Are both of these valid?
on the periphery
in the periphery
What would be the appropriate usage? Would it be sufficient to use peripheral as a replacement?
Macmillan Dictionary defines periphery as:
It includes this phrase as an example usage:
You can also say at the periphery (the party moved so far to the right that what had been at the periphery was now the center) or along the periphery (an invisible electric fence runs along the periphery of my property).
You can say in the periphery if you are referring to the very edges of something (he drove until he caught the outline of a shed in the periphery of the truck's headlights). It's common to see economics or history textbooks (like this one) discussing countries in the core, or center, and countries in the periphery.
If something is on, in, or at the periphery, then you can say it is peripheral (peripheral republics were far from Moscow, geographically, culturally, and linguistically).