Abstract: The Mg I b2 line at 5173 Å forms over a large range of heights but its core, which forms under conditions of non-local thermodynamic equilibrium, is most sensitive to heights near the temperature minimum, a region of the solar atmosphere that has not been sufficiently explored. The next-generation solar observatories will have access to this spectral line and will allow for multi-line observations to study the different layers of the solar atmosphere simultaneously and with unprecedented polarimetric sensitivity. We will present a morphological classification of the intensity and circular polarization profiles of this spectral line at high-spatial-resolution, using observations from the Swedish 1-m Solar Telescope. We will also discuss the results of the weak field approximation applied to the Mg I b2 line, and their comparison with inversion results of the Fe I 6173 Å line to understand how the magnetic field changes with height in the solar atmosphere.