In mixed line rate (MLR) networks, different line rates can coexist on the same fiber, on different wavelengths. Each lightpath can be established end-to-end, with requested line rate. Advanced modulation techniques are required for high line rates. Signals being propagated over transparent paths are exposed to detrimental effects of physical layer impairments (PLI). Advanced modulation techniques are more susceptible to impairments, especially to the cross-phase modulation (XPM) induced by intensity-modulated channels. In this study, we investigate the impairment-aware lightpath provisioning problem for MLR networks. We consider a transparent optical network where we aim to maximize the number of established connections, while avoiding disruption of existing lightpaths. We propose a weighted approach for impairment-aware lightpath provisioning in MLR networks. We employ an auxiliary graph to capture the PLIs on differently modulated channels with a weight assignment scheme. Simulation results show that the performance of our approach is noteworthy, in terms of blocking probability, bandwidth blocking ratio, and resource consumption.