Primordial atmospheres are found on young Terrestrial or similar worlds, and are a mix of various compounds, mostly of Hydrogen. They include Methane, Hydrogen Sulphide, Ammonia and Water Vapour, along with Neon.
Such atmospheres are often the second a terrestrial world has, since they initially have Hydrogen atmospheres gathered during the planetary formation phase. Over time, the hydrogen either binds with other gases ejected from the crust or is lost to space, leaving behind a Primordial atmosphere.
A Primordial atmosphere provides the basic environment which can lead to life. As time progresses, they can give way to an atmosphere dominated by CarbonDioxide.