  In this work we study the properties of a relativistic mixture of two
non-reacting dilute species in thermal local equilibrium. Following the
conventional ideas in kinetic theory, we use the concept of chaotic velocity.
In particular, we address the nature of the density, or pressure gradient term
that arises in the solution of the linearized Boltzmann equation in this
context. Such effect, also present for the single component problem, has so far
not been analyzed from the point of view of the Onsager resciprocity relations.
In order to address this matter, we propose two alternatives for the Onsagerian
matrix which comply with the corresponding reciprocity relations and also show
that, as in the non-relativistic case, the chemical potential is not an
adequate thermodynamic force. The implications of both representations are
briefly analyzed.
