Long term soil deformations and stress losses associated with Civil Engineering works are consequences of time-dependent processes, e.g., creep and stress relaxation. Although it is well known that all soils exhibit rheological behavior, most previous studies strive to model the time-dependent behavior and/or examine the time effects in clayey soils. There are few investigations that center on stress relaxation in granular materials, and studies that relate the effects of mineral composition on stress relaxation are non-existent. Thus, this research aims to contribute toward an improved understanding of the time-dependent behavior of sands with laboratory experiments of stress relaxation of heterogeneous sand mixtures loaded in one-dimensional compression. The evolution of stress relaxation at various maximum imposed stress levels, along with accompanying particle breakage factors, are documented and compared for specimens with varying degrees of mineral heterogeneity. Experimental results show that, at a fixed loading strain rate, the extent of relaxation is controlled primarily by the stress-strain state of the soil and previously incurred particle breakage. Similar mechanical response dependency trends are observed when heterogeneity is increased with 'weak' grains.