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Prediction of the composition dependence of Calcium aluminosilicate (CAS) glasses mechanical properties is essential to aid in the development of new glasses with tuned properties for specific applications. In this investigation, we use microindentation to measure the hardness (H) and modulus (M) and the inelastic volume that dissipates the inelastic energy is elucidated using annealing and AFM to decompose it into contributions from densification and shear flow. We chose a family of CAS glasses with a constant CaO/Al2O3 ratio but varying SiO2 content, as well as a commercial >99% SiO2 glass. We observe a decoupling between M & H, M systematically decreases upon increasing SiO2 content, however, H exhibits a non-monotonic behavior with a minimum around 79% SiO2. The inelastically deformed volumes show a high contribution from shear flow for SiO2-poor glasses and a high contribution of densification for SiO2-rich glasses, demonstrating a shear to densification dominated transition between these two extremes.