![]() ![]() ![]() Tensile stresses are produced on the crystals by the effect of the different strength of Teflon compared to steel. The system developed to analyze the signals coming from the fracturing of minerals is a “piston-cylinder” consisting of a Teflon cylinder and two steel pistons. For these purposes, equipment capable of crushing mineral samples has been developed, together with a system for the detection and recording of electromagnetic emissions. An image illustrating some preliminary results has been included in the Supplementary materials (Figure S2 of Supplementary Materials). ![]() In the same period, other minerals were also tested in order to verify the difference in intensity and spectral characteristics of the triboemissions of quartz and other solid phases subjected to the same stresses. The aim of this work is to improve our understanding of the spectral characteristics of the signals generated by quartz crystals subjected to shear stresses, in order to correlate the intensity of the stress with the morphological quantities and the electromagnetic radiation emitted during the formation of the new fracture surfaces. In order to cause the reduction of minerals, the energy supplied is consumed not only in the opening of new crack surfaces, through the breaking up of atomic bonds, but also in the plastic deformation of the reticular regions around the crack tip: part of the fracture energy is dissipated to create a permanent deformation of the crystal lattice, with the consequent formation of amorphous zones around the crack tip and, in part, re-emitted as EMR. In general, to obtain the reduction of crystals, the application of a tensile stress is more effective than that of a compressive nature. The solid compounds are broken down by the extension of the microfractures, already present inside the gratings, through relaxing efforts, until they are separated into smaller fragments. Consequently, it is crucial to understand which fingerprints are from EMR from stress, connected with deformation processes of the earth’s crust, even of a possible precursor of seismic events, discriminating them from other emissions of natural and human origin. These emissions bring with them a series of useful information to understand, in a remote and non-invasive way, what happens inside rocky bodies and crystals and, in particular, the state and size of the fractures, the degree of plastic deformation suffered, and more. In particular, previous laboratory experiments have allowed us to verify that EMR induced by mechanical stress on rocks and single crystals are emitted over a very wide energy spectrum, with frequencies ranging from a few Hz to heat emissions in thermal infrared and optical band emissions in visible and UV and, for some solids, in the X-ray range. Research of this type is made possible by laboratory simulations of natural phenomena. ![]() However, there is a possibility that the recorded signals are not of tectonic origin but may be produced by other processes of geophysical or anthropogenic origin, so it seems appropriate to develop investigation techniques that allow us to distinguish the origin of the signal by tracing, where possible, any signatures characteristic of signals of a different origin from the tectonic one. This migration could be used to distinguish possible natural signals emitted by quartz in tectonically active environments from possible signals of other geophysical and possibly anthropogenic origin.Įlectromagnetic emission monitoring (EMR) has long been proposed for earthquake forecasting–oriented research. In particular, the continuous recording of the radio emission spectra shows a migration of the peaks toward higher frequencies, as stress continues over time and smaller and larger fractures form. For the first time, a characteristic migration of peak frequencies was observed, proportional to the evolution of the fracturing process. The spectrum of radio emissions consists of continuous radiation and overlapping peaks. These emissions represent a tiny fraction of the total energy dissipated in the fracturing process. The data obtained indicate that shear-stressed quartz crystals can generate electromagnetic emissions in the LF–MF range. For these tests, a new type of piston-cylinder has been developed, instrumented to collect the electromagnetic signals generated by the quartz during shear stress tests and that allows energy measurements on electromagnetic emissions (EMR) to be performed. Shear tests on quartz rocks and single quartz crystals have been conducted to understand the possible relationship between the intensity of detectable stress in fault areas and the energy released in the form of electromagnetic waves in the range 30 KHz-1 MHz (LF–MF). ![]()
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