α/β phase transition in quartz monitored using acoustic emissions

Abstract

SUMMARY It is usually suggested that thermal cracking in a quartz‐bearing rock results from the anomalously high volumetric expansion coefficients of quartz (e.g. Simmons & Cooper 1978). It has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient (Friedrich & Wong 1986). Experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures (e.g. Friedrich & Johnson 1978; Homand‐Etienne & Troalan 1984; Atkinson, McDonald & Meredith 1984; Meredith & Atkinson 1985). Here we report on acoustic‐emission experiments that monitor the process of thermal cracking as it occurs during heating, supported by measurements of crack surface area, pore‐fluid permeability, porosity and surface conductivity carried out on rock samples treated to various peak temperatures. The acoustic‐emission measurements show a strong peak of microcracking at the phase transition temperature for quartz (˜573°C) superimposed upon a background of microcracking due to thermal expansion. There is also a clear peak of microcracking at higher temperatures (˜800°C) that can be attributed to oxidation‐dehydroxylation reactions of hornblende and chlorite. Measurements of fluid permeability, pore surface area, porosity and electrical conductivity, made on samples that have been heat treated to various maximum temperatures, show increases associated with a major episode of cracking in the 500‐600°C temperature range, indicating that the new cracks form a well‐interconnected network. This has been confirmed by SEM and optical microscopy These results have implications for the electrical conductivity of the continental crust, providing a mechanism enabling the high pore‐fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth. It should be recognized, however, that th***se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures.

Thermal weakening of the carbonate basement under Mt. Etna volcano (Italy): Implications for volcano instability

“…We can conclude that (1) mineralogical changes (and in particular decarbonation) and phase transitions give raise to the most significant changes in the elastic moduli of rocks: e.g., decarbonation (in the case of carbonates, this study), the α-β quartz transition (in granites and sandstones, see Glover et al, 1995), and the loss of water from zeolites (in pyroclastic deposits, see Heap et al, 2012). In the absence of chemical alteration, and when the rocks already contain an extensive thermal microcrack network [the case for the basalt of Heap et al (2009) and the andesite of Petrakova et al, 2012], no changes were observed.…”

Strength Failure and Crack Evolution Behavior of Rock Materials Containing Pre-existing Fissures

“…should not be expected between 100 and 200 °C, but become more significant above 400 or 500 °C (Tian et al 2012). Indeed, thermal cracking experiments have shown that the greatest changes are likely to be achieved once the temperature has exceeded 573 °C, i.e., the α/β transition of quartz (Glover et al 1995). Li et al (2011) investigated the change in the longitudinal wave velocity and mechanical behavior of sandstone after different high-temperature treatments using triaxial unloading experiments.…”

“…Many experimental investigations have demonstrated that high temperature significantly affects the physical properties of sandstones (Heuze 1983;Glover et al 1995;Chopra 1997;Zhang et al 2001;Tang et al 2011;Zhao et al 2012). Analytical and numerical modeling of thermomechanical processes in sandstones have shown that variations in rock physical properties (such as bulk density, porosity, compressional wave velocity, etc.)…”

Influence of temperature on brittle creep in sandstones

“…Temperature equilibrium in the sample was reached after approximately 75 minutes, and the thermal gradient along the sample was less than 1°C/cm. Experiments were performed at temperatures from 20°to 75°C because previous studies suggest (1) that thermal cracking in many crustal rocks commences around 100°C [Glover et al, 1995], so is likely to be minimised or precluded over this range, and (2) that this range is sufficient to significantly influence the rate of stress corrosion [Meredith and Atkinson, 1983].…”