  A propagating torsion model is derived from the requirement of compatibility
between minimal action principle and minimal coupling procedure in
Riemann-Cartan spacetimes. In the proposed model, the trace of the torsion
tensor is derived from a scalar potential that determines the volume element of
the spacetime. The equations of the model are written down for the vacuum and
for various types of matter fields. Some of their properties are discussed. In
particular, we show that gauge fields can interact minimally with the torsion
without the breaking of gauge symmetry.
