Temperature-induced, low-frequency acoustic emission of quartz related to formation of microcracks, fluid inclusion decrepitation, and phase transitions in the crystal structure

Abstract

Quartz, as the most ubiquitously occurring mineral, was chosen for investigations of the acoustic emission upon heating. Various samples from a wide range of conditions of formation were analyzed in the frequency range of 400–10 000 Hz with a high-resolution decrepitometer. The acoustic emission patterns showed a variation of 5000–80 000 total signals over the entire temperature range of 90 °C-610 °C (1-cm3 sample, grainsize fraction 250–500 μm). Microthermometric investigations of the sound-emitting processes and correlation with microcrack patterns determined by scanning electron microscopy (SEM) showed that various sources contribute to the sometimes vehement noise production. Stress built up because of thermal anisotropy causes the formation of microcracks along inter- and intragranular grain boundaries over most of the observed temperature range. Accumulation of signals from the decrepitation of fluid inclusions reveals information about the conditions of formation and subsequent alterations of the investigated sample. Acoustic emission caused by phase transitions in the crystal structure (α〈 − 〉β-transition at 573 °C) is related to changes in the twinning state of the sample. Measurement of the complex patterns of acoustic emission produces a “fingerprint” of the conditions of formation and subsequent alterations the sample underwent in its geological history. They can be used to determine the extension of mineralizations and alterations or detection of geological structures even when covered by soil. Real-time frequency analysis of the signals shall reveal further information about the origin of single acoustic signals. [Work supported by the DFG.]