Adamant is a gleaming, glossy black. Any reflections seen in it acquire rainbow edges, and this peculiar optical property is the sure-fire way to identify this surprisingly light, valuable metal.

Adamantine: This alloy, of five-eighths adamant to two-eighths silver and one-eighth electrum (itself a natural alloy of silver and gold) retains the hardness of adamant, but combines it with a rugged durability that makes adamantine so hard to shatter that it is the favored substance for the making of war hammer heads, the best nonmithral armor, and harbor chains. (By one of the miracles granted by the gods, adamantine can also be derived by combining steel and mithral if one knows how.⁵¹)

Adamantine is black, but has a clear green sheen in candlelight a sheen that
sharpens to purple-white under the light given off by most magical radiances and by will-o-wisps.

[...]

Essentially, adamantite is the ore, adamant is the adamantite ore worked into a metal and adamantine is an alloy of adamantite, silver and electrum.

The Drow Connection: AD&D 2e - The Drow of the Underdark

The Drow connection you have in your notes likely came from the AD&D 2e book The Drow of the Underdark (1991), which makes the same distinction between adamantite and adamantine as Volo's Guide to all things Magical.

One point to note, and potential difference to VGtatM (1996), is they do not make a distinction in tDotU (1991) between adamantite and adamant. This references in this product to adamantite (as opposed to adamantite ore) may actually be referring to the worked metal dubbed adamant in the 1996 release.

There are however some sections about a drow specific alloy of adamantite (also called adamantine confusingly), with additional properties.

Clothing And Weaponry

Drow weapons and armor are usually crafted from a dark, flexible metal known as adamantine, an alloy of adamantite. Most drow garb does not radiate a magical dweomer, but has magical effects.

Drow craftsmen, armorers, and wizards have developed careful processes to expose the materials from which these items are made to a specific combination of radiations and enchanted materials, and to handle the raw materials in exacting, largely secret ways.

This process takes a long time, and hence is ongoing: the adamantite ore that will be used in the adamantine alloy of a drow buckler may spend a year "baking" in the cold, hard radiations of a guarded, highly-prized natural cyst-cavern, in the heart of what was once a lava flow

Direct sunlight destroys most drow cloth, footwear, armor and weapons.

[...]

When most (90%) of the items listed above are exposed to the light of the sun for more than 2 rounds at a time (and any exposure totaling 5 rounds in a 5-day period, even if composed of brief instants), irreversible decay begins.

Within 2d6 days, the items lose their magical properties, and crumble to worthless powder. Drow items away from the Underdark's radiations that are carefully protected from full sunlight lose their quasi-magical properties in
and to handle the raw materials in exact- 1d20 + 30 days.

(Page 88, Chapter 7: Drow Craftwork)

Notably it also has sections which mirror the quotes in the other answers taken from 3rd edition about specific weapons and armor:

Drow Clothing

[...]

The non-encumbering, black mesh adamantite-alloy armor worn by almost all drow warriors encountered (equivalent to chain mail +1 to +5), and the bucklers many of them use (which carry a magical bonus of +1 to +3), are also imbued with their abilities thanks to the strange radiations of the Underdark.

(Page 69, Chapter 6: Drow Magical Items)

Whip of Fangs:

These belt whips have adamantite handles, but their tendrils are living snake heads, 2-5 in number

(Page 82, Chapter 6: Drow Magical Items)

They are basically interchangeable, somewhere around 3rd edition it became adamantine from adamantite. For example, there 4 references in the 1st edition Dungeon Masters Guide (DMG) to adamantite (see below for one instance comparison, emphasis mine), and zero references to adamantine.

DMG 1st Edition - Daern's Instant Fortress description (Page 142):

[...] The adamantite walls of Daern's Instant Fortress are totally unaffected by normal weapons other than those of catapult type. [...]

DMG 2nd Edition - Daern's Instant Fortress description (Page 165):

[...] The adamantite walls of Daern's Instant Fortress are totally unaffected by normal weapons other than those of catapult type. [...]

DMG 3.5 Edition - Daern's Instant Fortress description (Page 254):

[...] The adamantine walls of Daern's Instant Fortress have 100 hit points and hardness 20. [...]

Drow magical items do gradually lose their abilities and integrity when exposed to sunlight, however.

One online wikia that I looked at makes the claim that it was changed around 2000, however when I look through the Book of Aritifacts, copyright 1993, it refers to a couple of items as adamantine, and no references to adamantite.

Axe of the Emperors - Book of Artifacts (Page 19) -

The Axe of the Emperors is a double-bitted, two handed war axe. The head is of the brightest adamantine, forged so that it looks like a rising phoenix, the talons wrapping around the socket to clutch a sizeable ruby.

The BoA also refers to adamantine in creating items as well (Page 126):

If other materials are needed beyond the stick, the most obvious additions are ornamentations fashioned to it. Gems, beads, pearls, and bands of silver or gold are common. More uncommon components are feathers, exotic leather, adamantine wire, skeletal claws, and the like. These may sit in, dangle, rattle, wrap,
or point; a wand can have more than just a stick, too.

So it appears that somewhere in the early 1990's it was changed from adamantite to adamantine.

The disintegration is contained in the Monstrous Manual, 1994 (Pages 112-13) - Also note that it refers to their weapons and armor here as adamantite:

Drow wear finely crafted, non-encumbering. black mesh armor. This extremely strong mail is made with a special alloy of steel containing adamantite. The special alloy, when worked by a drow armorer, yields mail that has the same properties of chain mail +1 to +5, although it does not radiate magic.

And:

Direct sunlight utterly destroys drow cloth, boots, weapons,and armor. When any item produced by them is exposed to the light of the sun, irreversible decay begins. Within 2d6 days, the items lose their magical properties and rot, becoming totally worthless. Drow artifacts, protected from sunlight, retain their special properties for 1d20 + 30 days before becoming normal items. If a drow item is protected from direct sunlight and exposed to the radiations of the drow underworld for one week out of every four, it will retain its properties indefinitely.