“Pre-Earthquake” Micro-Structural Effects Induced by Shear Stress on α-Quartz in Laboratory Experiments

Abstract

This paper presents the results of measurements performed on α-quartz subjected to shear stress in dry conditions, to understand the relationship between the shear intensity and the resulting physical and chemical effects. If a shear stress of intensity higher than 100 MPa is applied continuously to alpha quartz crystals, they will tend to lose their crystallinity, progressively reduce their friction coefficient (Cof) and change into a low-order material, apparently amorphous under X-ray diffraction, but with a structure different from silica glass. Raman and Pair Distribution Function analyses suggested a structure like cristobalite, a silica polymorph well-known for its auxetic behavior, i.e., having a negative Poisson ratio. This elastic parameter pre-eminently controls the friction coefficient of the material and, if it is negative, the Cof lowering. As a result, the increase in low crystallinity cristobalite is sufficientto explain the lowering of the quartz friction coefficient up to values able to contribute, in principle, to the triggering processes of active faults. This allows hypothesizing a slip induction mechanism that does not include the need to have the interposition of layers of hydrated silica, as invoked by many authors, to justify the low friction coefficients that are achieved in shear stress tests on rocks abundant in quartz.

Fracture Analysis of α-Quartz Crystals Subjected to Shear Stress

“…The data obtained in this study should be seen in the light of the most recent literature on the effects of shear stress on alpha quartz, literature to which the authors have also contributed [5,6]. The fractures we have observed show an evolution which, we believe, depends substantially on the stress rate rather than on the pressure itself.…”

“…The task of this experimental work is to understand the mechanisms that link the progressive demolition of the crystal structure of quartz and the appearance of polymorphic phases during the application of sliding stresses. In two previous works [5,6], the authors of this work reported details on the polymorphic phases of silica that are formed during fracturing and on the correlations between fractures and the amount of very low frequency electromagnetic energy emitted by the crystals during the fracturing.…”

“…The automated scanning system allows one to measure and analyze about 1 million particles per hour and to know the values of the perimeter, area and volume of each particle, from 300 µm to 1 µm. This scanning microscope has already been used previously by the authors for investigations on asbestos fibers in soils and recently on quartz crystals [5,6]. To determine the morphologies, the particles are dispersed on the surface of the optical plate.…”

“…In a recent publication, we formulated another hypothesis to explain the lowering of the coefficient of friction in experiments on quartz-containing rocks. This hypothesis is based on the progressive formation of nanocrystalline cristobalite, which works as a solid lubricant [5,6]. The silica polymorph called cristobalite has the particularity of being an auxetic material, i.e., with a negative Poisson ratio, both in the form of low temperature (alpha) and in that of high temperature (beta).…”

“…This phase would be nanocrystalline, not visible by X diffraction but visible by Raman spectroscopy. The accumulation of this phase in the interface between the sliding surfaces would cause the friction coefficient to decay more or less rapidly, depending on the intensity of the dynamometamorphic action [6]. Furthermore, quartz's structural decay is marked by the decay of the electromagnetic signals generated by its fracturing, which diminishes over time as fracturing and damage of the original crystalline structure proceed [5,6].…”