Fractional abundance of SiO (full curves), as determined when including grain-grain processing, using the present model (in red) and the multi-fluid model of Paper III (in blue); the shock parameters are n_H   =   10⁵ cm^-3, V_s = 30 km s^-1 and b = 1.5. The temperature of the neutral fluid is shown also (broken curves; right-hand ordinate).