Hello, By geothermal, do you mean letting the ground warm, or temper water in the PEX line, or something along the lines of a geothermal system using refrigerant… ? I know next to nothing about the latter 😕 As to the former, any type of pipe would work as long as it can take the pressure and is sized for the flow with minimal pressure loss. I like to use the B&G System Syzer for that sort of figuring. http://bellgossett.com/selection-sizing-cad-tools/system-syzer/
One concern about buried PEX has been that it can absorb chemicals that may be in the ground, and transfer them to the water. Likely all plastics do this somewhat, but PEX got singled out when California was debating whether to allow PEX. So, don’t bury PEX in old auto wrecking yards or Superfund sites!