There's a % methanol used as anti-freeze in the geo loops. So a water / methanol mix.
The 53 degree fluid coming in from the geo field is then used inside, where it is "concentrated" to deliver whatever temp I need, from 85F in the floors to 120 for domestic hot water. Very simple, very efficient.
As a side note, the geo field methanol / water fluids never actually circulate in the house. The geo fluid runs to a heat exchanger, and the floor fluid also runs to the heat exchanger. There the geo fluids give up the energy to the floor fluids. This cools the geo fluids, and then the geo is pumped back outside for re-warming.
Just under 1 mile of this pipe with methanol / water circulating slowly.