While reading through the Inquisition War trilogy, there were a few instances where Space Marines were noted to have a psychic shielding in their helmets as proof against the mind-affecting influence of Warp entities, and thus making them less likely to need mind wiping after an engagement with said foes, operating in much the same manner as a tin foil hat.
The context by which this substance (known as psycurium) is referenced leads me to believe it is some form of processed mineral that lines an Astartes helmet. To render psycurium into game mechanics needn't be too awful difficult…
Psycurium, Requisition 10, Respected
A Space Marine wearing a helmet lined with Psycurium gains a +10 bonus to WP Tests made to resist Psychic Powers that influence or control the mind. Psycurium will also offset negative WP modifiers associated with being in close proximity to creatures with the Daemonic Trait, such as those caused by Daemonic Aura or Mask of Seeming (note that psycurium merely reduces the WP Test penalty by one step, and will not provide a positive bonus).
As psycurium behaves as a baffle between the human mind and energies/influences of the Warp, Librarians (or other human Psykers) attempting to manifest Psychic Powers while wearing a helmet lined with psycurium will find it more difficult to fully manipulate said energies, reducing their base Psy Rating by 1.