The logK is the equilibrium constant in the mass action equation of interest. For instance, K aqueous speciation in Equation 5 of:
http://documentation.pflotran.org/theory_guide/mode_reactive_transport.html
Or the K in transition state theory for mineral precipitation-dissolution (Equation 17 below)
In the database, these are temperature dependent if the values outside of 25 degrees C are not equal to “500.0”. 500 is the undefined value.
Glenn