Iridium Armour is highly advanced and very special form of Tau battlesuit armour, coming in the form of extra iridium armour plates spread across the surface of the battlesuit. The extra armour makes the suit a formidable enemy and very hard to kill, though the extra weight makes it potentially move more slowly and extremely expensive.
Battlesuits who wear these are called XV8-02 Crisis 'Iridium' Battlesuit. They sort of resemble additional armoured plates situated around the neck and thigh regions of the Battlesuit, although the XV8-02 Iridium Suit is covered entirely of this shit. In effect, they function almost identical to Slat armour rather than an APS (Active Protection System), in which it is a static barrier meant to 'tripwire' a shaped-charge warhead before it actually impacts the actual armour.
In real life, Iridium is a very hard and dense metal with the unique properties of being extraordinarily resistant to both heat and corrosion. It is a brittle metal, however, and works best in conjunction as an alloy with other metals. As such, we can presume that the Iridium Armour is a form of Iridium alloy.
Regular XV8s are known to wear these, albeit limited in the aforementioned neck and thigh areas. On the tabletop, this is granted as an armour upgrade, without using a Signature System slot to do so, giving them a shiny 2+ armour save. Nice.
|Armour of Warhammer 40,000|
|Imperials:||Flak Armour - Carapace Armour - Power Armour (Human Power Armour - Dragon Scale Armour |
Exo-Power Armour - Space Marine Power Armour - Artificer Armour - Aegis Armour - Auramite Armour)
Terminator Armour - Centurion Armour
|Chaos:||Flak Armour - Carapace Armour - Power Armour (Chaos Power Armour - Warp-Forged Armour) |
Chaos Terminator Armour - Fleshmetal Armour
|Asuryani:||Mesh Armour - Aspect Armour - Exarch Armour - Phoenix Armour - Rune Armour|
|Drukhari:||Kabalite Armour - Wychsuit - Incubus Warsuit - Ghostplate Armour|
|Harlequins:||Holo-Suit - Death's Legacy|
|Orks:||Studded Armour - Flak Armour - 'Eavy Armour - Mega Armour|
|Tau:||Kroot Armour - Recon Armour - Combat Armour - Battlesuit - Iridium Armour|
|Necrons:||Necrodermis - Sempiternal Weave|
|Tyranids:||Chitin - Carapace - Armoured Exoskeleton - Armoured Shell|