Mor'tonium is a highly-radioactive alloy discovered and manufactured by the Tau primarily to power their Ion Weapons. A fairly recent discovery, Mor'tonium rapidly decays upon exposure to air and releases extremely large numbers of ion particles in the process. Tau Ion Weapons can choose use this property in overcharging their shots and making them far more explosively powerful, simply by exposing their power source to the atmosphere.
For many years the Tau Earth Caste has been seeking ways to produce Ion Weaponry usable by the infantry. Previously, in order to generate the high-energy particles emitted by Tau Ion Weapons, a powerful generator or small reactor was necessary. Thus, only vehicles or Tau Battlesuits were large enough to utilise the potent Ion Weapons, as only they could carry the bulky power sources required. Recent developments, however, have produced a new finding -- a new alloy was discovered after enriching Iridium far past the point previously thought possible. This highly reactive material was named Mor'tonium and, although dangerous, it has proven highly useful.
When exposed to air, Mor'tonium immediately begins to deteriorate, creating a profusion of ion streams that can be harnessed and focused into a beam to fire a highly charged shot. This discovery meant that a heavy generator was no longer needed, allowing for the development of infantry-portable versions of Tau Ion Weapons that immediately went into Earth Caste production. The most notable result of this program was the Ion Rifle.
A cylindrical slab of Mor'tonium powers an Ion Rifle, and its power source can be recharged or overcharged simply by exposing the unstable alloy to the atmosphere. When the alloy has entirely degraded, a new slab can be quickly fitted into the weapon's protective housing. While there are issues -- the average Fire Warrior’s vital functions become compromised within a matter of solar months under the barrage of such intense radiation exposure when the weapon is used -- the high strength and long range of the Ion Rifle have made such inherent risks worthwhile. Already the Earth Caste is working on developing greater degrees of protection for Tau infantry who use Mor'tonium-powered Ion Weapons, although thus far no results of the program have proven cost-effective.
- Codex: Tau Empire (6th Edition), pg. 72