Experimentelle Verformung von natürlichen Quarzeinkristallen : Einfluss der Wasserneuverteilung auf die Deformation und mikrostrukturelle Untersuchungen

Quartz is one of the common minerals present in the Earth’s crust that controls its rheology and has a simple chemical composition. There is a lot of data available for quartz, which makes it one of the best studied minerals in nature as well as in experiments. Griggs and Blacic (1965) first described the phenomenon of “hydrolytic weakening” in experimentally deformed natural quartz. They found that “dry” natural quartz deformed at much lower flow stresses in a water containing environment compared to “dry” quartz without added water.
The aim of this study is to investigate the effect of temperature, confining pressure and water on the deformation of natural quartz single crystal. For this purpose, axial compression experiments were performed in a modified solid medium (NaCl) Griggs apparatus at different temperatures (700 –1000°C) and different confining pressures (700 MPa, 1000 MPa and 1500 MPa) with constant strain rates of ~10^-6s^-1. Samples, 6.5 mm in diameter and 10 mm to 13.5 mm in length, were cored from a narrow zone of a constant fluid inclusion density in a large single crystal in two different orientations: 1) normal to one of the prism planes (⊥m) and 2) 45° to <a> and 45° to [c] (O+). These orientation were use to achieve the highest resolved shear stress to activate 1) prism <a> slip and 2) basal <a> slip and prism [c] slip.
The strengths of the samples of both orientations at 900°C are between 150 MPa and 250 MPa. At lower temperatures the strength is accordingly higher (800°C – ~800 MPa ⊥m or 550 MPa O+, 700°C - ~1600 MPa ⊥m or 1400 MPa O+). Changes in confining pressure show only very small variations in strength. In deformed samples and both orientations microstructures like undulatory extinction, deformation lamellae and deformation bands can be observed. Recrystallized quartz grains are extremely rare and isolated. The microstructures as well as the sample shape and the strength are typical for crystal plastic deformation by dislocation glide. Sample shape, orientation of microstructures, EBSD measurements and [c]-axis rotation (by CIP) are used to define the active slip systems. In ⊥m samples the dominant slip system is prism <a> slip, but [c]-axis misorientations indicate the activity of a secondary slip system. Basal <a> slip is the active slip system in O+ samples.
The observation of fluid inclusions and the measurements of water content by FTIR, point out that the fluid inclusions of the starting material are very heterogeneous. The quartz starting material, away from fluid inclusions is characterized by flat FTIR-absorption and the water content calculated from these spectra yields low values (~50 H/10^6 Si). Therefore it can be described as a “two phase” material of dry quartz and water rich fluid inclusions. During pressurization and deformation the fluid inclusions of the starting material change shape and size dramatically (several 100 µm down to nm scale). Especially pressurization leads to the decrepitation of fluid inclusions and the formation of microcracks, which are the pathways for water. Fast crack healing due to high temperatures facilitates the local wetting of quartz. Pressurized samples show broad absorption band, typical for molecular water. The water content varies between several thousands H/10^6 Si and very low water contents of only tens of H/10^6 Si. These variations are probably caused by the redistribution of fluid inclusions that cannot be avoided during the FTIR measurements. After deformation a broad absorption band is still present but differs from pressurized samples, in that it is more pointed. Additionally a sharp absorption band at 3585 cm^-1 appears. This band is only present in deformed regions.
Due to the redistribution of fluid inclusions and therefore water a “one phase” material develops which behaves like “wet” quartz (mechanically weak). High temperatures, fluid inclusions decrepitation and the fast crack healing are the precursor for plastic deformation by dislocation glide in quartz single crystals in our experiments.