Example for a CLOUDY photoionization model of Galactic HVC gas at a distance of D = 50 kpc from the Galactic disc, with an H i column density of log N(H i) = 19 and a metallicity of 1.0 solar. Shown are the expected column densities of Si ii and Mg ii as a function of the gas density, n_H, and the ionization parameter, U. Because of the similar ionization potentials and cosmic abundances of both elements/ions the column densities of Si ii and Mg ii in HVCs are expected to be very similar.