The quartz α → β transition is a displacive phase transition associated with a significant change in elastic properties. However, the elastic properties of quartz at high-pressure and temperature remain poorly constrained experimentally, particularly within the field of β-quartz. Here, we conducted an experimental study on the quartz α → β transition during which P-wave velocities were measured in-situ at pressure (from 0.5 to 1.25 GPa) and temperature (200–900°C) conditions of the continental lower crust. Experiments were carried out on samples of microcrystalline material (grain size of 3–6 μm) and single-crystals. In all these, the transition was observed as a minimum in P-wave velocities, preceded by an important softening while P-wave velocities measured in the β-quartz field were systematically lower than that predicted by thermodynamic databases. Additional experiments during which acoustic emission (AE) were monitored showed no significant peak of AEs near or at the transition temperature. Microstructural analysis nevertheless revealed the importance of microcracking while Electron Back-Scatter Diffraction (EBSD) imaging on polycrystalline samples revealed a prevalence of Dauphiné twinning in samples that underwent the transition. Our results suggest that the velocity change due to the transition known at low pressure might be less important at higher pressure, implying a change in the relative compressibilities of α and β quartz. If true, the velocity changes related to the α → β quartz transition at lower crustal conditions might be lower than that expected in thickened continental crust.

中文翻译：

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较低大陆地壳压力和温度条件下 α → β 石英转变的纵波速度

石英α  →  β转变是一种位移相变，与弹性特性的显着变化相关。然而，石英在高压和高温下的弹性特性在实验上仍然很难受到限制，特别是在β-石英领域。在这里，我们对石英α  →  β转变进行了实验研究，在大陆下地壳的压力（0.5至1.25 GPa）和温度（200至900°C）条件下现场测量了P波速度。对微晶材料（晶粒尺寸为 3-6 μm）和单晶样品进行了实验。在所有这些中，观察到 P 波速度的转变为最小值，随后出现重要的软化，而在 β石英场中测量的 P 波速度系统地低于热力学数据库预测的速度。监测声发射 (AE) 的其他实验表明，在接近或处于转变温度时没有明显的 AE 峰值。然而，微观结构分析揭示了微裂纹的重要性，而多晶样品的电子背散射衍射 (EBSD) 成像揭示了经历转变的样品中普遍存在多菲涅孪晶。我们的结果表明，由于低压下已知的转变而导致的速度变化在高压下可能不太重要，这意味着α和β石英的相对压缩率发生了变化。如果属实，则在较低地壳条件下与α  →  β石英转变相关的速度变化可能低于在增厚的大陆地壳中预期的速度变化。