From dirt to diamonds: A mineralogical comparison of granular rheologies
Physical and Biological Sciences
Earth & Planetary Sciences
The geologic shear zones of the Earth contain a diverse array of mineral species in pulverized, granular form. From active fault zones to slope failure planes, sheared granular flows are a component of many human hazards. Despite this, relatively little is known about granular rheology, which can resemble a solid mass at rest, or, if significantly perturbed, a gaseous fluid. Recent research has centered on the transition between these two regimes, as a result of increasing shear stress, with wide applications to fault friction and earthquake nucleation. In this paper, I further characterize the rheology of granular media by exploring and quantifying the impact of grain strength and elasticity on flow dilation and compaction, both in a long-term sense and as a response to variable shear rate. To do this, I subject granular samples of different mineralogies to shear stress by means of a commercial torsional rheometer. Hard, stiff materials such as natural diamond showed dilation with increasing shear rate, as well as a large degree of compaction over time. Softer, elastic minerals showed more complicated behavior in the transitional regime, and compacted less with time. The experiment revealed convincing evidence that grain hardness and elasticity have a significant impact on long-term compaction. To the best of our knowledge, this effect has not been previously shown experimentally or suggested theoretically. A similar, but weaker relationship also exists between transitional, shear-rate dependent behaviors. I conclude that the mineralogy of shear zones will significantly affect volumetric behavior, and postulate several mechanical explanations.
